add command substitution $(...)

- $(command) is replaced by the output of command, which runs in a
  subshell (gosh -c), so cd/set in it do not change the shell
- trailing newlines are removed; unquoted the output is split into words
  at blanks and newlines, "$(...)" stays one word; wildcards in the
  output are not expanded
- nested substitutions, quotes and operators inside are handled; the
  list, alias and brace scanners skip $(...) as a whole
- a command that consists only of a substitution has its exit status,
  Ctrl+C in a substitution aborts the command
- $((...)) is reported as not supported

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -13,6 +13,12 @@ Built-in commands:
   set VAR=val   - Set environment variable
   export VAR=val - Export environment variable
   source file   - Run the commands of file in this shell (also: . file)
+  jobs          - List background and stopped jobs
+  fg [%n]       - Continue job n (default: current job) in the foreground
+  bg [%n]       - Continue stopped job n in the background
+  wait [%n]     - Wait for job n or all background jobs
+  alias [name[=value] ...] - Define or show aliases (alias ll='ls -l')
+  unalias name  - Remove an alias (unalias -a: all aliases)
   help          - Show this help
 
 Features:
@@ -24,19 +30,30 @@ Features:
     override with $GOSH_HISTFILE); lines starting with a space are not saved
   - Tab completion for commands and file paths
   - Tilde expansion (~, ~/path) at the start of unquoted words
+  - Brace expansion: a{b,c}d -> abd acd, {1..5}, {01..10..2}, {a..e}
   - Wildcards: * ? [abc] [a-z] [!abc] match file names (not hidden files
     unless the pattern starts with a dot); quoted wildcards and values of
     variables are not expanded, a pattern without match is kept as is
-  - Colored prompt (user@host green, red for root; directory blue),
+  - Prompt user@host:dir $, with [N] before the $ if the last command
-    configurable with $GOSH_COLORS, e.g. GOSH_COLORS='user=1;33:path=36'
+    failed with exit status N
-    (keys user, root, path; values are ANSI SGR codes, empty = no color);
+  - Colored prompt (user@host green, red for root; directory blue; exit
-    disabled with $NO_COLOR, TERM=dumb or when output is not a terminal
+    status red), configurable with $GOSH_COLORS, e.g.
+    GOSH_COLORS='user=1;33:path=36' (keys user, root, path, error; values
+    are ANSI SGR codes, empty = no color); disabled with $NO_COLOR,
+    TERM=dumb or when output is not a terminal
   - Environment variable expansion ($VAR, ${VAR}, ${VAR:-default});
     using an unset variable is an error, use ${VAR:-} for an empty default
+  - Command substitution: $(command) is replaced by the output of command
+    (run in a subshell); unquoted it is split into words, "$(command)"
+    stays one word
   - Command lists: a ; b (run both), a && b (b only if a succeeded),
     a || b (b only if a failed); $? is the exit status of the last command
   - Pipes (a | b) and redirections: < file, > file, >> file, 2> file,
     2>> file, 2>&1, >&2, &> file (stdout and stderr), &>> file
+  - Job control: command & runs in the background, Ctrl+Z stops the
+    foreground job; jobs, fg, bg and wait manage the jobs
+  - Aliases replace the first word of a command in interactive shells
+    (define them in ~/.goshrc); \name or 'name' skips the alias
   - Non-interactive use: gosh -c 'command', gosh script, command | gosh;
     the exit status is the one of the last command
   - Startup files: login shells (-l or argv[0] starting with -) read
@@ -90,6 +107,10 @@ Beispiel für `~/.goshrc`:
 export PATH=$HOME/bin:$HOME/go/bin:$PATH
 export EDITOR=vim
 
+# Aliase
+alias ll='ls -l'
+alias ..='cd ..'
+
 # Prompt-Farben (Anführungszeichen wegen des ;)
 export GOSH_COLORS='user=1;38;5;208:path=38;5;205'
 
@@ -104,6 +125,26 @@ export LANG=de_DE.UTF-8
 export PATH=$HOME/bin:$PATH
 ```
 
+### Befehlsersetzung
+
+`$(befehl)` wird durch die Ausgabe von `befehl` ersetzt:
+
+```
+$ echo "Heute ist $(date +%A)"
+Heute ist Montag
+$ kill $(pgrep -f mein-server)
+$ cd $(git rev-parse --show-toplevel)
+$ vim $(grep -l TODO *.go)
+$ echo "Dateien: $(ls | wc -l)"
+```
+
+- Der Befehl läuft in einer eigenen Subshell (einem neuen gosh-Prozess): `cd`, `set` oder `export` darin ändern die laufende Shell nicht. Er darf alles enthalten, was gosh kann (Pipes, `;`, `&&`, Umleitungen, verschachtelte `$(...)`), und liest von der gleichen Eingabe, Fehlermeldungen gehen auf stderr.
+- Abschließende Zeilenumbrüche der Ausgabe werden entfernt.
+- Ohne Anführungszeichen wird die Ausgabe an Leerzeichen, Tabs und Zeilenumbrüchen in mehrere Wörter geteilt, `"$(befehl)"` bleibt ein einziges Wort (mit den Zeilenumbrüchen darin).
+- Anders als in `sh` werden Wildcards in der Ausgabe nicht expandiert, und Zeichen wie `;` oder `|` darin sind normaler Text: Die Ausgabe ist immer nur Daten.
+- Besteht ein Befehl nur aus einer Ersetzung (`$(false)`), ist ihr Exit-Status `$?`, sonst zählt der Status des Befehls. Ctrl+C während der Ersetzung bricht die ganze Zeile ab.
+- In `'...'` oder mit `\$(` wird nichts ersetzt. Die alte Schreibweise mit Backticks (`` `befehl` ``) und Arithmetik `$((1+2))` gibt es nicht.
+
 ### Befehlslisten und Exit-Status
 
 Mehrere Befehle in einer Zeile werden wie in `sh` verknüpft:
@@ -128,6 +169,35 @@ Status 127
 
 Schlägt ein Programm fehl, gibt gosh selbst keine Meldung mehr aus (das Programm meldet den Fehler selbst), nur bei Fehlern von gosh, z. B. wenn ein Befehl nicht gefunden wird.
 
+### Klammer-Expansion
+
+Wie in bash erzeugt `{…}` aus einem Wort mehrere Wörter:
+
+| Ausdruck          | ergibt                          |
+|-------------------|---------------------------------|
+| `a{b,c}d`         | `abd acd`                       |
+| `file{,.bak}`     | `file file.bak`                 |
+| `{a,b}{1,2}`      | `a1 a2 b1 b2`                   |
+| `x{a,{b,c}}`      | `xa xb xc` (verschachtelt)      |
+| `{1..5}`          | `1 2 3 4 5`                     |
+| `{5..1}`          | `5 4 3 2 1`                     |
+| `{1..10..3}`      | `1 4 7 10` (Schrittweite)       |
+| `{01..10..4}`     | `01 05 09` (mit führenden Nullen) |
+| `{a..e}`          | `a b c d e`                     |
+
+```
+$ mkdir -p projekt/{src,doc,test}
+$ cp config.yaml{,.bak}          # Sicherungskopie
+$ ls *.{go,md}
+$ touch log-{01..12}.txt
+```
+
+- Die Klammer-Expansion kommt zuerst, die Ergebnisse durchlaufen danach `~`, Variablen und Wildcards: `*.{go,md}` wird zu `*.go *.md`, `{${A},b}` zu `$A b`.
+- Ohne Komma (und ohne gültige Folge) bleiben Klammern stehen: `{a}`, `{}`, `{a..1}`. Auch `${VAR}` wird nicht angefasst.
+- In Anführungszeichen oder mit `\` davor sind die Zeichen normale Zeichen: `"{a,b}"`, `\{a,b\}`. Ein Komma lässt sich mit `\,` schützen: `{a\,b,c}` ergibt `a,b c`.
+- Bei einer Umleitung muss genau ein Wort herauskommen, `> {a,b}` meldet `ambiguous redirect`.
+- Ein Wort darf höchstens 100 000 Wörter erzeugen.
+
 ### Wildcards
 
 Wörter mit `*`, `?` oder `[...]` werden durch die passenden Dateinamen ersetzt, sortiert:
@@ -179,7 +249,87 @@ $ grep -c error < build.log
 - Operatoren in Anführungszeichen oder mit `\` davor sind normale Zeichen: `echo "a|b" 'c>d' e\>f`.
 - Der Exit-Status einer Pipeline ist der des letzten Befehls. Beendet sich ein Befehl am Ende (z. B. `head`), bekommt der schreibende Befehl davor `SIGPIPE`, `yes | head -1` endet also sofort.
 - Die Built-ins `pwd` und `help` lassen sich umleiten und in Pipes verwenden (`help | less`). `cd`, `set`, `export`, `exit` und `quit` verändern die Shell selbst und sind in einer Pipeline nicht erlaubt.
-- Ein einzelnes `&` (Hintergrundprozesse) gibt es noch nicht, es wird als normales Zeichen übergeben.
+
+### Aliase
+
+Ein Alias ist ein Kurzname für einen Befehl, am besten in `~/.goshrc` definiert:
+
+```bash
+alias ll='ls -l'
+alias la='ls -la'
+alias ..='cd ..'
+alias gs='git status'
+alias ls='ls --color=auto'      # ein Alias darf den eigenen Namen enthalten
+alias please='sudo '            # endet mit Leerzeichen: das nächste Wort wird auch ersetzt
+```
+
+| Befehl                  | Bedeutung                                         |
+|-------------------------|---------------------------------------------------|
+| `alias name='wert'`     | definiert einen Alias (mehrere in einem Aufruf möglich) |
+| `alias`                 | listet alle Aliase (in einer Form, die sich wieder einlesen lässt) |
+| `alias name`            | zeigt einen Alias                                 |
+| `unalias name`          | entfernt einen Alias, `unalias -a` alle           |
+
+- Ersetzt wird nur das erste Wort eines Befehls, also am Zeilenanfang und nach `;`, `&&`, `||`, `|` und `&`: `ll | less` funktioniert, `echo ll` gibt `ll` aus.
+- Der Wert darf selbst mehrere Befehle enthalten (`alias up='cd .. && ls'`) und wird wiederum auf Aliase geprüft. Ein Alias, der gerade ersetzt wird, wird darin nicht noch einmal ersetzt, deshalb funktioniert `alias ls='ls --color=auto'`.
+- Endet der Wert mit einem Leerzeichen, wird auch das nächste Wort als Alias geprüft: Mit den Aliasen oben wird `please ll` zu `sudo ls --color=auto -l`.
+- Mit `\ll` oder `'ll'` umgeht man den Alias und ruft das Programm direkt auf.
+- Aliase gelten nur in interaktiven Shells (wie in bash), also auch in `~/.goshrc`, aber nicht in Skripten und bei `gosh -c`. Ein Alias wirkt erst ab der nächsten Zeile, nicht in der Zeile, in der er definiert wird.
+- Erlaubt sind Namen aus Buchstaben, Ziffern und `_ - . + : , @ % ^`.
+- Aliase werden auch mit Tab vervollständigt.
+
+### Job-Control
+
+In einer interaktiven Shell läuft jede Befehlszeile (bzw. jede Pipeline) als eigener *Job* in einer eigenen Prozessgruppe:
+
+```
+$ sleep 100 &                 # im Hintergrund starten
+[1] 12345
+$ vim notizen.txt             # Ctrl+Z hält vim an
+^Z
+[2]+  Stopped                 vim notizen.txt
+$ jobs
+[1]-  Running                 sleep 100 &
+[2]+  Stopped                 vim notizen.txt
+$ fg                          # vim wieder im Vordergrund
+$ bg %2                       # oder: angehaltenen Job im Hintergrund weiterlaufen lassen
+```
+
+| Befehl / Taste | Bedeutung                                                                 |
+|----------------|---------------------------------------------------------------------------|
+| `befehl &`     | startet den Befehl im Hintergrund, gosh wartet nicht auf ihn (`$?` ist 0) |
+| Ctrl+Z         | hält den Vordergrund-Job an (`$?` wird 128+SIGTSTP, unter Linux 148)      |
+| Ctrl+C         | bricht nur den Vordergrund-Job ab, nicht die Shell                        |
+| `jobs`         | listet laufende und angehaltene Jobs                                      |
+| `fg [%n]`      | holt Job `n` in den Vordergrund (und setzt ihn fort, falls angehalten)    |
+| `bg [%n]`      | setzt einen angehaltenen Job im Hintergrund fort                          |
+| `wait [%n]`    | wartet auf Job `n` bzw. auf alle Hintergrund-Jobs                         |
+
+- Ohne Angabe gilt der *aktuelle* Job (in `jobs` mit `+` markiert, der zuletzt angehaltene oder in den Hintergrund geschickte), `%-` ist der vorherige (`-`). `%n` und `n` meinen Job Nummer `n`.
+- Beendete Hintergrund-Jobs meldet gosh vor dem nächsten Prompt (`[1]+  Done   sleep 100`, bei Fehlern `Exit N`).
+- Liest ein Hintergrund-Job vom Terminal, wird er angehalten (wie in bash), mit `fg` bekommt er die Eingabe.
+- Die Terminal-Einstellungen werden gesichert und wiederhergestellt: Nach dem Anhalten von `vim` oder `less` funktioniert der Prompt normal, nach `fg` bekommt das Programm seine Einstellungen zurück.
+- `exit` (oder Ctrl+D) mit angehaltenen Jobs warnt einmal (`there are stopped jobs`), ein zweites `exit` direkt danach beendet gosh.
+- `cd`, `set`, `export`, `exit`, `source`, `fg`, `bg` und `wait` lassen sich nicht mit `&` im Hintergrund starten.
+- In Skripten und mit `gosh -c` gibt es keine Job-Control (kein Ctrl+Z, kein `fg`/`bg`), `&` und `wait` funktionieren aber. Hintergrund-Jobs lesen dort von `/dev/null`.
+- Wird ein angehaltener Job von außen fortgesetzt (`kill -CONT`), zeigt `jobs` ihn weiter als `Stopped`, bis er endet oder erneut angehalten wird; `fg` und `bg` funktionieren trotzdem.
