add custom prompts with PS1 and PS2

PS1 and PS2 support the backslash escapes of bash (\u \h \w \W \$ \t \j
\e \[ \] ...) and expand $VAR, ${...}, $(...) and $((...)) before each
prompt. Values of escapes and expansions are sanitized. Lines before the
last one and non-SGR escape sequences are written before readline's
prompt. Without PS1, the default prompt is unchanged.

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

README.md

@@ -59,6 +59,13 @@ Features:
     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
+  - Custom prompt with $PS1 (and $PS2 for continuation lines, default
+    "> ") using the escapes of bash: \u user, \h/\H host, \w/\W directory,
+    \$ (# for root), \t \T \@ \A \d time/date, \j jobs, \n newline,
+    \e escape, \[ \]; $VAR, $(...) and $((...)) are expanded, e.g.
+    PS1='\[\e[1;32m\]\u@\h\[\e[0m\]:\w\$ '
+  - Reports the working directory to the terminal (OSC 7) before each
+    prompt, so new tabs/windows open in the same directory
   - Variables: name=value sets a variable (like set), name=value command
     sets it only for that command; the value is not split or expanded as
     a pattern
@@ -723,7 +730,7 @@ $ bg %2                       # oder: angehaltenen Job im Hintergrund weiterlauf
 
 ### 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:
+Der Standard-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
@@ -739,7 +746,7 @@ Typische Werte: `1` allgemeiner Fehler, `2` Syntaxfehler, `126` nicht ausführba
 
 ### Farben
 
-Der Prompt wird über die Umgebungsvariable `GOSH_COLORS` eingefärbt. Sie besteht aus durch `:` getrennten Einträgen `schlüssel=code`:
+Der Standard-Prompt wird über die Umgebungsvariable `GOSH_COLORS` eingefärbt (für einen eigenen Prompt mit `PS1` gilt sie nicht, siehe unten). Sie besteht aus durch `:` getrennten Einträgen `schlüssel=code`:
 
 | Schlüssel | Teil des Prompts               | Standard            |
 |-----------|--------------------------------|---------------------|
@@ -794,6 +801,42 @@ GOSH_COLORS='root=1;97;41'
 GOSH_COLORS='user=:path=4'
 ```
 
+### Eigener Prompt: PS1 und PS2
+
+Wie in bash lässt sich der Prompt mit der Variablen `PS1` frei gestalten, am besten in `~/.goshrc`. Ist `PS1` nicht gesetzt, gilt der Standard-Prompt von oben; `unset PS1` holt ihn zurück. `PS2` ist der Prompt für Folgezeilen unvollständiger Befehle (Standard `> `).
+
+```bash
+PS1='\u@\h:\w\$ '                               # wie der Standard, ohne Farben
+PS1='\[\e[1;32m\]\u@\h\[\e[0m\]:\[\e[1;34m\]\w\[\e[0m\]\$ '   # mit Farben
+PS1='\[\e[2m\]\t\[\e[0m\] \W \$ '               # Uhrzeit und nur der Verzeichnisname
+PS1='[$?] \w\n\$ '                              # Exit-Status, Eingabe in eigener Zeile
+PS1='\w$(git branch --show-current 2>/dev/null | sed "s/.*/ (&)/") \$ '   # git-Branch
+PS2='… '
+```
+
+| Escape        | Bedeutung                                                   |
+|---------------|-------------------------------------------------------------|
+| `\u`          | Benutzername                                                |
+| `\h` / `\H`   | Rechnername bis zum ersten `.` / vollständig                |
+| `\w` / `\W`   | aktuelles Verzeichnis (`~` für das Home-Verzeichnis) / nur der letzte Teil |
+| `\$`          | `#` für root, sonst `$`                                     |
+| `\t` `\T` `\@` `\A` | Uhrzeit: `14:07:09`, `02:07:09`, `02:07 PM`, `14:07`  |
+| `\d`          | Datum, z. B. `Mon Oct 05`                                   |
+| `\j`          | Anzahl der Jobs                                             |
+| `\l`          | Terminal (z. B. `pts/3`)                                    |
+| `\s` / `\v` / `\V` | `gosh` / Version `0.19` / `0.19.0`                    |
+| `\n` `\r` `\a` `\e` | Zeilenumbruch, Wagenrücklauf, Klingel, Escape         |
+| `\nnn`        | Zeichen mit dem Oktalcode `nnn`, z. B. `\033`               |
+| `\\`          | Backslash                                                   |
+| `\[` `\]`     | umschließen Escape-Sequenzen (wie in bash, siehe unten)     |
+
+- Danach werden `$VAR`, `${...}`, `$(...)` und `$((...))` vor jedem Prompt neu ausgewertet. Deshalb `PS1` in einfachen Anführungszeichen setzen, sonst wird z. B. `$(…)` nur einmal beim Zuweisen ausgeführt. `$?` ist der Exit-Status des letzten Befehls; Befehle im Prompt ändern ihn nicht.
