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

@@ -34,12 +34,18 @@ Features:
   - 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); $? is the exit status of the last command
   - Pipes (a | b) and redirections: < file, > file, >> file, 2> file,
@@ -119,6 +125,26 @@ export LANG=de_DE.UTF-8
 export PATH=$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:
@@ -289,6 +315,22 @@ $ bg %2                       # oder: angehaltenen Job im Hintergrund weiterlauf
 - 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
 
 Der Prompt wird über die Umgebungsvariable `GOSH_COLORS` eingefärbt. Sie besteht aus durch `:` getrennten Einträgen `schlüssel=code`:
@@ -298,6 +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:
 
@@ -335,6 +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

@@ -96,6 +96,16 @@ func expandAliasesIn(line string, active map[string]bool) string {
 			}
 		}
 
+		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:

brace.go

@@ -15,7 +15,8 @@ import (
 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.
+// text of a pipeline). Quoted or escaped braces, ${...} and $(...) are left
+// alone.
 // A redirection target must expand to a single word.
 func expandBraceWords(input string) (string, error) {
 	var (
@@ -69,6 +70,10 @@ func wordEnd(s string, i int) int {
 			}
 		case ch == '\\':
 			i++
+		case isSubstStart(s, i):
+			if j := substEnd(s, i); j >= 0 {
+				i = j
+			}
 		case ch == '$' && i+1 < len(s) && s[i+1] == '{':
 			if j := strings.IndexByte(s[i:], '}'); j >= 0 {
 				i += j
@@ -103,6 +108,12 @@ func braceScan(s string, fn func(i, depth int) bool) {
 		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

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
@@ -1275,6 +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 {
@@ -1355,6 +1400,7 @@ func execInput(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
@@ -1363,8 +1409,11 @@ func execCommand(input string, background bool) error {
 	// 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 {
@@ -1375,6 +1424,10 @@ func execCommand(input string, background bool) error {
 		return err
 	}
 	if len(stages) == 0 {
+		// A command that consists only of $(...) has its status
+		if substStatus != 0 {
+			return statusError(substStatus)
+		}
 		return nil
 	}
 	// "exit" with stopped jobs only warns, unless it is repeated right away
@@ -1793,12 +1846,18 @@ Features:
   - 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,

main_test.go

@@ -3,6 +3,7 @@ package main
 import (
 	"bytes"
 	"errors"
+	"fmt"
 	"io"
 	"os"
 	"os/exec"
@@ -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)
 	}

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)
+		}
+	}
+}