+- Einen Job beenden lässt sich mit `kill` über die Prozess-ID (z. B. `kill 12345`); `kill %1` versteht gosh noch nicht.
+
+### Prompt
+
+Der Prompt zeigt Benutzer, Rechner und Verzeichnis. Ist der letzte Befehl fehlgeschlagen, steht sein Exit-Status in eckigen Klammern davor, bis wieder ein Befehl erfolgreich war:
+
+```
+flow@host:~/src $ false
+flow@host:~/src [1] $ gibtsnicht
+gosh: gibtsnicht: command not found
+flow@host:~/src [127] $ sleep 100     # mit Ctrl+C abgebrochen
+^C
+flow@host:~/src [130] $ true
+flow@host:~/src $
+```
+
+Typische Werte: `1` allgemeiner Fehler, `2` Syntaxfehler, `126` nicht ausführbar, `127` Befehl nicht gefunden, `128+n` durch Signal `n` beendet (`130` = Ctrl+C, `148` = mit Ctrl+Z angehalten unter Linux). Eine leere Eingabe ändert den Status nicht.
 
 ### Farben
 
@@ -190,6 +340,7 @@ Der Prompt wird über die Umgebungsvariable `GOSH_COLORS` eingefärbt. Sie beste
 | `user`    | `user@host`                    | `1;32` (fett grün)  |
 | `root`    | `user@host`, wenn man root ist | `1;31` (fett rot)   |
 | `path`    | aktuelles Verzeichnis          | `1;34` (fett blau)  |
+| `error`   | Exit-Status `[N]` nach einem Fehler | `1;31` (fett rot) |
 
 Die Werte sind [ANSI-SGR-Codes](https://en.wikipedia.org/wiki/ANSI_escape_code#SGR_(Select_Graphic_Rendition)_parameters), mehrere werden mit `;` kombiniert:
 
@@ -227,6 +378,9 @@ GOSH_COLORS='user=1;38;2;136;192;208:path=38;2;216;222;233'
 # Ozean: türkis und kursiv blau
 GOSH_COLORS='user=1;96:path=3;94'
 
+# Fehler-Status gelb auf rotem Hintergrund
+GOSH_COLORS='error=1;33;41'
+
 # Root-Warnung: weiß auf rotem Hintergrund
 GOSH_COLORS='root=1;97;41'
 

alias.go

@@ -0,0 +1,201 @@
+package main
+
+// Aliases: like in bash, an alias replaces the first word of a command and
+// is only expanded in interactive shells (including the startup files), not
+// in scripts or with gosh -c.
+
+import (
+	"fmt"
+	"io"
+	"sort"
+	"strings"
+	"sync"
+)
+
+var (
+	aliasMu sync.Mutex
+	aliases = map[string]string{}
+)
+
+// isAliasName reports whether s can be an alias name: letters, digits and a
+// few punctuation characters, but nothing that is special to the shell.
+func isAliasName(s string) bool {
+	if s == "" {
+		return false
+	}
+	for _, r := range s {
+		switch {
+		case r >= 'a' && r <= 'z', r >= 'A' && r <= 'Z', r >= '0' && r <= '9':
+		case strings.ContainsRune("_-.+:,@%^", r):
+		default:
+			return false
+		}
+	}
+	return true
+}
+
+// lookupAlias returns the value of an alias.
+func lookupAlias(name string) (string, bool) {
+	aliasMu.Lock()
+	defer aliasMu.Unlock()
+	value, ok := aliases[name]
+	return value, ok
+}
+
+// aliasNames returns the names of all aliases, sorted.
+func aliasNames() []string {
+	aliasMu.Lock()
+	defer aliasMu.Unlock()
+	names := make([]string, 0, len(aliases))
+	for name := range aliases {
+		names = append(names, name)
+	}
+	sort.Strings(names)
+	return names
+}
+
+// expandAliases replaces aliases at the start of every command in line (at
+// the start and after ;, &, &&, || and |). The value of an alias is expanded
+// again, except for aliases that are already being expanded, so that e.g.
+// alias ls='ls --color' works. If the value ends with a blank, the next word
+// is checked for an alias too. Quoted or escaped words (\ls, 'ls') are never
+// replaced.
+func expandAliases(line string) string {
+	return expandAliasesIn(line, map[string]bool{})
+}
+
+func expandAliasesIn(line string, active map[string]bool) string {
+	var (
+		out        strings.Builder
+		quote      byte
+		escaped    bool
+		commandPos = true // the next word is the name of a command
+	)
+
+	for i := 0; i < len(line); i++ {
+		ch := line[i]
+		if quote == 0 && !escaped && commandPos {
+			if ch == ' ' || ch == '\t' {
+				out.WriteByte(ch)
+				continue
+			}
+			commandPos = false
+			end := i
+			for end < len(line) && strings.IndexByte(" \t\n;&|<>", line[end]) < 0 {
+				end++
+			}
+			name := line[i:end]
+			if value, ok := lookupAlias(name); ok && isAliasName(name) && !active[name] {
+				active[name] = true
+				out.WriteString(expandAliasesIn(value, active))
+				delete(active, name)
+				// A trailing blank makes the next word a command name too
+				commandPos = strings.HasSuffix(value, " ") || strings.HasSuffix(value, "\t")
+				i = end - 1
+				continue
+			}
+		}
+
+		if quote != '\'' && !escaped && isSubstStart(line, i) {
+			// $(...) is copied as it is; the subshell does not expand
+			// aliases
+			if end := substEnd(line, i); end >= 0 {
+				out.WriteString(line[i : end+1])
+				i = end
+				continue
+			}
+		}
+
+		out.WriteByte(ch)
+		switch {
+		case escaped:
+			escaped = false
+		case quote == '\'':
+			if ch == '\'' {
+				quote = 0
+			}
+		case ch == '\\':
+			escaped = true
+		case quote == '"':
+			if ch == '"' {
+				quote = 0
+			}
+		case ch == '"' || ch == '\'':
+			quote = ch
+		case ch == '#' && (i == 0 || strings.IndexByte(" \t", line[i-1]) >= 0):
+			// Comment until the end of the line
+			out.WriteString(line[i+1:])
+			return out.String()
+		case ch == ';' || ch == '|':
+			commandPos = true
+		case ch == '&':
+			// & and && separate commands, &>, >& and <& do not
+			if !(i+1 < len(line) && line[i+1] == '>') && !(i > 0 && strings.IndexByte("<>", line[i-1]) >= 0) {
+				commandPos = true
+			}
+		}
+	}
+	return out.String()
+}
+
+// quoteAlias quotes a value for the output of alias, so that it can be read
+// back: 'value' with ' written as '\”.
+func quoteAlias(value string) string {
+	return "'" + strings.ReplaceAll(value, "'", `'\''`) + "'"
+}
+
+// builtinAlias defines aliases (alias name=value ...), prints them (alias
+// name ...) or lists all of them (alias).
+func builtinAlias(args []string, stdout io.Writer) error {
+	if len(args) == 1 {
+		for _, name := range aliasNames() {
+			value, _ := lookupAlias(name)
+			fmt.Fprintf(stdout, "alias %s=%s\n", name, sanitize(quoteAlias(value)))
+		}
+		return nil
+	}
+
+	var err error
+	for _, arg := range args[1:] {
+		name, value, define := strings.Cut(arg, "=")
+		if !isAliasName(name) {
+			err = fmt.Errorf("alias: `%s': invalid alias name", sanitize(name))
+			continue
+		}
+		if define {
+			aliasMu.Lock()
+			aliases[name] = value
+			aliasMu.Unlock()
+			continue
+		}
+		if value, ok := lookupAlias(name); ok {
+			fmt.Fprintf(stdout, "alias %s=%s\n", name, sanitize(quoteAlias(value)))
+		} else {
+			err = fmt.Errorf("alias: %s: not found", sanitize(name))
+		}
+	}
+	return err
+}
+
+// builtinUnalias removes aliases (unalias name ...) or all of them
+// (unalias -a).
+func builtinUnalias(args []string) error {
+	if len(args) < 2 {
+		return fmt.Errorf("usage: unalias [-a] name ...")
+	}
+	aliasMu.Lock()
+	defer aliasMu.Unlock()
+	if args[1] == "-a" {
+		aliases = map[string]string{}
+		return nil
+	}
+	var err error
+	for _, name := range args[1:] {
+		if _, ok := aliases[name]; !ok {
+			err = fmt.Errorf("unalias: %s: not found", sanitize(name))
+			continue
+		}
+		delete(aliases, name)
+	}
+	return err
+}

alias_test.go

@@ -0,0 +1,164 @@
+package main
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+// withAliases sets the aliases for a test and restores them afterwards.
+func withAliases(t *testing.T, defs map[string]string) {
+	t.Helper()
+	aliasMu.Lock()
+	saved := aliases
+	aliases = map[string]string{}
+	for name, value := range defs {
+		aliases[name] = value
+	}
+	aliasMu.Unlock()
+	t.Cleanup(func() {
+		aliasMu.Lock()
+		aliases = saved
+		aliasMu.Unlock()
+	})
+}
+
+func TestExpandAliases(t *testing.T) {
+	withAliases(t, map[string]string{
+		"ll":    "ls -l",
+		"ls":    "ls --color",
+		"..":    "cd ..",
+		"e":     "echo ",
+		"w":     "world",
+		"a":     "b",
+		"b":     "a",
+		"multi": "cd /tmp && ls",
+	})
+
+	tests := []struct {
+		input, want string
+	}{
+		{"ll", "ls --color -l"},
+		{"  ll -a", "  ls --color -l -a"},
+		{"ls", "ls --color"},
+		{"..", "cd .."},
+		// Only the command name is replaced
+		{"echo ll", "echo ll"},
+		{"ll ll", "ls --color -l ll"},
+		// ... also after ;, &&, ||, | and &
+		{"ll; ll && ll || ll | ll & ll", "ls --color -l; ls --color -l && ls --color -l || ls --color -l | ls --color -l & ls --color -l"},
+		// but not after redirections with &
+		{"ll 2>&1 ll", "ls --color -l 2>&1 ll"},
+		{"ll &> ll", "ls --color -l &> ll"},
+		// Quoted or escaped words are not replaced
+		{`\ll`, `\ll`},
+		{`'ll'`, `'ll'`},
+		{`"ll"`, `"ll"`},
+		{`echo "a; ll"`, `echo "a; ll"`},
+		{`echo 'a | ll'`, `echo 'a | ll'`},
+		// A trailing blank makes the next word a command name too
+		{"e w", "echo  world"},
+		{"e e w", "echo  echo  world"},
+		// Recursion stops at aliases that are already being expanded
+		{"a", "a"},
+		{"b", "b"},
+		// The value can contain several commands
+		{"multi", "cd /tmp && ls --color"},
+		// Comments are left alone
+		{"echo x # ll", "echo x # ll"},
+	}
+	for _, tc := range tests {
+		if got := expandAliases(tc.input); got != tc.want {
+			t.Errorf("expandAliases(%q) = %q, want %q", tc.input, got, tc.want)
+		}
+	}
+}
+
+func TestBuiltinAlias(t *testing.T) {
+	withAliases(t, nil)
+
+	run := func(args ...string) (string, error) {
+		var out bytes.Buffer
+		err := builtinAlias(append([]string{"alias"}, args...), &out)
+		return out.String(), err
+	}
+
+	if _, err := run("ll=ls -l", "q=echo it's", "..=cd .."); err != nil {
+		t.Fatal(err)
+	}
+	out, _ := run()
+	want := "alias ..='cd ..'\nalias ll='ls -l'\nalias q='echo it'\\''s'\n"
+	if out != want {
+		t.Errorf("alias = %q, want %q", out, want)
+	}
+	if out, _ := run("ll"); out != "alias ll='ls -l'\n" {
+		t.Errorf("alias ll = %q", out)
+	}
+	if _, err := run("nope"); err == nil || err.Error() != "alias: nope: not found" {
+		t.Errorf("alias nope: %v", err)
+	}
+	for _, name := range []string{"a/b", "a=b", "$x", "a b", ""} {
+		if isAliasName(name) {
+			t.Errorf("isAliasName(%q) = true", name)
+		}
+	}
+	if _, err := run("a/b=x"); err == nil || !strings.Contains(err.Error(), "invalid alias name") {
+		t.Errorf("alias a/b=x: %v", err)
+	}
+
+	if err := builtinUnalias([]string{"unalias", "ll", "nope"}); err == nil || err.Error() != "unalias: nope: not found" {
+		t.Errorf("unalias nope: %v", err)
+	}
+	if _, ok := lookupAlias("ll"); ok {
+		t.Error("ll still defined after unalias")
+	}
+	builtinUnalias([]string{"unalias", "-a"})
+	if names := aliasNames(); len(names) != 0 {
+		t.Errorf("aliases after unalias -a: %q", names)
+	}
+}
+
+func TestExecInput_Aliases(t *testing.T) {
+	withAliases(t, nil)
+	origDir, _ := os.Getwd()
+	defer os.Chdir(origDir)
+	dir := t.TempDir()
+	os.Chdir(dir)
+	defer func(saved bool) { interactiveShell = saved }(interactiveShell)
+
+	// Aliases are expanded in interactive shells only
+	interactiveShell = true
+	execInput(`alias say='echo hello' to=out`)
+	execInput("say world > out")
+	if got, _ := os.ReadFile("out"); string(got) != "hello world\n" {
+		t.Errorf("alias in interactive shell: out = %q", got)
+	}
+
+	interactiveShell = false
+	os.Remove("out")
+	if err := execInput("say world > out"); err == nil {
+		t.Error("alias was expanded in a non-interactive shell")
+	}
+
+	// alias works in pipelines (listing), unalias does not
+	interactiveShell = true
+	execInput("alias | grep say > out")
+	if got, _ := os.ReadFile(filepath.Join(dir, "out")); string(got) != "alias say='echo hello'\n" {
+		t.Errorf("alias | grep = %q", got)
+	}
+	if err := execInput("echo | unalias say"); err == nil {
+		t.Error("unalias in a pipeline must fail")
+	}
+}
+
+func TestShellCompleter_Aliases(t *testing.T) {
+	withAliases(t, map[string]string{"gst": "git status"})
+	t.Setenv("PATH", t.TempDir())
+	got, length := completeLine("gs")
+	if !reflect.DeepEqual(got, []string{"t "}) || length != 2 {
+		t.Errorf("completion of gs = %q, %d", got, length)
+	}
+}

brace.go

@@ -0,0 +1,279 @@
+package main
+
+// Brace expansion like in bash: a{b,c}d becomes abd acd, {1..5} becomes
+// 1 2 3 4 5. It happens first, on the text of the command line, so that the
+// resulting words go through the other expansions (~, variables, wildcards).