+- Eine nicht gesetzte Variable im Prompt ist ein Fehler (`gosh: PS1: X: unbound variable`), dann erscheint der Standard-Prompt. `${X:-}` vermeidet das.
+- Andere Escapes (z. B. `\D{…}`, `\!`, `\#`) kennt gosh nicht, sie bleiben unverändert stehen.
+- `\[` und `\]` braucht gosh nicht, sie werden ignoriert, damit Prompts aus der `.bashrc` unverändert funktionieren. gosh erkennt Farbsequenzen (`\e[…m`) selbst; andere Steuersequenzen wie der Fenstertitel `\e]0;titel\a` werden vor dem Prompt ausgegeben.
+- Die Werte von `\w`, `\u`, `\h` usw. und aller Expansionen werden wie der Standard-Prompt bereinigt: Steuerzeichen erscheinen escaped (`\x1b`). Ein Verzeichnis- oder Branch-Name kann so keine Escape-Sequenzen ins Terminal schreiben – Farben müssen deshalb direkt in `PS1` stehen, nicht in der Ausgabe von `$(...)`.
+- Wie alle Variablen in gosh ist `PS1` eine Umgebungsvariable und wird an gestartete Programme vererbt (eine bash darin versteht dieselben Escapes).
+
 ### Sicherheitshinweise
 
 - Die History liegt in `$XDG_STATE_HOME/gosh/history` (Standard: `~/.local/state/gosh/history`) und ist nur für den eigenen Benutzer lesbar (Verzeichnis `0700`, Datei `0600`). Mit `GOSH_HISTFILE` lässt sich ein anderer Pfad setzen, mit `GOSH_HISTFILE=` wird die History abgeschaltet.

main.go

@@ -668,8 +668,13 @@ func prepareHistoryFile(path string) error {
 	return f.Chmod(0600)
 }
 
-// getPrompt returns a dynamic prompt string like "user@host:dir $"
+// getPrompt returns the expanded $PS1, or if it is not set the default
+// prompt "user@host:dir $".
 func getPrompt() string {
+	if prompt, ok := promptString("PS1"); ok {
+		return prompt
+	}
+
 	// Get current working directory
 	wd, err := os.Getwd()
 	if err != nil {
@@ -690,10 +695,7 @@ func getPrompt() string {
 	}
 	
 	// Replace the home directory with ~
-	home := getHomeDir()
+	wd = tildePath(wd, getHomeDir())
-	if home != "" && (wd == home || strings.HasPrefix(wd, home+"/")) {
-		wd = "~" + strings.TrimPrefix(wd, home)
-	}
 	
 	var colors *promptColors
 	if useColor() {
@@ -795,6 +797,49 @@ func useColor() bool {
 	return readline.IsTerminal(int(os.Stdout.Fd()))
 }
 
+// reportCwd tells the terminal the current working directory with the OSC 7
+// escape sequence, so that new tabs or windows open in the same directory
+// (VTE terminals like GNOME Terminal, kitty, WezTerm, foot, iTerm2, ...).
+// Terminals without support ignore it. Nothing is written if stdout is not a
+// terminal or TERM is "dumb".
+func reportCwd() {
+	if os.Getenv("TERM") == "dumb" || !readline.IsTerminal(int(os.Stdout.Fd())) {
+		return
+	}
+	wd, err := os.Getwd()
+	if err != nil {
+		return
+	}
+	hostname, _ := os.Hostname()
+	fmt.Fprint(os.Stdout, osc7Sequence(hostname, wd))
+}
+
+// osc7Sequence returns ESC ] 7 ; file://host/path BEL. Host and path are
+// percent-encoded, so control characters in a directory name cannot end the
+// sequence early or inject other escape sequences.