+
+import (
+	"errors"
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+// maxBraceWords limits the number of words a single word may expand to.
+const maxBraceWords = 100000
+
+// expandBraceWords applies brace expansion to every word of a command (the
+// text of a pipeline). Quoted or escaped braces, ${...} and $(...) are left
+// alone.
+// A redirection target must expand to a single word.
+func expandBraceWords(input string) (string, error) {
+	var (
+		out           strings.Builder
+		afterRedirect bool // the next word is the target of a redirection
+	)
+
+	for i := 0; i < len(input); {
+		ch := input[i]
+		switch {
+		case ch == ' ' || ch == '\t' || ch == '\n':
+			out.WriteByte(ch)
+			i++
+		case strings.IndexByte(";&|<>", ch) >= 0:
+			if ch == '<' || ch == '>' {
+				afterRedirect = true
+			}
+			out.WriteByte(ch)
+			i++
+		case ch == '#':
+			// A comment, until the end of the line
+			out.WriteString(input[i:])
+			return out.String(), nil
+		default:
+			end := wordEnd(input, i)
+			words, err := expandBraces(input[i:end])
+			if err != nil {
+				return "", err
+			}
+			if afterRedirect && len(words) > 1 {
+				return "", fmt.Errorf("%s: ambiguous redirect", sanitize(input[i:end]))
+			}
+			afterRedirect = false
+			out.WriteString(strings.Join(words, " "))
+			i = end
+		}
+	}
+	return out.String(), nil
+}
+
+// wordEnd returns the end of the word starting at i: the next unquoted blank
+// or operator character.
+func wordEnd(s string, i int) int {
+	var quote byte
+	for ; i < len(s); i++ {
+		ch := s[i]
+		switch {
+		case quote == '\'':
+			if ch == '\'' {
+				quote = 0
+			}
+		case ch == '\\':
+			i++
+		case isSubstStart(s, i):
+			if j := substEnd(s, i); j >= 0 {
+				i = j
+			}
+		case ch == '$' && i+1 < len(s) && s[i+1] == '{':
+			if j := strings.IndexByte(s[i:], '}'); j >= 0 {
+				i += j
+			}
+		case quote == '"':
+			if ch == '"' {
+				quote = 0
+			}
+		case ch == '"' || ch == '\'':
+			quote = ch
+		case strings.IndexByte(" \t\n;&|<>", ch) >= 0:
+			return i
+		}
+	}
+	return len(s)
+}
+
+// braceScan calls fn for every unquoted, unescaped character of s that is
+// not part of a ${...} reference, with its index and the brace depth.
+// Scanning stops when fn returns false.
+func braceScan(s string, fn func(i, depth int) bool) {
+	var quote byte
+	depth := 0
+	for i := 0; i < len(s); i++ {
+		ch := s[i]
+		switch {
+		case quote == '\'':
+			if ch == '\'' {
+				quote = 0
+			}
+			continue
+		case ch == '\\':
+			i++
+			continue
+		case isSubstStart(s, i):
+			// The subshell does its own brace expansion
+			if j := substEnd(s, i); j >= 0 {
+				i = j
+			}
+			continue
+		case ch == '$' && i+1 < len(s) && s[i+1] == '{':
+			if j := strings.IndexByte(s[i:], '}'); j >= 0 {
+				i += j
+			}
+			continue
+		case quote == '"':
+			if ch == '"' {
+				quote = 0
+			}
+			continue
+		case ch == '"' || ch == '\'':
+			quote = ch
+			continue
+		}
+		if ch == '}' {
+			depth--
+		}
+		if !fn(i, depth) {
+			return
+		}
+		if ch == '{' {
+			depth++
+		}
+	}
+}
+
+// expandBraces expands the first valid brace expression of a word and then
+// recursively the results: a{b,c}d -> abd acd, x{1..3} -> x1 x2 x3. A brace
+// expression is valid if it contains an unquoted comma at its top level or
+// is a sequence; other braces ({}, {a}, ${x}) are kept as they are.
+func expandBraces(word string) ([]string, error) {
+	// Find the candidates: unquoted { with their matching }
+	var opens []int
+	pairs := map[int]int{}
+	braceScan(word, func(i, depth int) bool {
+		switch word[i] {
+		case '{':
+			opens = append(opens, i)
+		case '}':
+			if len(opens) > 0 {
+				pairs[opens[len(opens)-1]] = i
+				opens = opens[:len(opens)-1]
+			}
+		}
+		return true
+	})
+
+	for start := 0; start < len(word); start++ {
+		end, ok := pairs[start]
+		if !ok {
+			continue
+		}
+		inner := word[start+1 : end]
+		alternatives := splitTopLevelCommas(inner)
+		if len(alternatives) < 2 {
+			seq, ok, err := braceSequence(inner)
+			if err != nil {
+				return nil, err
+			}
+			if !ok {
+				continue
+			}
+			alternatives = seq
+		}
+
+		prefix, suffix := word[:start], word[end+1:]
+		var words []string
+		for _, alt := range alternatives {
+			expanded, err := expandBraces(prefix + alt + suffix)
+			if err != nil {
+				return nil, err
+			}
+			words = append(words, expanded...)
+			if len(words) > maxBraceWords {
+				return nil, errors.New("brace expansion: too many words")
+			}
+		}
+		return words, nil
+	}
+	return []string{word}, nil
+}
+
+// splitTopLevelCommas splits s at its unquoted commas that are not inside
+// nested braces.
+func splitTopLevelCommas(s string) []string {
+	var parts []string
+	last := 0
+	braceScan(s, func(i, depth int) bool {
+		if s[i] == ',' && depth == 0 {
+			parts = append(parts, s[last:i])
+			last = i + 1
+		}
+		return true
+	})
+	return append(parts, s[last:])
+}
+
+// braceSequence expands a sequence expression: x..y or x..y..step with
+// integers (zero-padded to the same width if one of them has a leading zero)
+// or single letters. ok is false if s is no sequence.
+func braceSequence(s string) (words []string, ok bool, err error) {
+	parts := strings.Split(s, "..")
+	if len(parts) != 2 && len(parts) != 3 {
+		return nil, false, nil
+	}
+	step := 1
+	if len(parts) == 3 {
+		step, err = strconv.Atoi(parts[2])
+		if err != nil {
+			return nil, false, nil
+		}
+		if step < 0 {
+			step = -step
+		}
+		if step == 0 {
+			step = 1
+		}
+	}
+
+	from, errFrom := strconv.Atoi(parts[0])
+	to, errTo := strconv.Atoi(parts[1])
+	isLetter := func(p string) bool {
+		return len(p) == 1 && (p[0] >= 'a' && p[0] <= 'z' || p[0] >= 'A' && p[0] <= 'Z')
+	}
+	format := func(n int) string { return strconv.Itoa(n) }
+	switch {
+	case errFrom == nil && errTo == nil:
+		// {01..10}: pad to the width of the longer number
+		if hasLeadingZero(parts[0]) || hasLeadingZero(parts[1]) {
+			width := max(len(parts[0]), len(parts[1]))
+			format = func(n int) string {
+				if n < 0 {
+					return fmt.Sprintf("-%0*d", width-1, -n)
+				}
+				return fmt.Sprintf("%0*d", width, n)
+			}
+		}
+	case isLetter(parts[0]) && isLetter(parts[1]):
+		from, to = int(parts[0][0]), int(parts[1][0])
+		format = func(n int) string { return string(rune(n)) }
+	default:
+		return nil, false, nil
+	}
+
+	count := (max(from, to)-min(from, to))/step + 1
+	if count > maxBraceWords {
+		return nil, false, errors.New("brace expansion: too many words")
+	}
+	if to < from {
+		step = -step
+	}
+	for n := from; (step > 0 && n <= to) || (step < 0 && n >= to); n += step {
+		words = append(words, format(n))
+	}
+	return words, true, nil
+}
+
+// hasLeadingZero reports whether a number is written with a leading zero
+// (01, -05), which makes a sequence zero-padded.
+func hasLeadingZero(s string) bool {
+	s = strings.TrimPrefix(s, "-")
+	return len(s) > 1 && s[0] == '0'
+}

brace_test.go

@@ -0,0 +1,118 @@
+package main
+
+import (
+	"os"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+func TestExpandBraces(t *testing.T) {
+	tests := []struct {
+		word string
+		want []string
+	}{
+		{"a{b,c}d", []string{"abd", "acd"}},
+		{"{a,b}{1,2}", []string{"a1", "a2", "b1", "b2"}},
+		{"x{a,{b,c}}y", []string{"xay", "xby", "xcy"}},
+		{"file{,.bak}", []string{"file", "file.bak"}},
+		{"{1..5}", []string{"1", "2", "3", "4", "5"}},
+		{"{3..1}", []string{"3", "2", "1"}},
+		{"{1..10..3}", []string{"1", "4", "7", "10"}},
+		{"{10..1..-4}", []string{"10", "6", "2"}},
+		{"{01..10..4}", []string{"01", "05", "09"}},
+		{"{8..010..1}", []string{"008", "009", "010"}},
+		{"{-2..2}", []string{"-2", "-1", "0", "1", "2"}},
+		{"{-01..1}", []string{"-01", "000", "001"}},
+		{"{a..e}", []string{"a", "b", "c", "d", "e"}},
+		{"{C..A}", []string{"C", "B", "A"}},
+		{"*.{go,md}", []string{"*.go", "*.md"}},
+		{`{"a b",c}`, []string{`"a b"`, "c"}},
+		{`{a\,b,c}`, []string{`a\,b`, "c"}},
+		{"{$HOME,~}/x", []string{"$HOME/x", "~/x"}},
+		// Not expanded: quoted, escaped, no comma, ${...}, invalid sequences
+		{`"{a,b}"`, []string{`"{a,b}"`}},
+		{`'{a,b}'`, []string{`'{a,b}'`}},
+		{`\{a,b\}`, []string{`\{a,b\}`}},
+		{"{a}", []string{"{a}"}},
+		{"{}", []string{"{}"}},
+		{"{a,b", []string{"{a,b"}},
+		{"a,b}", []string{"a,b}"}},
+		{"${HOME}", []string{"${HOME}"}},
+		{"${X:-a,b}", []string{"${X:-a,b}"}},
+		{"{a..1}", []string{"{a..1}"}},
+		{"{1..2..x}", []string{"{1..2..x}"}},
+		{"{ab..cd}", []string{"{ab..cd}"}},
+		// An invalid brace before a valid one
+		{"{a}{b,c}", []string{"{a}b", "{a}c"}},
+	}
+	for _, tc := range tests {
+		got, err := expandBraces(tc.word)
+		if err != nil {
+			t.Errorf("expandBraces(%q) returned error: %v", tc.word, err)
+			continue
+		}
+		if !reflect.DeepEqual(got, tc.want) {
+			t.Errorf("expandBraces(%q) = %q, want %q", tc.word, got, tc.want)
+		}
+	}
+
+	if _, err := expandBraces("{1..1000000}"); err == nil {
+		t.Error("expected an error for a too large sequence")
+	}
+	if _, err := expandBraces("{1..11}{1..100}{1..100}"); err == nil {
+		t.Error("expected an error for too many words")
+	}
+}
+
+func TestExpandBraceWords(t *testing.T) {
+	tests := []struct {
+		input, want string
+	}{
+		{"echo a{b,c} d", "echo ab ac d"},
+		{"echo {1..3}|wc -w", "echo 1 2 3|wc -w"},
+		{"echo a > {f}", "echo a > {f}"},
+		{"echo {a,b} 2>&1", "echo a b 2>&1"},
+		{"echo x # {a,b}", "echo x # {a,b}"},
+		{`echo "{a,b} c" {d,e}`, `echo "{a,b} c" d e`},
+	}
+	for _, tc := range tests {
+		got, err := expandBraceWords(tc.input)
+		if err != nil || got != tc.want {
+			t.Errorf("expandBraceWords(%q) = %q, %v; want %q", tc.input, got, err, tc.want)
+		}
+	}
+
+	for _, input := range []string{"echo a > {f,g}", "cat < x{1..2}", "echo a 2>> {a,b}"} {
+		if _, err := expandBraceWords(input); err == nil || !strings.Contains(err.Error(), "ambiguous redirect") {
+			t.Errorf("expandBraceWords(%q): %v, want ambiguous redirect", input, err)
+		}
+	}
+}
+
+func TestExecInput_BraceExpansion(t *testing.T) {
+	origDir, _ := os.Getwd()
+	defer os.Chdir(origDir)
+	dir := t.TempDir()
+	os.Chdir(dir)
+
+	if err := execInput("mkdir -p d/{x,y}/{1..2} && touch d/x/1/f{a,b}.txt"); err != nil {
+		t.Fatal(err)
+	}
+	for _, path := range []string{"d/x/1", "d/x/2", "d/y/1", "d/y/2", "d/x/1/fa.txt", "d/x/1/fb.txt"} {
+		if _, err := os.Stat(path); err != nil {
+			t.Errorf("%s was not created", path)
+		}
+	}
+
+	// Brace expansion comes before wildcards and variables
+	t.Setenv("GOSH_BRACE", "v")
+	execInput("echo d/x/1/*.{txt,go} {${GOSH_BRACE},w}1 > out")
+	if got, _ := os.ReadFile("out"); string(got) != "d/x/1/fa.txt d/x/1/fb.txt d/x/1/*.go v1 w1\n" {
+		t.Errorf("out = %q", got)
+	}
+
+	if err := execInput("echo a > {f,g}"); err == nil || lastStatus != 1 {
+		t.Errorf("ambiguous redirect: %v, $? = %d", err, lastStatus)
+	}
+}

go.mod

@@ -7,4 +7,4 @@ require (
 	github.com/google/shlex v0.0.0-20191202100458-e7afc7fbc510
 )
 
-require golang.org/x/sys v0.48.0 // indirect
+require golang.org/x/sys v0.48.0

jobs.go

@@ -0,0 +1,536 @@
+package main
+
+// Job control: in an interactive shell every pipeline runs in its own process
+// group, which gets the terminal while it runs in the foreground. Ctrl+Z stops
+// the foreground job, "&" starts a job in the background, and the builtins
+// jobs, fg, bg and wait manage the jobs. Without a terminal (scripts, gosh -c)
+// there is no job control, but "&" and wait still work.
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"os/exec"
+	"os/signal"
+	"strconv"
+	"strings"
+	"syscall"
+
+	"github.com/chzyer/readline"
+	"golang.org/x/sys/unix"
+)
+
+// proc is a stage of a job: a process, or a builtin or a stage that failed to
+// start (both without process).
+type proc struct {
+	cmd      *exec.Cmd  // nil if the stage is no process
+	done     chan error // the result of a stage without process
+	finished bool
+	stopped  bool
+	err      error
+}
+
+// job is a pipeline that was started by the shell.
+type job struct {
+	id    int    // number in the job table, 0 if not in the table
+	pgid  int    // process group, 0 without job control or processes
+	text  string // the command line
+	procs []*proc
+	tty   *readline.State // terminal modes of the job when it was stopped
+}
+
+var (
+	jobControl       bool            // job control is enabled
+	interactiveShell bool            // the shell reads commands from a user
+	ttyFd            int             // the terminal of the shell
+	shellPgid        int             // the process group of the shell
+	origPgid         int             // the foreground process group at start
+	shellTTY         *readline.State // terminal modes of the shell
+	jobTable         []*job          // background and stopped jobs
+	exitWarned       bool            // "there are stopped jobs" was shown
+)
+
+// initJobControl enables job control if stdin is a terminal: the shell moves
+// into its own process group and takes the terminal.
+func initJobControl() {
+	fd := int(os.Stdin.Fd())
+	if !readline.IsTerminal(fd) {
+		return
+	}
+	pgrp, err := unix.IoctlGetInt(fd, unix.TIOCGPGRP)
+	if err != nil {
+		return
+	}
+	pid := os.Getpid()
+	if syscall.Getpgrp() != pid {
+		if err := syscall.Setpgid(0, 0); err != nil {
+			return
+		}
+	}
+	origPgid, ttyFd, shellPgid = pgrp, fd, pid
+	if err := setForeground(pid); err != nil {
+		return
+	}
+	// Ctrl+Z must not stop the shell itself. (signal.Ignore would be
+	// inherited by the child processes.)
+	signal.Notify(make(chan os.Signal, 1), syscall.SIGTSTP)
+	jobControl = true
+}
+
+// releaseTerminal gives the terminal back to the process group that had it
+// when the shell started.
+func releaseTerminal() {
+	if jobControl && origPgid > 0 && origPgid != shellPgid {
+		setForeground(origPgid)
+	}
+}
+
+// setForeground makes pgid the foreground process group of the terminal.
+func setForeground(pgid int) error {
+	// The shell gets SIGTTOU if it does this while it is in the background
+	// (when it takes the terminal back from a job), unless the signal is
+	// ignored. It is only ignored for this call, since ignored signals are
+	// inherited by child processes.
+	signal.Ignore(syscall.SIGTTOU)
+	defer signal.Reset(syscall.SIGTTOU)
+	return unix.IoctlSetPointerInt(ttyFd, unix.TIOCSPGRP, pgid)
+}
+
+// ttyIndex returns the index of the file in fds that is the terminal of the
+// shell, or -1.
+func ttyIndex(fds [3]*os.File) int {
+	tty, err := os.Stdin.Stat()
+	if err != nil {
+		return -1
+	}
+	for i, f := range fds {
+		if info, err := f.Stat(); err == nil && os.SameFile(info, tty) {
+			return i
+		}
+	}
+	return -1
+}
+
+// finishedProc returns a proc without process that already has its result.
+func finishedProc(err error) *proc {
+	p := &proc{done: make(chan error, 1)}
+	p.done <- err
+	return p
+}
+
+// running reports whether a process of the job is running.
+func (j *job) running() bool {
+	for _, p := range j.procs {
+		if p.cmd != nil && !p.finished && !p.stopped {
+			return true
+		}
+	}
+	return false
+}
+
+// stopped reports whether the job is stopped: no process is running, but
+// one is stopped.
+func (j *job) stopped() bool {
+	if j.running() {
+		return false
+	}
+	for _, p := range j.procs {
+		if p.stopped {
+			return true
+		}
+	}
+	return false
+}
+
+// finished reports whether all stages of the job have finished.
+func (j *job) finished() bool {
+	for _, p := range j.procs {
+		if !p.finished {
+			return false
+		}
+	}
+	return true
+}
+
+// update records a status change of process pid.
+func (j *job) update(pid int, ws syscall.WaitStatus) {
+	for _, p := range j.procs {
+		if p.cmd == nil || p.cmd.Process.Pid != pid {
+			continue
+		}
+		switch {
+		case ws.Stopped():
+			p.stopped = true
+		case ws.Exited() || ws.Signaled():
+			p.finished, p.stopped = true, false
+			p.err = waitStatusError(ws)
+			p.cmd.Process.Release()
+		}
+	}
+}
+
+// waitStatusError converts the status of a finished process into an error:
+// nil for exit code 0, otherwise a statusError with the exit code, or 128+n
+// if the process was killed by signal n.
+func waitStatusError(ws syscall.WaitStatus) error {
+	switch {
+	case ws.Signaled():
+		return statusError(128 + int(ws.Signal()))
+	case ws.ExitStatus() != 0:
+		return statusError(ws.ExitStatus())
+	}
+	return nil
+}
+
+// waitFlags returns the options for wait4: WUNTRACED to see stopped
+// processes (with job control) and WNOHANG to not block.
+//
+// WCONTINUED is not used: Go's syscall.WaitStatus decodes the "continued"
+// status wrongly on FreeBSD (as killed by SIGCONT) and macOS (as stopped).
+// The shell marks jobs as running itself when it continues them (fg, bg); a
+// job continued from outside (kill -CONT) stays "Stopped" in the job table
+// until it stops again or exits.
+func waitFlags(j *job, nohang bool) int {
+	flags := 0
+	if jobControl && j.pgid != 0 {
+		flags |= syscall.WUNTRACED
+	}
+	if nohang {
+		flags |= syscall.WNOHANG
+	}
+	return flags
+}
+
+// wait waits until no process of the job is running any more (all finished
+// or, with job control, stopped). With nohang it only collects the status
+// changes that are already available and does not block.
+func (j *job) wait(nohang bool) {
+	flags := waitFlags(j, nohang)
+
+	for {
+		// With job control all processes of the job are in its process
+		// group, otherwise they are waited for one by one.
+		target := 0
+		if jobControl && j.pgid != 0 {
+			if !j.running() && !(nohang && j.stopped()) {
+				break
+			}
+			target = -j.pgid
+		} else {
+			if nohang {
+				// Check every process once
+				for _, p := range j.procs {
+					if p.cmd == nil || p.finished {
+						continue
+					}
+					var ws syscall.WaitStatus
+					if pid, err := syscall.Wait4(p.cmd.Process.Pid, &ws, flags, nil); err == nil && pid > 0 {
+						j.update(pid, ws)
+					}
+				}
+				break
+			}
+			for _, p := range j.procs {
+				if p.cmd != nil && !p.finished {
+					target = p.cmd.Process.Pid
+					break
+				}
+			}
+			if target == 0 {
+				break
+			}
+		}
+
+		var ws syscall.WaitStatus
+		pid, err := syscall.Wait4(target, &ws, flags, nil)
+		if errors.Is(err, syscall.EINTR) {
+			continue
+		}
+		if err != nil {
+			// Should not happen; do not wait forever
+			for _, p := range j.procs {
+				if p.cmd != nil && !p.finished {
+					p.finished, p.stopped = true, false
+				}
+			}
+			break
+		}
+		if pid == 0 {
+			break // nohang and nothing changed
+		}
+		j.update(pid, ws)
+	}
+
+	// Stages without process (builtins in pipelines)
+	for _, p := range j.procs {
+		if p.cmd != nil || p.finished {
+			continue
+		}
+		if nohang || j.stopped() {
+			select {
+			case p.err = <-p.done:
+				p.finished = true
+			default:
+			}
+		} else {
+			p.err = <-p.done
+			p.finished = true
+		}
+	}
+}
+
+// result returns the error of the last stage of a finished job (so $? is its
+// exit status, like in sh) and prints the errors of the other stages.
+func (j *job) result() error {
+	for _, p := range j.procs[:len(j.procs)-1] {
+		printError(p.err)
+	}
+	return j.procs[len(j.procs)-1].err
+}
+
+// waitForeground waits for a job in the foreground. If it is stopped, it is
+// added to the job table.
+func waitForeground(j *job) error {
+	j.wait(false)
+	if jobControl && j.pgid != 0 {
+		setForeground(shellPgid)
+		if j.stopped() {
+			j.tty, _ = readline.GetState(ttyFd)
+		}
+		if shellTTY != nil {
+			readline.Restore(ttyFd, shellTTY)
+		}
+	}
+
+	if j.stopped() {
+		addJob(j)
+		fmt.Fprintln(os.Stderr)
+		fmt.Fprintln(os.Stderr, j.describe("Stopped"))
+		return statusError(128 + int(syscall.SIGTSTP))
+	}
+	removeJob(j)
+	j.wait(false) // wait for the builtins, if any
+	return j.result()
+}
+
+// addJob adds j to the job table (if it is not there yet) and makes it the
+// current job.
+func addJob(j *job) {
+	removeJob(j)
+	if j.id == 0 {
+		for _, other := range jobTable {
+			j.id = max(j.id, other.id)
+		}
+		j.id++
+	}
+	jobTable = append(jobTable, j)
+}
+
+// removeJob removes j from the job table.
+func removeJob(j *job) {
+	for i, other := range jobTable {
+		if other == j {
+			jobTable = append(jobTable[:i], jobTable[i+1:]...)
+			return
+		}
+	}
+}
+
+// describe formats a job for jobs and the notifications, like bash:
+// "[1]+  Stopped                 sleep 30". The current job (the last one in
+// the table, i.e. the one stopped or put into the background most recently) is
+// marked with +, the previous one with -.
+func (j *job) describe(state string) string {
+	mark := ' '
+	if n := len(jobTable); n > 0 && jobTable[n-1] == j {
+		mark = '+'
+	} else if n > 1 && jobTable[n-2] == j {
+		mark = '-'
+	}
+	text := j.text
+	if state == "Running" {
+		text += " &"
+	}
+	return fmt.Sprintf("[%d]%c  %-24s%s", j.id, mark, state, sanitize(text))
+}
+
+// state returns the state of a job for jobs and the notifications.
+func (j *job) state() string {
+	switch {
+	case j.stopped():
+		return "Stopped"
+	case !j.finished():
+		return "Running"
+	}
+	if status := exitStatus(j.procs[len(j.procs)-1].err); status != 0 {
+		return "Exit " + strconv.Itoa(status)
+	}
+	return "Done"
+}
+
+// hasStoppedJobs reports whether a job in the table is stopped.
+func hasStoppedJobs() bool {
+	for _, j := range jobTable {
+		if j.stopped() {
+			return true
+		}
+	}
+	return false
+}
+
+// reapJobs collects the status changes of the background jobs, reports
+// finished and newly stopped jobs (if notify is set) and removes finished
+// jobs from the table.
+func reapJobs(notify bool) {
+	for _, j := range append([]*job(nil), jobTable...) {
+		wasStopped := j.stopped()
+		j.wait(true)
+		switch {
+		case j.finished():
+			if notify {
+				for _, p := range j.procs {
+					printError(p.err)
+				}
+				fmt.Fprintln(os.Stderr, j.describe(j.state()))
+			}
+			removeJob(j)
+		case j.stopped() && !wasStopped:
+			addJob(j) // the current job now
+			if notify {
+				fmt.Fprintln(os.Stderr, j.describe("Stopped"))
+			}
+		}
+	}
+}
+
+// findJob returns the job for a job spec: %N or N for job N, %+, %% or nothing
+// for the current job, %- for the previous one.
+func findJob(cmd string, args []string) (*job, error) {
+	spec := "%+"
+	if len(args) > 1 {
+		spec = args[1]
+	}
+	n := len(jobTable)
+	switch spec {
+	case "%+", "%%", "%":
+		if n > 0 {
+			return jobTable[n-1], nil
+		}
+		return nil, fmt.Errorf("%s: current: no such job", cmd)
+	case "%-":
+		if n > 1 {
+			return jobTable[n-2], nil
+		}
+		return nil, fmt.Errorf("%s: previous: no such job", cmd)
+	}
+	id, err := strconv.Atoi(strings.TrimPrefix(spec, "%"))
+	if err == nil {
+		for _, j := range jobTable {
+			if j.id == id {
+				return j, nil
+			}
+		}
+	}
+	return nil, fmt.Errorf("%s: %s: no such job", cmd, sanitize(spec))
+}
+
+// continueJob sends SIGCONT to a stopped job.
+func continueJob(j *job) {
+	for _, p := range j.procs {
+		p.stopped = false
+	}
+	if j.pgid != 0 {
+		syscall.Kill(-j.pgid, syscall.SIGCONT)
+	}
+}
+
+// builtinJobs lists the jobs.
+func builtinJobs(stdout io.Writer) error {
+	for _, j := range jobTable {
+		j.wait(true)
+	}
+	for _, j := range append([]*job(nil), jobTable...) {
+		fmt.Fprintln(stdout, j.describe(j.state()))
+		if j.finished() {
+			removeJob(j)
+		}
+	}
+	return nil
+}
+
+// builtinFg continues a job in the foreground and waits for it.