+func osc7Sequence(hostname, dir string) string {
+	return "\x1b]7;file://" + percentEncode(hostname, false) + percentEncode(dir, true) + "\a"
+}
+
+// percentEncode encodes every byte except unreserved URI characters (and '/'
+// if keepSlash) as %XX.
+func percentEncode(s string, keepSlash bool) string {
+	const hex = "0123456789ABCDEF"
+	var b strings.Builder
+	for i := 0; i < len(s); i++ {
+		c := s[i]
+		if c >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z' || c >= '0' && c <= '9' ||
+			c == '-' || c == '.' || c == '_' || c == '~' || c == '/' && keepSlash {
+			b.WriteByte(c)
+		} else {
+			b.WriteByte('%')
+			b.WriteByte(hex[c>>4])
+			b.WriteByte(hex[c&15])
+		}
+	}
+	return b.String()
+}
+
 // shellCompleter implements readline.AutoCompleter for contextual completion
 type shellCompleter struct {
 	commands []string
@@ -1326,18 +1371,25 @@ func interactive() {
 	defer l.Close()
 
 	// Main loop. A command that is not complete yet (e.g. if without fi)
-	// is continued on the next line with the prompt "> ".
+	// is continued on the next line with the prompt $PS2 (default "> ").
 	var buf strings.Builder
 	for {
+		var prompt string
 		if buf.Len() == 0 {
 			runPendingTraps()
 			// Report finished and stopped background jobs
 			reapJobs(true)
 			// Update prompt dynamically (in case directory changed)
-			l.SetPrompt(getPrompt())
+			reportCwd()
+			prompt = getPrompt()
+		} else if ps2, ok := promptString("PS2"); ok {
+			prompt = ps2
 		} else {
-			l.SetPrompt("> ")
+			prompt = "> "
 		}
+		prefix, prompt := splitPrompt(prompt)
+		fmt.Print(prefix)
+		l.SetPrompt(prompt)
 
 		line, err := l.Readline()
 		if err != nil {
@@ -2137,6 +2189,13 @@ Features:
     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
+  - Custom prompt with $PS1 (and $PS2 for continuation lines, default
+    "> ") using the escapes of bash: \u user, \h/\H host, \w/\W directory,
+    \$ (# for root), \t \T \@ \A \d time/date, \j jobs, \n newline,
+    \e escape, \[ \]; $VAR, $(...) and $((...)) are expanded, e.g.
+    PS1='\[\e[1;32m\]\u@\h\[\e[0m\]:\w\$ '
+  - Reports the working directory to the terminal (OSC 7) before each
+    prompt, so new tabs/windows open in the same directory
   - Variables: name=value sets a variable (like set), name=value command
     sets it only for that command; the value is not split or expanded as
     a pattern

main_test.go

@@ -399,6 +399,7 @@ func TestExecInput_CdDashNoPrevious(t *testing.T) {
 // =============================================================================
 
 func TestGetPrompt_Format(t *testing.T) {
+	unsetPS1(t)
 	prompt := getPrompt()
 
 	// Should contain username@hostname:path $ pattern
@@ -414,6 +415,7 @@ func TestGetPrompt_Format(t *testing.T) {
 }
 
 func TestGetPrompt_IncludesHome(t *testing.T) {
+	unsetPS1(t)
 	// Save and restore current directory
 	origDir, _ := os.Getwd()
 	defer os.Chdir(origDir)
@@ -1037,6 +1039,7 @@ func TestSanitize(t *testing.T) {
 }
 
 func TestGetPrompt_EscapesControlCharacters(t *testing.T) {
+	unsetPS1(t)
 	t.Setenv("NO_COLOR", "1")
 	origDir, _ := os.Getwd()
 	defer os.Chdir(origDir)
@@ -1133,6 +1136,31 @@ func TestBuildPrompt_ColorEscapesControlCharacters(t *testing.T) {
 	}