+func builtinFg(args []string, stdout io.Writer) error {
+	if !jobControl {
+		return errors.New("fg: no job control")
+	}
+	j, err := findJob("fg", args)
+	if err != nil {
+		return err
+	}
+	fmt.Fprintln(stdout, sanitize(j.text))
+
+	if j.pgid != 0 {
+		shellTTY, _ = readline.GetState(ttyFd)
+		if j.stopped() && j.tty != nil {
+			readline.Restore(ttyFd, j.tty)
+		}
+		setForeground(j.pgid)
+	}
+	continueJob(j)
+	return waitForeground(j)
+}
+
+// builtinBg continues a stopped job in the background.
+func builtinBg(args []string, stdout io.Writer) error {
+	if !jobControl {
+		return errors.New("bg: no job control")
+	}
+	j, err := findJob("bg", args)
+	if err != nil {
+		return err
+	}
+	if !j.stopped() {
+		return fmt.Errorf("bg: job %d already in background", j.id)
+	}
+	continueJob(j)
+	addJob(j) // the current job now
+	fmt.Fprintln(stdout, j.describe("Running"))
+	return nil
+}
+
+// builtinWait waits for a job, or for all background jobs. Stopped jobs are
+// not waited for.
+func builtinWait(args []string) error {
+	jobs := append([]*job(nil), jobTable...)
+	if len(args) > 1 {
+		j, err := findJob("wait", args)
+		if err != nil {
+			return err
+		}
+		jobs = []*job{j}
+	}
+
+	var err error
+	for _, j := range jobs {
+		j.wait(false)
+		err = nil
+		if j.finished() {
+			removeJob(j)
+			err = j.result()
+		}
+	}
+	return err
+}
+
+// exitShell exits the shell, unless there are stopped jobs and the user was
+// not warned yet (like bash).
+func exitShell(code int) error {
+	if interactiveShell && hasStoppedJobs() && !exitWarned {
+		exitWarned = true
+		return errors.New("there are stopped jobs")
+	}
+	releaseTerminal()
+	os.Exit(code)
+	return nil
+}

jobs_linux_test.go

@@ -0,0 +1,204 @@
+package main
+
+import (
+	"bytes"
+	"os"
+	"os/exec"
+	"strconv"
+	"strings"
+	"sync"
+	"syscall"
+	"testing"
+	"time"
+
+	"golang.org/x/sys/unix"
+)
+
+// openPty opens a new pseudo terminal and returns the master and slave side.
+func openPty(t *testing.T) (*os.File, *os.File) {
+	t.Helper()
+	master, err := os.OpenFile("/dev/ptmx", os.O_RDWR, 0)
+	if err != nil {
+		t.Skipf("no pty available: %v", err)
+	}
+	if err := unix.IoctlSetPointerInt(int(master.Fd()), unix.TIOCSPTLCK, 0); err != nil {
+		t.Fatal(err)
+	}
+	n, err := unix.IoctlGetInt(int(master.Fd()), unix.TIOCGPTN)
+	if err != nil {
+		t.Fatal(err)
+	}
+	slave, err := os.OpenFile("/dev/pts/"+strconv.Itoa(n), os.O_RDWR|syscall.O_NOCTTY, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return master, slave
+}
+
+// ptyShell is the shell running on a pseudo terminal.
+type ptyShell struct {
+	t      *testing.T
+	master *os.File
+	mu     sync.Mutex
+	out    bytes.Buffer
+	cmd    *exec.Cmd
+}
+
+func startPtyShell(t *testing.T) *ptyShell {
+	bin := buildShell(t)
+	master, slave := openPty(t)
+	cmd := exec.Command(bin, "--norc")
+	cmd.Stdin, cmd.Stdout, cmd.Stderr = slave, slave, slave
+	cmd.Env = append(os.Environ(), "NO_COLOR=1")
+	// New session with the pty as controlling terminal, like a terminal
+	// emulator does
+	cmd.SysProcAttr = &syscall.SysProcAttr{Setsid: true, Setctty: true, Ctty: 0}
+	if err := cmd.Start(); err != nil {
+		t.Fatal(err)
+	}
+	slave.Close()
+
+	s := &ptyShell{t: t, master: master, cmd: cmd}
+	go func() {
+		buf := make([]byte, 4096)
+		for {
+			n, err := master.Read(buf)
+			s.mu.Lock()
+			s.out.Write(buf[:n])
+			s.mu.Unlock()
+			if err != nil {
+				return
+			}
+		}
+	}()
+	t.Cleanup(func() {
+		cmd.Process.Kill()
+		cmd.Wait()
+		master.Close()
+	})
+	s.expect("$ ")
+	return s
+}
+
+func (s *ptyShell) send(text string) {
+	s.master.WriteString(text)
+}
+
+// expect waits until want appears in the output after the position of the
+// last match and consumes the output up to it.
+func (s *ptyShell) expect(want string) {
+	s.t.Helper()
+	deadline := time.Now().Add(5 * time.Second)
+	for time.Now().Before(deadline) {
+		s.mu.Lock()
+		out := s.out.String()
+		if i := strings.Index(out, want); i >= 0 {
+			s.out.Next(i + len(want))
+			s.mu.Unlock()
+			return
+		}
+		s.mu.Unlock()
+		time.Sleep(20 * time.Millisecond)
+	}
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.t.Fatalf("timeout waiting for %q, output:\n%s", want, s.out.String())
+}
+
+func TestWaitFlags_NoWCONTINUED(t *testing.T) {
+	defer func(saved bool) { jobControl = saved }(jobControl)
+	jobControl = true
+	j := &job{pgid: 1}
+	for _, nohang := range []bool{false, true} {
+		flags := waitFlags(j, nohang)
+		if flags&syscall.WCONTINUED != 0 {
+			t.Errorf("waitFlags(nohang=%v) contains WCONTINUED, which Go decodes wrongly on BSD", nohang)
+		}
+		if flags&syscall.WUNTRACED == 0 {
+			t.Errorf("waitFlags(nohang=%v) misses WUNTRACED", nohang)
+		}
+	}
+}
+
+func TestJobControl(t *testing.T) {
+	s := startPtyShell(t)
+
+	// Ctrl+Z stops the foreground job, the shell gets the terminal back
+	s.send("sleep 30\n")
+	time.Sleep(300 * time.Millisecond)
+	s.send("\x1a")
+	s.expect("[1]+  Stopped                 sleep 30")
+	s.send("jobs\n")
+	s.expect("[1]+  Stopped                 sleep 30")
+
+	// bg continues it in the background
+	s.send("bg\n")
+	s.expect("[1]+  Running                 sleep 30 &")
+
+	// After bg the job keeps running and is not reported as finished
+	// (regression: on FreeBSD the "continued" status was read as killed by
+	// SIGCONT, "Exit 147")
+	time.Sleep(300 * time.Millisecond)
+	s.send("jobs\n")
+	s.expect("[1]+  Running                 sleep 30 &")
+
+	// fg brings it back, Ctrl+C reaches only the job
+	s.send("fg %1\n")
+	s.expect("sleep 30")
+	time.Sleep(300 * time.Millisecond)
+	s.send("\x03")
+	s.send("echo status=$?\n")
+	s.expect("status=130")
+
+	// A pipeline is stopped and continued as a whole
+	s.send("sleep 30 | cat\n")
+	time.Sleep(300 * time.Millisecond)
+	s.send("\x1a")
+	s.expect("Stopped                 sleep 30 | cat")
+	s.send("fg\n")
+	time.Sleep(300 * time.Millisecond)
+	s.send("\x03")
+	s.send("echo pipeline=$?\n")
+	s.expect("pipeline=130")
+
+	// A background job that reads from the terminal is stopped (SIGTTIN),
+	// in the foreground it gets the input
+	s.send("cat &\n")
+	s.expect("[1] ")
+	time.Sleep(300 * time.Millisecond)
+	s.send("echo next\n")
+	s.expect("[1]+  Stopped                 cat")
+	s.send("fg\n")
+	time.Sleep(300 * time.Millisecond)
+	s.send("hello cat\n")
+	s.expect("hello cat\r\nhello cat")
+	s.send("\x04")
+	s.send("echo cat=$?\n")
+	s.expect("cat=0")
+
+	// Finished background jobs are reported before the next prompt
+	s.send("sleep 0.2 &\n")
+	time.Sleep(500 * time.Millisecond)
+	s.send("echo tick\n")
+	s.expect("Done                    sleep 0.2")
+
+	// exit warns about stopped jobs once
+	s.send("sleep 30\n")
+	time.Sleep(300 * time.Millisecond)
+	s.send("\x1a")
+	s.expect("Stopped")
+	s.send("exit\n")
+	s.expect("there are stopped jobs")
+	s.send("exit 0\n")
+
+	done := make(chan error, 1)
+	go func() { done <- s.cmd.Wait() }()
+	select {
+	case err := <-done:
+		if err != nil {
+			t.Errorf("shell exited with %v", err)
+		}
+	case <-time.After(5 * time.Second):
+		t.Fatal("shell did not exit")
+	}
+}

main.go

@@ -131,6 +131,8 @@ type token struct {
 //   - '...' is taken literally, nothing is expanded inside
 //   - "..." expands $VAR and ${VAR}; \ only escapes $ " \ and `
 //   - outside quotes \ escapes any character
+//   - $(command) is replaced by the output of command (see commandSubst);
+//     unquoted, the output is split into words at blanks and newlines
 //   - $VAR, ${VAR} and ${VAR:-default} are expanded outside single quotes;
 //     an unset variable is an error, so that e.g. "rm -rf $DIR/x" never runs
 //     with an empty $DIR
@@ -190,6 +192,32 @@ func tokenize(input string) ([]token, error) {
 		case ch == '\\':
 			startRaw(i)
 			escaped = true
+		case isSubstStart(input, i):
+			startRaw(i)
+			end := substEnd(input, i)
+			if end < 0 {
+				return nil, errors.New("missing ) in command substitution")
+			}
+			out, err := commandSubst(input[i+2 : end])
+			if err != nil {
+				return nil, err
+			}
+			if quote == '"' {
+				literal(out)
+				inWord = true
+			} else {
+				// Unquoted, the output is split into words at blanks and
+				// newlines (but, unlike in sh, wildcards in it are not
+				// expanded)
+				for k, field := range strings.Fields(out) {
+					if k > 0 {
+						endWord()
+					}
+					literal(field)
+					inWord = true
+				}
+			}
+			i = end
 		case ch == '$':
 			startRaw(i)
 			value, n, err := expandVar(input[i:])
@@ -612,21 +640,23 @@ func getPrompt() string {
 		c := parseColors(os.Getenv("GOSH_COLORS"))
 		colors = &c
 	}
-	return buildPrompt(username, hostname, wd, os.Geteuid() == 0, colors)
+	return buildPrompt(username, hostname, wd, os.Geteuid() == 0, lastStatus, colors)
 }
 
 // promptColors holds the SGR parameters (e.g. "1;32") for the parts of the
 // prompt. An empty value means no color for that part.
 type promptColors struct {
-	user string // user@host
+	user  string // user@host
-	root string // user@host when running as root
+	root  string // user@host when running as root
-	path string // working directory
+	path  string // working directory
+	error string // exit status of the last command, if it failed
 }
 
-// defaultColors: bold green user, bold red root, bold blue directory
+// defaultColors: bold green user, bold red root, bold blue directory, bold
-var defaultColors = promptColors{user: "1;32", root: "1;31", path: "1;34"}
+// red exit status
+var defaultColors = promptColors{user: "1;32", root: "1;31", path: "1;34", error: "1;31"}
 
-// parseColors reads a GOSH_COLORS value like "user=1;33:root=1;31:path=36"
+// parseColors reads a GOSH_COLORS value like "user=1;33:path=36:error=31"
 // and overrides the corresponding defaults. Unknown keys and values that are
 // not plain SGR parameters (digits and ';') are ignored, so the variable
 // cannot be used to write arbitrary escape sequences to the terminal.
@@ -644,6 +674,8 @@ func parseColors(spec string) promptColors {
 			colors.root = value
 		case "path":
 			colors.path = value
+		case "error":
+			colors.error = value
 		}
 	}
 	return colors
@@ -669,20 +701,28 @@ func colorize(text, sgr string) string {
 	return "\x1b[" + sgr + "m" + text + "\x1b[0m"
 }
 
-// buildPrompt formats the prompt as user@host:path $. The parts are sanitized
+// buildPrompt formats the prompt as user@host:path $, or user@host:path [N] $
+// if the last command failed with exit status N. The parts are sanitized
 // before the color codes are added, so only our own escape sequences end up
 // in the prompt. colors == nil disables colors.
-func buildPrompt(username, hostname, path string, root bool, colors *promptColors) string {
+func buildPrompt(username, hostname, path string, root bool, status int, colors *promptColors) string {
 	userHost := sanitize(username + "@" + hostname)
 	path = sanitize(path)
+	var failed string
+	if status != 0 {
+		failed = " [" + strconv.Itoa(status) + "]"
+	}
 	if colors == nil {
-		return userHost + ":" + path + " $ "
+		return userHost + ":" + path + failed + " $ "
 	}
 	userColor := colors.user
 	if root {
 		userColor = colors.root
 	}
-	return colorize(userHost, userColor) + ":" + colorize(path, colors.path) + " $ "
+	if failed != "" {
+		failed = " " + colorize(failed[1:], colors.error)
+	}
+	return colorize(userHost, userColor) + ":" + colorize(path, colors.path) + failed + " $ "
 }
 
 // useColor reports whether the prompt should be colored: stdout must be a
@@ -702,7 +742,7 @@ type shellCompleter struct {
 
 func newShellCompleter() *shellCompleter {
 	return &shellCompleter{
-		commands: []string{"cd", "pwd", "echo", "exit", "quit", "set", "export", "source", "help"},
+		commands: []string{"cd", "pwd", "echo", "exit", "quit", "set", "export", "source", "jobs", "fg", "bg", "wait", "alias", "unalias", "help"},
 	}
 }
 
@@ -726,7 +766,8 @@ func (c *shellCompleter) Do(line []rune, pos int) ([][]rune, int) {
 		// Complete commands
 		var candidates [][]rune
 		seen := make(map[string]bool)
-		for _, cmd := range append(c.commands, pathExecutables(word)...) {
+		names := append(append(append([]string(nil), c.commands...), aliasNames()...), pathExecutables(word)...)