 }
 
+func TestOSC7Sequence(t *testing.T) {
+	tests := []struct{ host, dir, want string }{
+		{"host", "/home/flow", "\x1b]7;file://host/home/flow\a"},
+		{"host", "/tmp/a b/ä", "\x1b]7;file://host/tmp/a%20b/%C3%A4\a"},
+		{"host", "/x%y#z?", "\x1b]7;file://host/x%25y%23z%3F\a"},
+		{"", "/", "\x1b]7;file:///\a"},
+	}
+	for _, tt := range tests {
+		if got := osc7Sequence(tt.host, tt.dir); got != tt.want {
+			t.Errorf("osc7Sequence(%q, %q) = %q, want %q", tt.host, tt.dir, got, tt.want)
+		}
+	}
+}
+
+func TestOSC7Sequence_EscapesControlCharacters(t *testing.T) {
+	got := osc7Sequence("ho/st\x07", "/evil\x07\x1b]0;title\x1b\\")
+	body := strings.TrimSuffix(strings.TrimPrefix(got, "\x1b]7;"), "\a")
+	if strings.ContainsAny(body, "\x07\x1b") {
+		t.Errorf("sequence contains control characters: %q", got)
+	}
+	if !strings.HasPrefix(body, "file://ho%2Fst%07/evil%07%1B") {
+		t.Errorf("unexpected encoding: %q", got)
+	}
+}
+
 func TestUseColor_NoColor(t *testing.T) {
 	t.Setenv("NO_COLOR", "1")
 	if useColor() {

packaging/man/gosh.1

@@ -445,7 +445,9 @@ and 2 are supported.
 .PP
 Control structures may span several lines; an incomplete command shows
 the continuation prompt
-.BR "> " .
+.B $PS2
+(default
+.BR "> " ).
 .B !
 negates the exit status of a command.
 .B for
@@ -731,7 +733,9 @@ Exit the shell.
 .PP
 Lines starting with a space are not saved in the history.
 .PP
-The prompt shows
+If
+.B PS1
+is not set, the prompt shows
 .IR user @ host : dir
 followed by
 .BR $ .
@@ -739,6 +743,62 @@ If the last command failed, its exit status is shown as
 .BI [ N ]
 before the
 .BR $ .
+.PP
+Before each prompt, gosh reports the working directory to the terminal with
+the OSC 7 escape sequence
+.RB ( "ESC ] 7 ; file://" \fIhost\fP/\fIpath\fP " BEL" ),
+so terminals like GNOME Terminal open new tabs and windows in the same
+directory. Nothing is sent if the output is not a terminal or
+.BR TERM=dumb .
+.SS Custom prompts
+.B PS1
+and
+.B PS2
+are expanded before each prompt. They may contain the backslash escapes
+of
+.BR bash (1):
+.PP
+.RS
+.nf
+\fB\e\&u\fP        user name
+\fB\e\&h\fP, \fB\e\&H\fP    host name up to the first \(oq.\(cq, full host name
+\fB\e\&w\fP, \fB\e\&W\fP    working directory (\fB~\fP for $HOME), its last component
+\fB\e\&$\fP        \fB#\fP for root, otherwise \fB$\fP
+\fB\e\&t\fP, \fB\e\&T\fP    time as HH:MM:SS (24 and 12 hours)
+\fB\e\&@\fP, \fB\e\&A\fP    time as HH:MM AM/PM and HH:MM (24 hours)
+\fB\e\&d\fP        date, e.g. Mon Oct 05
+\fB\e\&j\fP        number of jobs
+\fB\e\&l\fP        terminal name
+\fB\e\&s\fP, \fB\e\&v\fP, \fB\e\&V\fP shell name, version (major.minor), full version
+\fB\e\&n\fP, \fB\e\&r\fP    newline, carriage return
+\fB\e\&a\fP, \fB\e\&e\fP    bell, escape
+\fB\e\&\fP\fInnn\fP      character with octal code \fInnn\fP
+\fB\e\e\fP        backslash
+\fB\e[\fP, \fB\e]\fP    ignored (needed by bash around escape sequences)
+.fi
+.RE
+.PP
+Unknown escapes are kept as they are. Then
+.BI $ VAR\fR,\fP
+.BR ${...} ,
+.B $(...)
+and
+.B $((...))
+are expanded;
+.B $?
+is the status of the last command. Control characters in the values of
+escapes and expansions are shown escaped, so colors must be written in
+the prompt itself, e.g.
+.PP
+.RS
+.nf
+PS1=\(aq\e[\ee[1;32m\e]\eu@\eh\e[\ee[0m\e]:\ew\e$ \(aq
+.fi
+.RE
+.PP
+If the prompt cannot be expanded (e.g. because of an unset variable), an
+error is printed and the default prompt is used. Escape sequences other
+than colors (e.g. the window title) are written before the prompt.
 .SH ENVIRONMENT
 .TP
 .B GOSH_HISTFILE
@@ -748,8 +808,18 @@ Path of the history file. If set to an empty value, no history is saved.
 Base directory of the default history file
 .IR $XDG_STATE_HOME/gosh/history .
 .TP
+.B PS1
+Prompt of the interactive shell, see
+.BR "Custom prompts" .
+.TP
+.B PS2
+Prompt for continuation lines (default
+.BR "> " ).
+.TP
 .B GOSH_COLORS
-Prompt colors as colon-separated
+Colors of the default prompt (if
+.B PS1
+is not set) as colon-separated
 .IB key = code
 entries. Keys are
 .B user

prompt.go

@@ -0,0 +1,270 @@
+package main
+
+// Custom prompts with $PS1 and $PS2, using the backslash escapes of bash:
+//
+//	\u user name          \h host name up to the first '.'   \H host name
+//	\w working directory  \W its last component (~ for home) \$ # for root, else $
+//	\t time 24h HH:MM:SS  \T 12h HH:MM:SS  \@ 12h am/pm  \A 24h HH:MM  \d date
+//	\j number of jobs     \l terminal      \s shell name  \v \V version
+//	\n newline  \r carriage return  \a bell  \e escape  \nnn octal  \\ backslash
+//	\[ ... \]   begin/end of characters that take no space (escape sequences)
+//
+// After the escapes, $VAR, ${...}, $(...) and $((...)) are expanded (in the
+// same pass, so the values of escapes are not expanded again).
+
+import (
+	"fmt"
+	"os"
+	"os/user"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"time"
+)
+
+// promptInfo holds the values for the escapes of a prompt string.
+type promptInfo struct {
+	user, host, wd, home string
+	root                 bool
+	jobs                 int
+	tty                  string
+	now                  time.Time
+}
+
+// currentPromptInfo collects the promptInfo of the running shell.
+func currentPromptInfo() promptInfo {
+	info := promptInfo{user: "?", host: "unknown", wd: "?", home: getHomeDir(),
+		root: os.Geteuid() == 0, jobs: len(jobTable), now: time.Now()}
+	if wd, err := os.Getwd(); err == nil {
+		info.wd = wd
+	}
+	if usr, err := user.Current(); err == nil && usr.Username != "" {
+		info.user = usr.Username
+	}
+	if host, err := os.Hostname(); err == nil {
+		info.host = host
+	}
+	if tty, err := os.Readlink("/proc/self/fd/0"); err == nil && strings.HasPrefix(tty, "/dev/") {
+		info.tty = filepath.Base(tty)
+	}
+	return info
+}
+
+// expandPrompt expands the escapes and $ expressions of a PS1 or PS2 value.
+// The values of escapes like \w and of expansions are sanitized, so a
+// directory or git branch name cannot write escape sequences to the
+// terminal; colors have to be written as \e[...m in the prompt itself.
+func expandPrompt(ps string, info promptInfo) (string, error) {
+	// $(...) in the prompt must not change $? or the status of the next
+	// command line
+	defer func(status int) { substStatus = status }(substStatus)
+
+	var b strings.Builder
+	for i := 0; i < len(ps); i++ {
+		ch := ps[i]
+		if ch == '$' {
+			value, n, err := expandDollar(ps[i:])
+			if err != nil {
+				return "", err
+			}
+			b.WriteString(sanitize(value))
+			i += n - 1
+			continue
+		}
+		if ch != '\\' || i+1 == len(ps) {
+			b.WriteByte(ch)
+			continue
+		}
+		i++
+		switch esc := ps[i]; esc {
+		case 'u':
+			b.WriteString(sanitize(info.user))
+		case 'h':
+			host, _, _ := strings.Cut(info.host, ".")