+		for _, cmd := range names {
 			if strings.HasPrefix(cmd, word) && !seen[cmd] {
 				seen[cmd] = true
 				candidates = append(candidates, []rune(escapeWord(cmd[len(word):])+" "))
@@ -814,7 +855,7 @@ func currentWord(s string) (word string, start int, quote byte) {
 func escapeWord(s string) string {
 	var b strings.Builder
 	for _, r := range s {
-		if strings.ContainsRune(" \t\\'\";|&<>*?[$", r) {
+		if strings.ContainsRune(" \t\\'\";|&<>*?[${", r) {
 			b.WriteByte('\\')
 		}
 		b.WriteRune(r)
@@ -951,6 +992,8 @@ flags:
 		script = func() int { return runScript(os.Stdin, "") }
 	}
 
+	// Aliases are expanded in interactive shells, also in their startup files
+	interactiveShell = interactiveMode
 	for _, file := range startupFiles(login && !noProfile, interactiveMode && !noRC) {
 		sourceStartupFile(file)
 	}
@@ -1109,6 +1152,10 @@ func exitStatus(err error) int {
 // interactive runs the interactive shell with line editing, history and
 // completion until the user exits.
 func interactive() {
+	interactiveShell = true
+	initJobControl()
+	defer releaseTerminal()
+
 	// Ctrl+C sends SIGINT to the whole foreground process group. Catch it so
 	// that it only stops the running command and not the shell. (signal.Ignore
 	// would be inherited by the child processes.)
@@ -1139,6 +1186,9 @@ func interactive() {
 
 	// Main loop
 	for {
+		// Report finished and stopped background jobs
+		reapJobs(true)
+
 		// Update prompt dynamically (in case directory changed)
 		l.SetPrompt(getPrompt())
 		
@@ -1151,6 +1201,11 @@ func interactive() {
 			// Handle EOF (Ctrl+D)
 			if err == io.EOF {
 				fmt.Println()
+				if hasStoppedJobs() && !exitWarned {
+					exitWarned = true
+					fmt.Fprintln(os.Stderr, "gosh: there are stopped jobs")
+					continue
+				}
 				break
 			}
 			fmt.Fprintln(os.Stderr, "Error reading input:", err)
@@ -1211,11 +1266,12 @@ func printError(err error) {
 // listItem is a command of a command list together with the operator that
 // connects it to the previous command ("" for the first one).
 type listItem struct {
-	op      string // "", ";", "&&" or "||"
+	op         string // "", ";", "&&" or "||"
-	command string
+	command    string
+	background bool // the command ends with &
 }
 
-// splitList splits input at unquoted ;, && and ||. Quotes, escapes, ${...}
+// splitList splits input at unquoted ;, &, && and ||. Quotes, escapes, ${...}
 // and comments are handled like in splitWords, but nothing is expanded yet:
 // that happens per command when it runs, so "cd /tmp && echo $PWD" works.
 func splitList(input string) ([]listItem, error) {
@@ -1259,6 +1315,11 @@ scan:
 			}
 		case ch == '\\':
 			escaped = true
+		case isSubstStart(input, i):
+			// Skip $(...) as a whole, its ; | & belong to the subshell
+			if j := substEnd(input, i); j >= 0 {
+				i = j
+			}
 		case ch == '$' && i+1 < len(input) && input[i+1] == '{':
 			// Skip ${...} as a whole, like expandVar does
 			if j := strings.IndexByte(input[i:], '}'); j >= 0 {
@@ -1275,6 +1336,17 @@ scan:
 		case ch == '#' && isWordStart:
 			end = i
 			break scan
+		case ch == '&' && !(i+1 < len(input) && strings.IndexByte("&>", input[i+1]) >= 0) &&
+			!(i > 0 && strings.IndexByte("<>", input[i-1]) >= 0):
+			// A single & (not &&, &>, >& or <&) runs the command in the
+			// background
+			if err := add(i, "&"); err != nil {
+				return nil, err
+			}
+			items[len(items)-1].background = true
+			op = ";"
+			start = i + 1
+			wordStart = true
 		case ch == ';',
 			ch == '&' && i+1 < len(input) && input[i+1] == '&',
 			ch == '|' && i+1 < len(input) && input[i+1] == '|':
@@ -1303,6 +1375,9 @@ scan:
 // and ||. It returns the error of the last command that was run; errors of
 // earlier commands are printed. $? is updated after each command.
 func execInput(input string) error {
+	if interactiveShell {
+		input = expandAliases(input)
+	}
 	items, err := splitList(input)
 	if err != nil {
 		lastStatus = exitStatus(err)
@@ -1315,19 +1390,30 @@ func execInput(input string) error {
 			continue
 		}
 		printError(lastErr)
-		lastErr = execCommand(item.command)
+		lastErr = execCommand(item.command, item.background)
 		lastStatus = exitStatus(lastErr)
 	}
 	return lastErr
 }
 
-// execCommand executes a pipeline of simple commands with redirections.
+// execCommand executes a pipeline of simple commands with redirections, in
-func execCommand(input string) error {
+// the background if background is set.
+func execCommand(input string, background bool) error {
+	// Brace expansion comes first, so that its results are expanded further
+	substStatus = 0
+	input, err := expandBraceWords(input)
+	if err != nil {
+		return err
+	}
+
 	// Split the input into words and operators, expanding variables and ~ on
 	// the way.
 	tokens, err := tokenize(input)
-	var synErr syntaxError
+	var (
-	if errors.As(err, &synErr) {
+		synErr    syntaxError
+		statusErr statusError
+	)
+	if errors.As(err, &synErr) || errors.As(err, &statusErr) {
 		return err
 	}
 	if err != nil {
@@ -1338,9 +1424,17 @@ func execCommand(input string) error {
 		return err
 	}
 	if len(stages) == 0 {
+		// A command that consists only of $(...) has its status
+		if substStatus != 0 {
+			return statusError(substStatus)
+		}
 		return nil
 	}
-	return runPipeline(stages)
+	// "exit" with stopped jobs only warns, unless it is repeated right away
+	if name := stages[0].args; len(name) == 0 || (name[0] != "exit" && name[0] != "quit") {
+		exitWarned = false
+	}
+	return runPipeline(stages, strings.TrimSpace(input), background)
 }
 
 // redirect is a redirection of a simple command, e.g. 2>>file or 2>&1.
@@ -1403,49 +1497,71 @@ func parsePipeline(tokens []token) ([]stage, error) {
 	return append(stages, cur), nil
 }
 
-// runPipeline runs the stages of a pipeline concurrently, connecting the
+// runPipeline runs the stages of a pipeline concurrently as a job,
-// stdout of each stage to the stdin of the next one. It returns the error of
+// connecting the stdout of each stage to the stdin of the next one. In the
-// the last stage (so $? is its exit status, like in sh); errors of the other
+// foreground it waits for the job and returns the error of the last stage (so
-// stages are printed.
+// $? is its exit status, like in sh); errors of the other stages are printed.
-func runPipeline(stages []stage) error {
+// In the background it adds the job to the job table and returns nil.
-	waits := make([]func() error, len(stages))
+func runPipeline(stages []stage, text string, background bool) error {
+	j := &job{text: text}
+	if jobControl && !background {
+		shellTTY, _ = readline.GetState(ttyFd)
+	}
+
 	stdin := os.Stdin
+	if background && !jobControl {
+		// Without job control background jobs must not read from the
+		// terminal, like in sh
+		if devNull, err := os.Open(os.DevNull); err == nil {
+			stdin = devNull
+		}
+	}
+	var startErr error
 	for i, st := range stages {
 		stdout := os.Stdout
 		var next *os.File
 		if i < len(stages)-1 {
 			r, w, err := os.Pipe()
 			if err != nil {
-				// Let the stages that already run finish
 				if stdin != os.Stdin {
 					stdin.Close()
 				}
-				for _, wait := range waits[:i] {
+				startErr = err
-					printError(wait())
+				break
-				}
-				return err
 			}
 			stdout, next = w, r
 		}
-		waits[i] = startStage(st, stdin, stdout, len(stages) > 1)
+		j.procs = append(j.procs, startStage(st, stdin, stdout, j, len(stages) > 1, background))
 		stdin = next
 	}
-
+	if startErr != nil {
-	var err error
+		// Let the stages that already run finish
-	for i, wait := range waits {
+		if len(j.procs) > 0 {
-		err = wait()
+			waitForeground(j)
-		if i < len(waits)-1 {
-			printError(err)
 		}
+		return startErr
 	}
-	return err
+
+	if background {
+		addJob(j)
+		if interactiveShell {
+			pid := 0
+			for _, p := range j.procs {
+				if p.cmd != nil {
+					pid = p.cmd.Process.Pid
+				}
+			}
+			fmt.Fprintf(os.Stderr, "[%d] %d\n", j.id, pid)
+		}
+		return nil
+	}
+	return waitForeground(j)
 }
 
-// startStage starts a stage of a pipeline with the given stdin and stdout and
+// startStage starts a stage of the pipeline of job j with the given stdin and
-// returns a function that waits for it to finish. Pipe ends passed in are
+// stdout. Pipe ends passed in are closed by startStage once the stage no
-// closed by startStage once the stage no longer needs them, so that the
+// longer needs them, so that the neighbors see EOF or get SIGPIPE.
-// neighbors see EOF or get SIGPIPE.
+func startStage(st stage, stdin, stdout *os.File, j *job, inPipeline, background bool) *proc {
-func startStage(st stage, stdin, stdout *os.File, inPipeline bool) func() error {
 	var owned []*os.File
 	if stdin != os.Stdin {
 		owned = append(owned, stdin)
@@ -1458,9 +1574,6 @@ func startStage(st stage, stdin, stdout *os.File, inPipeline bool) func() error
 			f.Close()
 		}
 	}
-	result := func(err error) func() error {
-		return func() error { return err }
-	}
 
 	// Connect the command to the terminal unless redirected. Without Stdin
 	// the command would read from /dev/null, so interactive programs (su,
@@ -1472,13 +1585,17 @@ func startStage(st stage, stdin, stdout *os.File, inPipeline bool) func() error
 	if err != nil || len(st.args) == 0 {
 		// A command without arguments only creates or truncates the files
 		closeOwned()
-		return result(err)
+		return finishedProc(err)
 	}
 
 	if builtin, ok := builtins[st.args[0]]; ok {
-		if inPipeline && builtin.changesState {
+		switch {
+		case inPipeline && builtin.changesState:
 			closeOwned()
-			return result(fmt.Errorf("%s: cannot be used in a pipeline", st.args[0]))
+			return finishedProc(fmt.Errorf("%s: cannot be used in a pipeline", st.args[0]))
+		case background && builtin.changesState:
+			closeOwned()
+			return finishedProc(fmt.Errorf("%s: cannot be run in the background", st.args[0]))
 		}
 		run := func() error {
 			defer closeOwned()
@@ -1490,16 +1607,29 @@ func startStage(st stage, stdin, stdout *os.File, inPipeline bool) func() error
 			}
 			return err
 		}
-		if !inPipeline {
+		if !inPipeline && !background {
-			return result(run())
+			return finishedProc(run())
 		}
-		done := make(chan error, 1)
+		p := &proc{done: make(chan error, 1)}
-		go func() { done <- run() }()
+		go func() { p.done <- run() }()
-		return func() error { return <-done }
+		return p
 	}
 
 	cmd := exec.Command(st.args[0], st.args[1:]...)
 	cmd.Stdin, cmd.Stdout, cmd.Stderr = fds[0], fds[1], fds[2]
+	if jobControl {
+		// All processes of the job share a process group: the one of the
+		// first process. In the foreground, the first process makes its
+		// group the foreground group of the terminal before it starts the
+		// program, so that it can use the terminal right away.
+		attr := &syscall.SysProcAttr{Setpgid: true, Pgid: j.pgid}
+		if j.pgid == 0 && !background {
+			if ctty := ttyIndex(fds); ctty >= 0 {
+				attr.Foreground, attr.Ctty = true, ctty
+			}
+		}
+		cmd.SysProcAttr = attr
+	}
 	err = cmd.Start()
 	// The child has its own copies of the files now
 	closeOwned()
@@ -1507,9 +1637,16 @@ func startStage(st stage, stdin, stdout *os.File, inPipeline bool) func() error
 		err = fmt.Errorf("%s: %w", sanitize(st.args[0]), errCommandNotFound)
 	}
 	if err != nil {
-		return result(err)
+		return finishedProc(err)
 	}
-	return cmd.Wait
+	if jobControl && j.pgid == 0 {
+		j.pgid = cmd.Process.Pid
+		if !background {
+			// In case the process could not do it (no terminal in fds)
+			setForeground(j.pgid)
+		}
+	}
+	return &proc{cmd: cmd}
 }
 
 // applyRedirects applies the redirections in order to fds (stdin, stdout,
@@ -1564,15 +1701,21 @@ func applyRedirects(fds *[3]*os.File, redirects []redirect) ([]*os.File, error)
 // the shell cannot be used in a pipeline, since there they would run
 // concurrently (sh runs them in a subshell where they have no effect).
 var builtins = map[string]struct{ changesState bool }{
-	"cd":     {changesState: true},
+	"cd":      {changesState: true},
-	"pwd":    {},
+	"pwd":     {},
-	"exit":   {changesState: true},
+	"exit":    {changesState: true},
-	"quit":   {changesState: true},
+	"quit":    {changesState: true},
-	"set":    {changesState: true},
+	"set":     {changesState: true},
-	"export": {changesState: true},
+	"export":  {changesState: true},
-	"help":   {},
+	"help":    {},
-	"source": {changesState: true},
+	"source":  {changesState: true},
-	".":      {changesState: true},
+	".":       {changesState: true},
+	"jobs":    {},
+	"fg":      {changesState: true},
+	"bg":      {changesState: true},
+	"wait":    {changesState: true},
+	"alias":   {},
+	"unalias": {changesState: true},
 }
 
 // runBuiltin executes a built-in command, writing its output to stdout.