+			b.WriteString(sanitize(host))
+		case 'H':
+			b.WriteString(sanitize(info.host))
+		case 'w':
+			b.WriteString(sanitize(tildePath(info.wd, info.home)))
+		case 'W':
+			b.WriteString(sanitize(baseDir(info.wd, info.home)))
+		case '$':
+			if info.root {
+				b.WriteByte('#')
+			} else {
+				b.WriteByte('$')
+			}
+		case 't':
+			b.WriteString(info.now.Format("15:04:05"))
+		case 'T':
+			b.WriteString(info.now.Format("03:04:05"))
+		case '@':
+			b.WriteString(info.now.Format("03:04 PM"))
+		case 'A':
+			b.WriteString(info.now.Format("15:04"))
+		case 'd':
+			b.WriteString(info.now.Format("Mon Jan 02"))
+		case 'j':
+			b.WriteString(strconv.Itoa(info.jobs))
+		case 'l':
+			b.WriteString(sanitize(info.tty))
+		case 's':
+			b.WriteString("gosh")
+		case 'v':
+			b.WriteString(shortVersion(version))
+		case 'V':
+			b.WriteString(sanitize(version))
+		case 'n':
+			b.WriteByte('\n')
+		case 'r':
+			b.WriteByte('\r')
+		case 'a':
+			b.WriteByte('\a')
+		case 'e':
+			b.WriteByte('\x1b')
+		case '\\':
+			b.WriteByte('\\')
+		case '[', ']':
+			// Markers for bash's readline. Our readline finds the width of
+			// the prompt itself (see splitPrompt).
+		default:
+			if n := octalLen(ps[i:]); n > 0 {
+				code, _ := strconv.ParseUint(ps[i:i+n], 8, 8)
+				b.WriteByte(byte(code))
+				i += n - 1
+			} else {
+				// Unknown escapes are kept, like in bash
+				b.WriteByte('\\')
+				b.WriteByte(esc)
+			}
+		}
+	}
+	return b.String(), nil
+}
+
+// tildePath replaces the home directory at the start of path with ~.
+func tildePath(path, home string) string {
+	if home != "" && home != "/" && (path == home || strings.HasPrefix(path, home+"/")) {
+		return "~" + strings.TrimPrefix(path, home)
+	}
+	return path
+}
+
+// baseDir returns the last component of path for \W: ~ for the home
+// directory, / for the root directory.
+func baseDir(path, home string) string {
+	if home != "" && path == home {
+		return "~"
+	}
+	if path == "/" {
+		return path
+	}
+	return filepath.Base(path)
+}
+
+// shortVersion returns major.minor of a version like v0.19.0 for \v.
+func shortVersion(v string) string {
+	v = strings.TrimPrefix(v, "v")
+	parts := strings.SplitN(v, ".", 3)
+	if len(parts) >= 2 {
+		return sanitize(parts[0] + "." + parts[1])
+	}
+	return sanitize(v)
+}
+
+// octalLen returns the length of the octal number (up to 3 digits, at most
+// 0377) at the start of s.
+func octalLen(s string) int {
+	n := 0
+	for n < len(s) && n < 3 && s[n] >= '0' && s[n] <= '7' {
+		n++
+	}
+	if n == 3 && s[0] > '3' {
+		n = 2
+	}
+	return n
+}
+
+// splitPrompt splits an expanded prompt into what is written to the terminal
+// before readline starts and the prompt that readline shows. readline only
+// handles a single line and only knows the width of SGR sequences
+// (\e[...m), so all lines but the last and every other control sequence of
+// the last line (e.g. \e]0;title\a for the window title) go into prefix.
+func splitPrompt(prompt string) (prefix, line string) {
+	var pre strings.Builder
+	if i := strings.LastIndexByte(prompt, '\n'); i >= 0 {
+		pre.WriteString(prompt[:i+1])
+		prompt = prompt[i+1:]
+	}
+	var b strings.Builder
+	for i := 0; i < len(prompt); {
+		ch := prompt[i]
+		switch {
+		case ch == '\x1b':
+			n := escapeLen(prompt[i:])
+			if seq := prompt[i : i+n]; isSGRSequence(seq) {
+				b.WriteString(seq)
+			} else {
+				pre.WriteString(seq)
+			}
+			i += n
+		case ch < ' ' || ch == 0x7f:
+			pre.WriteByte(ch)
+			i++
+		default:
+			b.WriteByte(ch)
+			i++
+		}
+	}
+	return pre.String(), b.String()
+}
+
+// escapeLen returns the length of the escape sequence at the start of s
+// (which starts with ESC): a CSI sequence ESC [ ... final byte, an OSC
+// sequence ESC ] ... BEL or ESC \, or ESC followed by one character.