@@ -1617,17 +1760,17 @@ func runBuiltin(args []string, stdout io.Writer) error {
 		if len(args) > 1 {
 			var exitCode int
 			fmt.Sscanf(args[1], "%d", &exitCode)
-			os.Exit(exitCode)
+			return exitShell(exitCode)
 		}
-		os.Exit(lastStatus)
+		return exitShell(lastStatus)
 	case "quit":
 		// quit with optional status code
 		if len(args) > 1 {
 			var exitCode int
 			fmt.Sscanf(args[1], "%d", &exitCode)
-			os.Exit(exitCode)
+			return exitShell(exitCode)
 		}
-		os.Exit(lastStatus)
+		return exitShell(lastStatus)
 	case "set":
 		// set VAR=value - set environment variable
 		if len(args) < 2 {
@@ -1652,6 +1795,18 @@ func runBuiltin(args []string, stdout io.Writer) error {
 		}
 		os.Setenv(parts[0], parts[1])
 		return nil
+	case "alias":
+		return builtinAlias(args, stdout)
+	case "unalias":
+		return builtinUnalias(args)
+	case "jobs":
+		return builtinJobs(stdout)
+	case "fg":
+		return builtinFg(args, stdout)
+	case "bg":
+		return builtinBg(args, stdout)
+	case "wait":
+		return builtinWait(args)
 	case "source", ".":
 		// source file - run the commands of file in the current shell
 		if len(args) < 2 {
@@ -1670,6 +1825,12 @@ Built-in commands:
   set VAR=val   - Set environment variable
   export VAR=val - Export environment variable
   source file   - Run the commands of file in this shell (also: . file)
+  jobs          - List background and stopped jobs
+  fg [%n]       - Continue job n (default: current job) in the foreground
+  bg [%n]       - Continue stopped job n in the background
+  wait [%n]     - Wait for job n or all background jobs
+  alias [name[=value] ...] - Define or show aliases (alias ll='ls -l')
+  unalias name  - Remove an alias (unalias -a: all aliases)
   help          - Show this help
 
 Features:
@@ -1681,19 +1842,30 @@ Features:
     override with $GOSH_HISTFILE); lines starting with a space are not saved
   - Tab completion for commands and file paths
   - Tilde expansion (~, ~/path) at the start of unquoted words
+  - Brace expansion: a{b,c}d -> abd acd, {1..5}, {01..10..2}, {a..e}
   - Wildcards: * ? [abc] [a-z] [!abc] match file names (not hidden files
     unless the pattern starts with a dot); quoted wildcards and values of
     variables are not expanded, a pattern without match is kept as is
-  - Colored prompt (user@host green, red for root; directory blue),
+  - Prompt user@host:dir $, with [N] before the $ if the last command
-    configurable with $GOSH_COLORS, e.g. GOSH_COLORS='user=1;33:path=36'
+    failed with exit status N
-    (keys user, root, path; values are ANSI SGR codes, empty = no color);
+  - Colored prompt (user@host green, red for root; directory blue; exit
-    disabled with $NO_COLOR, TERM=dumb or when output is not a terminal
+    status red), configurable with $GOSH_COLORS, e.g.
+    GOSH_COLORS='user=1;33:path=36' (keys user, root, path, error; values
+    are ANSI SGR codes, empty = no color); disabled with $NO_COLOR,
+    TERM=dumb or when output is not a terminal
   - Environment variable expansion ($VAR, ${VAR}, ${VAR:-default});
     using an unset variable is an error, use ${VAR:-} for an empty default
+  - Command substitution: $(command) is replaced by the output of command
+    (run in a subshell); unquoted it is split into words, "$(command)"
+    stays one word
   - Command lists: a ; b (run both), a && b (b only if a succeeded),
     a || b (b only if a failed); $? is the exit status of the last command
   - Pipes (a | b) and redirections: < file, > file, >> file, 2> file,
     2>> file, 2>&1, >&2, &> file (stdout and stderr), &>> file
+  - Job control: command & runs in the background, Ctrl+Z stops the
+    foreground job; jobs, fg, bg and wait manage the jobs
+  - Aliases replace the first word of a command in interactive shells
+    (define them in ~/.goshrc); \name or 'name' skips the alias
   - Non-interactive use: gosh -c 'command', gosh script, command | gosh;
     the exit status is the one of the last command
   - Startup files: login shells (-l or argv[0] starting with -) read

main_test.go

@@ -3,6 +3,7 @@ package main
 import (
 	"bytes"
 	"errors"
+	"fmt"
 	"io"
 	"os"
 	"os/exec"
@@ -226,7 +227,7 @@ func TestShell_EndToEnd(t *testing.T) {
 
 	for _, tc := range tests {
 		t.Run(tc.name, func(t *testing.T) {
-			cmd := exec.Command("go", "run", "main.go")
+			cmd := exec.Command("go", "run", ".")
 			cmd.Stdin = strings.NewReader(tc.input)
 			var stdout, stderr bytes.Buffer
 			cmd.Stdout = &stdout
@@ -259,7 +260,7 @@ func TestShell_EOF_ExitsGracefully(t *testing.T) {
 	// Simulate EOF by sending empty input
 	// Note: When using piped input, readline doesn't print prompts
 	// So we just verify the shell exits cleanly
-	cmd := exec.Command("go", "run", "main.go")
+	cmd := exec.Command("go", "run", ".")
 	cmd.Stdin = strings.NewReader("")
 	var stdout, stderr bytes.Buffer
 	cmd.Stdout = &stdout
@@ -278,7 +279,7 @@ func TestShell_EOF_ExitsGracefully(t *testing.T) {
 func TestShell_EOF_AfterCommand(t *testing.T) {
 	// Test that EOF after a command works
 	// Note: When using piped input, readline doesn't print prompts
-	cmd := exec.Command("go", "run", "main.go")
+	cmd := exec.Command("go", "run", ".")
 	cmd.Stdin = strings.NewReader("echo test\n")
 	var stdout, stderr bytes.Buffer
 	cmd.Stdout = &stdout
@@ -302,7 +303,7 @@ func TestShell_SIGINT_HandlerSetup(t *testing.T) {
 	// This test verifies that the shell doesn't panic when receiving signals
 	// We can't easily test SIGINT handling from a test, but we can verify
 	// the shell runs without crashing
-	cmd := exec.Command("go", "run", "main.go")
+	cmd := exec.Command("go", "run", ".")
 	cmd.Stdin = strings.NewReader("echo test\nquit\n")
 	var stdout, stderr bytes.Buffer
 	cmd.Stdout = &stdout
@@ -458,7 +459,7 @@ func TestShell_ExitWithStatusCode(t *testing.T) {
 	}
 	
 	// Build the binary once
-	build := exec.Command("go", "build", "-o", "gosh_test", "main.go")
+	build := exec.Command("go", "build", "-o", "gosh_test", ".")
 	if err := build.Run(); err != nil {
 		t.Fatalf("failed to build: %v", err)
 	}
@@ -494,7 +495,7 @@ func TestShell_ExitWithStatusCode(t *testing.T) {
 // =============================================================================
 
 func TestShell_EnvExpansion_Home(t *testing.T) {
-	cmd := exec.Command("go", "run", "main.go")
+	cmd := exec.Command("go", "run", ".")
 	cmd.Stdin = strings.NewReader("echo $HOME\nexit\n")
 	var stdout, stderr bytes.Buffer
 	cmd.Stdout = &stdout
@@ -513,7 +514,7 @@ func TestShell_EnvExpansion_Home(t *testing.T) {
 }
 
 func TestShell_EnvExpansion_Path(t *testing.T) {
-	cmd := exec.Command("go", "run", "main.go")
+	cmd := exec.Command("go", "run", ".")
 	cmd.Stdin = strings.NewReader("echo $PATH\nexit\n")
 	var stdout, stderr bytes.Buffer
 	cmd.Stdout = &stdout
@@ -532,7 +533,7 @@ func TestShell_EnvExpansion_Path(t *testing.T) {
 }
 
 func TestShell_EnvExpansion_Braces(t *testing.T) {
-	cmd := exec.Command("go", "run", "main.go")
+	cmd := exec.Command("go", "run", ".")
 	cmd.Stdin = strings.NewReader("echo ${HOME}\nexit\n")
 	var stdout, stderr bytes.Buffer
 	cmd.Stdout = &stdout
@@ -551,7 +552,7 @@ func TestShell_EnvExpansion_Braces(t *testing.T) {
 }
 
 func TestShell_EnvExpansion_CdWithVar(t *testing.T) {
-	cmd := exec.Command("go", "run", "main.go")
+	cmd := exec.Command("go", "run", ".")
 	cmd.Stdin = strings.NewReader("cd $HOME\npwd\nexit\n")
 	var stdout, stderr bytes.Buffer
 	cmd.Stdout = &stdout
@@ -570,7 +571,7 @@ func TestShell_EnvExpansion_CdWithVar(t *testing.T) {
 }
 
 func TestShell_TildeExpansion_Ls(t *testing.T) {
-	cmd := exec.Command("go", "run", "main.go")
+	cmd := exec.Command("go", "run", ".")
 	cmd.Stdin = strings.NewReader("ls ~\nexit\n")
 	var stdout, stderr bytes.Buffer
 	cmd.Stdout = &stdout
@@ -892,6 +893,11 @@ func TestMain(m *testing.M) {
 	// Never read the system's startup files; tests that read the user's
 	// files set $HOME themselves
 	sysConfDir = filepath.Join(dir, "etc")
+	// Command substitution runs gosh, not the test binary
+	subshellPath = filepath.Join(dir, "gosh")
+	if out, err := exec.Command("go", "build", "-o", subshellPath, ".").CombinedOutput(); err != nil {
+		panic(fmt.Sprintf("building gosh: %v\n%s", err, out))
+	}
 	code := m.Run()
 	os.RemoveAll(dir)
 	os.Exit(code)
@@ -1054,24 +1060,24 @@ func TestGetPrompt_EscapesControlCharacters(t *testing.T) {
 }
 
 func TestBuildPrompt_Color(t *testing.T) {
-	got := buildPrompt("flow", "host", "~/src", false, &defaultColors)
+	got := buildPrompt("flow", "host", "~/src", false, 0, &defaultColors)
 	want := "\x1b[1;32mflow@host\x1b[0m:\x1b[1;34m~/src\x1b[0m $ "
 	if got != want {
 		t.Errorf("buildPrompt() = %q, want %q", got, want)
 	}
 
-	got = buildPrompt("root", "host", "/", true, &defaultColors)
+	got = buildPrompt("root", "host", "/", true, 0, &defaultColors)
 	if !strings.HasPrefix(got, "\x1b[1;31mroot@host") {
 		t.Errorf("expected red user for root, got %q", got)
 	}
 
-	got = buildPrompt("flow", "host", "~/src", false, nil)
+	got = buildPrompt("flow", "host", "~/src", false, 0, nil)
 	if got != "flow@host:~/src $ " {
 		t.Errorf("buildPrompt() without color = %q", got)
 	}
 
 	colors := promptColors{user: "38;5;208", path: ""}
-	got = buildPrompt("flow", "host", "~/src", false, &colors)
+	got = buildPrompt("flow", "host", "~/src", false, 0, &colors)
 	if got != "\x1b[38;5;208mflow@host\x1b[0m:~/src $ " {
 		t.Errorf("buildPrompt() with custom colors = %q", got)
 	}
@@ -1083,13 +1089,14 @@ func TestParseColors(t *testing.T) {
 		want promptColors
 	}{
 		{"", defaultColors},
-		{"user=1;33", promptColors{user: "1;33", root: "1;31", path: "1;34"}},
+		{"user=1;33", promptColors{user: "1;33", root: "1;31", path: "1;34", error: "1;31"}},
-		{"user=33:root=41;97:path=38;5;208", promptColors{user: "33", root: "41;97", path: "38;5;208"}},
+		{"user=33:root=41;97:path=38;5;208", promptColors{user: "33", root: "41;97", path: "38;5;208", error: "1;31"}},
-		{"path=", promptColors{user: "1;32", root: "1;31", path: ""}},
+		{"error=33", promptColors{user: "1;32", root: "1;31", path: "1;34", error: "33"}},
+		{"path=", promptColors{user: "1;32", root: "1;31", path: "", error: "1;31"}},
 		{"path=0m\x1b[2J", defaultColors}, // escape sequences are rejected
 		{"path=red", defaultColors},       // only numeric codes
 		{"foo=1:user", defaultColors},     // unknown key, missing value
-		{"user=35::path=36", promptColors{user: "35", root: "1;31", path: "36"}},
+		{"user=35::path=36", promptColors{user: "35", root: "1;31", path: "36", error: "1;31"}},
 	}
 	for _, tc := range tests {
 		if got := parseColors(tc.spec); got != tc.want {
@@ -1098,8 +1105,27 @@ func TestParseColors(t *testing.T) {
 	}
 }
 
+func TestBuildPrompt_Status(t *testing.T) {
+	if got := buildPrompt("flow", "host", "~", false, 0, nil); got != "flow@host:~ $ " {
+		t.Errorf("status 0: %q", got)
+	}
+	if got := buildPrompt("flow", "host", "~", false, 127, nil); got != "flow@host:~ [127] $ " {
+		t.Errorf("status 127 without color: %q", got)
+	}
+	got := buildPrompt("flow", "host", "~", false, 1, &defaultColors)
+	want := "\x1b[1;32mflow@host\x1b[0m:\x1b[1;34m~\x1b[0m \x1b[1;31m[1]\x1b[0m $ "
+	if got != want {
+		t.Errorf("status 1 with color = %q, want %q", got, want)
+	}
+	colors := defaultColors
+	colors.error = ""
+	if got := buildPrompt("flow", "host", "~", false, 2, &colors); !strings.HasSuffix(got, " [2] $ ") {
+		t.Errorf("status without error color = %q", got)
+	}
+}
+
 func TestBuildPrompt_ColorEscapesControlCharacters(t *testing.T) {
-	got := buildPrompt("flow", "host", "evil\x1b[2J\x07", false, &defaultColors)
+	got := buildPrompt("flow", "host", "evil\x1b[2J\x07", false, 0, &defaultColors)
 	if strings.Contains(got, "\x1b[2J") || strings.Contains(got, "\x07") {
 		t.Errorf("prompt contains control characters from path: %q", got)
 	}
@@ -1369,16 +1395,23 @@ func TestSplitList(t *testing.T) {
 		{"", nil},
 		{"   ", nil},
 		{"# only a comment", nil},
-		{"ls", []listItem{{"", "ls"}}},
+		{"ls", []listItem{{"", "ls", false}}},
-		{"a; b", []listItem{{"", "a"}, {";", " b"}}},
+		{"a; b", []listItem{{"", "a", false}, {";", " b", false}}},
-		{"a;", []listItem{{"", "a"}}},
+		{"a;", []listItem{{"", "a", false}}},
-		{"a && b || c", []listItem{{"", "a "}, {"&&", " b "}, {"||", " c"}}},
+		{"a && b || c", []listItem{{"", "a ", false}, {"&&", " b ", false}, {"||", " c", false}}},
-		{`echo "a;b" 'c&&d' e\;f`, []listItem{{"", `echo "a;b" 'c&&d' e\;f`}}},
+		{`echo "a;b" 'c&&d' e\;f`, []listItem{{"", `echo "a;b" 'c&&d' e\;f`, false}}},
-		{`echo "x\"; y"`, []listItem{{"", `echo "x\"; y"`}}},
+		{`echo "x\"; y"`, []listItem{{"", `echo "x\"; y"`, false}}},
-		{"echo ${X:-a;b}; c", []listItem{{"", "echo ${X:-a;b}"}, {";", " c"}}},
+		{"echo ${X:-a;b}; c", []listItem{{"", "echo ${X:-a;b}", false}, {";", " c", false}}},
-		{"echo a # b; c", []listItem{{"", "echo a "}}},
+		{"echo a # b; c", []listItem{{"", "echo a ", false}}},
-		{"echo a#b; c", []listItem{{"", "echo a#b"}, {";", " c"}}},
+		{"echo a#b; c", []listItem{{"", "echo a#b", false}, {";", " c", false}}},
-		{"echo a|b", []listItem{{"", "echo a|b"}}},
+		{"echo a|b", []listItem{{"", "echo a|b", false}}},
+		{"a & b", []listItem{{"", "a ", true}, {";", " b", false}}},
+		{"a &", []listItem{{"", "a ", true}}},
+		{"a&", []listItem{{"", "a", true}}},
+		{"a 2>&1 &", []listItem{{"", "a 2>&1 ", true}}},
+		{"a &> f &", []listItem{{"", "a &> f ", true}}},
+		{"a >&2 && b", []listItem{{"", "a >&2 ", false}, {"&&", " b", false}}},
+		{`echo "a & b" \&`, []listItem{{"", `echo "a & b" \&`, false}}},
 	}
 	for _, tc := range tests {
 		got, err := splitList(tc.input)
@@ -1387,13 +1420,13 @@ func TestSplitList(t *testing.T) {
 			continue
 		}
 		if !reflect.DeepEqual(got, tc.want) {
-			t.Errorf("splitList(%q) = %q, want %q", tc.input, got, tc.want)
+			t.Errorf("splitList(%q) = %+v, want %+v", tc.input, got, tc.want)
 		}
 	}
 }
 
 func TestSplitList_SyntaxErrors(t *testing.T) {
-	for _, input := range []string{"; a", "a ;; b", "&& a", "a &&", "a ||", "a && ; b", "a || && b"} {
+	for _, input := range []string{"; a", "a ;; b", "&& a", "a &&", "a ||", "a && ; b", "a || && b", "& a", "a & && b", "a & & b"} {
 		if _, err := splitList(input); err == nil || !strings.Contains(err.Error(), "syntax error") {
 			t.Errorf("splitList(%q) = %v, want syntax error", input, err)
 		}
@@ -1855,3 +1888,75 @@ func TestGetHomeDir_UsesHOME(t *testing.T) {
 		t.Error("getHomeDir() is empty without $HOME")
 	}
 }
+
+func TestExecInput_Background(t *testing.T) {
+	origDir, _ := os.Getwd()
+	defer os.Chdir(origDir)
+	dir := t.TempDir()
+	os.Chdir(dir)
+
+	// The shell does not wait for a background job, wait does
+	start := time.Now()
+	if err := execInput("sh -c 'sleep 0.3; echo done > out' &"); err != nil {
+		t.Fatal(err)
+	}
+	if lastStatus != 0 {
+		t.Errorf("$? = %d after starting a background job, want 0", lastStatus)
+	}
+	if time.Since(start) > 200*time.Millisecond {
+		t.Error("the shell waited for the background job")
+	}
+	if _, err := os.Stat("out"); err == nil {
+		t.Error("background job finished too early")
+	}
+	if err := execInput("wait"); err != nil {
+		t.Fatal(err)
+	}
+	if got, _ := os.ReadFile("out"); string(got) != "done\n" {
+		t.Errorf("out = %q after wait", got)
+	}
+	if len(jobTable) != 0 {
+		t.Errorf("job table not empty after wait: %d jobs", len(jobTable))
+	}
+
+	// wait %n returns the status of the job
+	execInput("sh -c 'exit 3' &")
+	execInput("wait %1")
+	if lastStatus != 3 {
+		t.Errorf("$? = %d after wait %%1, want 3", lastStatus)
+	}
+
+	// Without job control, background jobs read from /dev/null
+	execInput("cat > stdin.txt &")
+	execInput("wait")
+	if got, err := os.ReadFile("stdin.txt"); err != nil || len(got) != 0 {
+		t.Errorf("background job read %q, %v from stdin", got, err)
+	}
+
+	// jobs lists running jobs, also into a redirection
+	execInput("sleep 0.3 &")
+	execInput("jobs > jobs.txt")
+	if got, _ := os.ReadFile("jobs.txt"); !strings.Contains(string(got), "Running                 sleep 0.3 &") {
+		t.Errorf("jobs output = %q", got)
+	}
+	execInput("wait")
+
+	// Builtins that change the shell cannot run in the background
+	if err := execInput("cd / &"); err != nil {
+		t.Fatal(err)
+	}
+	execInput("wait %1")
+	if wd, _ := os.Getwd(); wd != dir {
+		t.Errorf("cd in the background changed the directory to %s", wd)
+	}
+
+	// fg and bg need job control
+	for _, cmd := range []string{"fg", "bg"} {
+		if err := execInput(cmd); err == nil || err.Error() != cmd+": no job control" {
+			t.Errorf("%s without job control: %v", cmd, err)
+		}
+	}
+	if err := execInput("wait %9"); err == nil || err.Error() != "wait: %9: no such job" {
+		t.Errorf("wait %%9: %v", err)
+	}
+}

subst.go

@@ -0,0 +1,98 @@
+package main
+
+// Command substitution: $(command) is replaced by the output of command,
+// which runs in a subshell (a new gosh process), so that cd, set etc. in it
+// do not change the shell.
+
+import (
+	"bytes"
+	"errors"
+	"os"
+	"os/exec"
+	"strings"
+	"syscall"
+)
+
+// subshellPath is the gosh binary used for command substitution; empty means
+// the running executable. Tests set it, since there the running executable is
+// the test binary.
+var subshellPath string
+
+// substStatus is the exit status of the last command substitution of the
+// current command (a command that consists only of a substitution has this
+// status, like in sh).
+var substStatus int
+
+// isSubstStart reports whether a command substitution $( starts at s[i].
+func isSubstStart(s string, i int) bool {
+	return s[i] == '$' && i+1 < len(s) && s[i+1] == '('
+}
+
+// substEnd returns the index of the ) that ends the command substitution
+// starting with $( at s[i], or -1 if it is not terminated. Quotes, escapes
+// and nested substitutions inside are taken into account.
+func substEnd(s string, i int) int {
+	depth := 0
+	var quote byte
+	for j := i + 1; j < len(s); j++ {
+		ch := s[j]
+		switch {
+		case quote == '\'':
+			if ch == '\'' {
+				quote = 0
+			}
+		case ch == '\\':
+			j++
+		case quote == '"':
+			if ch == '"' {
+				quote = 0
+			} else if isSubstStart(s, j) {
+				end := substEnd(s, j)
+				if end < 0 {
+					return -1
+				}
+				j = end
+			}
+		case ch == '"' || ch == '\'':
+			quote = ch
+		case ch == '(':
+			depth++
+		case ch == ')':
+			depth--
+			if depth == 0 {
+				return j
+			}
+		}
+	}
+	return -1
+}
+
+// commandSubst runs command in a subshell and returns its output without
+// trailing newlines. stdin and stderr are those of the shell.
+func commandSubst(command string) (string, error) {
+	if strings.HasPrefix(command, "(") {
+		return "", errors.New("arithmetic expansion $((...)) is not supported")
+	}
+	path := subshellPath
+	if path == "" {
+		var err error
+		if path, err = os.Executable(); err != nil {
+			return "", err
+		}
+	}
+
+	var out bytes.Buffer
+	cmd := exec.Command(path, "-c", command)
+	cmd.Stdin, cmd.Stdout, cmd.Stderr = os.Stdin, &out, os.Stderr
+	err := cmd.Run()
+	substStatus = exitStatus(err)
+	if substStatus == 128+int(syscall.SIGINT) {
+		// Ctrl+C aborts the whole command, like in bash
+		return "", statusError(substStatus)
+	}
+	var exitErr *exec.ExitError
+	if err != nil && !errors.As(err, &exitErr) {
+		return "", err
+	}
+	return strings.TrimRight(out.String(), "\n"), nil
+}

subst_test.go

@@ -0,0 +1,116 @@
+package main
+
+import (
+	"os"
+	"reflect"
+	"testing"
+)
+
+func TestSubstEnd(t *testing.T) {
+	tests := []struct {
+		s    string
+		want int
+	}{
+		{"$(echo a)", 8},
+		{"$(echo a) b", 8},
+		{"$(echo $(date))x", 14},
+		{`$(echo ")")`, 10},
+		{`$(echo ')')`, 10},
+		{`$(echo \))`, 9},
+		{`$(echo "$(echo ")")")`, 20},
+		{"$(echo (a))", 10},
+		{"$(echo a", -1},
+		{`$(echo ")`, -1},
+	}
+	for _, tc := range tests {
+		if got := substEnd(tc.s, 0); got != tc.want {
+			t.Errorf("substEnd(%q) = %d, want %d", tc.s, got, tc.want)
+		}
+	}
+}
+
+func TestSplitList_SkipsSubstitution(t *testing.T) {
+	got, err := splitList(`echo $(a; b && c | d) "$(e || f)" & g`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := []listItem{{"", `echo $(a; b && c | d) "$(e || f)" `, true}, {";", " g", false}}
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("splitList = %+v, want %+v", got, want)
+	}
+}
+
+func TestExpandAliasesAndBraces_SkipSubstitution(t *testing.T) {
+	withAliases(t, map[string]string{"ll": "ls -l"})
+	if got := expandAliases("echo $(ll; ll) && ll"); got != "echo $(ll; ll) && ls -l" {
+		t.Errorf("expandAliases = %q", got)
+	}
+	got, err := expandBraceWords("echo $(echo {a,b}) {c,d}")
+	if err != nil || got != "echo $(echo {a,b}) c d" {
+		t.Errorf("expandBraceWords = %q, %v", got, err)
+	}
+}
+
+func TestExecInput_CommandSubstitution(t *testing.T) {
+	origDir, _ := os.Getwd()
+	defer os.Chdir(origDir)
+	dir := t.TempDir()
+	os.Chdir(dir)
+	os.WriteFile("a.go", nil, 0644)
+	os.WriteFile("b.go", nil, 0644)
+
+	tests := []struct {
+		input string
+		want  string
+	}{
+		{"echo $(echo hello) > out", "hello\n"},
+		// Unquoted, the output is split into words
+		{"printf '[%s]' $(printf 'a b\\nc\\n\\n') > out", "[a][b][c]"},
+		{"printf '[%s]' x$(echo a b)y > out", "[xa][by]"},
+		// Quoted, it is a single word, only trailing newlines are removed
+		{`printf '[%s]' "$(printf 'a  b\nc\n\n')" > out`, "[a  b\nc]"},
+		{`printf '[%s]' "$(true)" > out`, "[]"},
+		{"printf '[%s]' $(true) x > out", "[x]"},
+		// Nested, with quotes and operators inside
+		{"echo $(echo $(echo nested)) > out", "nested\n"},
+		{`echo "$(echo "a;b|c")" > out`, "a;b|c\n"},
+		{"echo $(echo a | tr a-z A-Z) > out", "A\n"},
+		// Wildcards in the output are not expanded, in the command they are
+		{`echo $(echo "*") > out`, "*\n"},
+		{"echo $(echo *.go) > out", "a.go b.go\n"},
+		// Quoted or escaped, $( is literal
+		{`echo '$(echo a)' \$(echo b) > out`, "$(echo a) $(echo b)\n"},
+		// The subshell does not change the shell
+		{"echo $(cd /; pwd) > out; pwd >> out", "/\n" + dir + "\n"},
+		{"set GOSH_SUBST=$(echo value); echo $GOSH_SUBST > out", "value\n"},
+	}
+	for _, tc := range tests {
+		if err := execInput(tc.input); err != nil {
+			t.Errorf("%q: %v", tc.input, err)
+		}
+		if got, _ := os.ReadFile("out"); string(got) != tc.want {
+			t.Errorf("%q: out = %q, want %q", tc.input, got, tc.want)
+		}
+	}
+	os.Unsetenv("GOSH_SUBST")
+
+	// A command that consists only of a substitution has its status
+	execInput("$(sh -c 'exit 3')")
+	if lastStatus != 3 {
+		t.Errorf("$? = %d after $(exit 3), want 3", lastStatus)
+	}
+	// Otherwise the status of the command counts
+	execInput("echo $(false) > out")
+	if lastStatus != 0 {
+		t.Errorf("$? = %d after echo $(false), want 0", lastStatus)
+	}
+
+	for input, wantErr := range map[string]string{
+		"echo $(echo a":  "invalid input: missing ) in command substitution",
+		"echo $((1 + 2))": "invalid input: arithmetic expansion $((...)) is not supported",
+	} {
+		if err := execInput(input); err == nil || err.Error() != wantErr {
+			t.Errorf("%q: error %v, want %q", input, err, wantErr)
+		}
+	}
+}