+func escapeLen(s string) int {
+	if len(s) < 2 {
+		return len(s)
+	}
+	switch s[1] {
+	case '[':
+		for i := 2; i < len(s); i++ {
+			if s[i] >= 0x40 && s[i] <= 0x7e {
+				return i + 1
+			}
+		}
+		return len(s)
+	case ']', 'P', '_', '^':
+		for i := 2; i < len(s); i++ {
+			if s[i] == '\a' {
+				return i + 1
+			}
+			if s[i] == '\x1b' && i+1 < len(s) && s[i+1] == '\\' {
+				return i + 2
+			}
+		}
+		return len(s)
+	}
+	return 2
+}
+
+// isSGRSequence reports whether seq is a complete ESC [ ... m sequence.
+func isSGRSequence(seq string) bool {
+	return len(seq) >= 3 && strings.HasPrefix(seq, "\x1b[") && strings.HasSuffix(seq, "m") && isSGR(seq[2:len(seq)-1])
+}
+
+// promptString returns the expanded value of the prompt variable name (PS1
+// or PS2), or ok == false if it is not set. An error in the prompt is reported and
+// the default prompt is used.
+func promptString(name string) (string, bool) {
+	ps, ok := os.LookupEnv(name)
+	if !ok {
+		return "", false
+	}
+	prompt, err := expandPrompt(ps, currentPromptInfo())
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "gosh: %s: %s\n", name, sanitize(err.Error()))
+		return "", false
+	}
+	return prompt, true
+}

prompt_test.go

@@ -0,0 +1,125 @@
+package main
+
+import (
+	"os"
+	"strings"
+	"testing"
+	"time"
+)
+
+// unsetPS1 removes $PS1 for the test, so that the default prompt is used.
+func unsetPS1(t *testing.T) {
+	t.Helper()
+	t.Setenv("PS1", "")
+	os.Unsetenv("PS1")
+}
+
+var testInfo = promptInfo{
+	user: "flow", host: "box.example.com", wd: "/home/flow/src/gosh", home: "/home/flow",
+	jobs: 2, tty: "pts/3", now: time.Date(2026, 10, 5, 14, 7, 9, 0, time.UTC),
+}
+
+func TestExpandPrompt_Escapes(t *testing.T) {
+	t.Setenv("GOSH_PS_VAR", "val")
+	rootInfo := testInfo
+	rootInfo.root, rootInfo.wd = true, "/"
+	homeInfo := testInfo
+	homeInfo.wd = "/home/flow"
+	tests := []struct {
+		ps   string
+		info promptInfo
+		want string
+	}{
+		{`\u@\h:\w\$ `, testInfo, "flow@box:~/src/gosh$ "},
+		{`\u@\H \W \$ `, rootInfo, "flow@box.example.com / # "},
+		{`\W`, homeInfo, "~"},
+		{`\W`, testInfo, "gosh"},
+		{`\t \T \@ \A \d`, testInfo, "14:07:09 02:07:09 02:07 PM 14:07 Mon Oct 05"},
+		{`\j \l \s`, testInfo, "2 pts/3 gosh"},
+		{`a\nb\\c`, testInfo, "a\nb\\c"},
+		{`\[\e[1;32m\]x\[\033[0m\]`, testInfo, "\x1b[1;32mx\x1b[0m"},
+		{`\101\400`, testInfo, "A 0"},
+		{`\q\`, testInfo, `\q\`},
+		{`$GOSH_PS_VAR ${GOSH_PS_VAR}-$((1+2)) $(echo sub) $?`, testInfo, "val val-3 sub 0"},
+		{`'$GOSH_PS_VAR' "x"`, testInfo, `'val' "x"`},
+		{`cost: 5$ \$`, testInfo, "cost: 5$ $"},
+	}
+	for _, tt := range tests {
+		got, err := expandPrompt(tt.ps, tt.info)
+		if err != nil {
+			t.Errorf("expandPrompt(%q): %v", tt.ps, err)
+		} else if got != tt.want {
+			t.Errorf("expandPrompt(%q) = %q, want %q", tt.ps, got, tt.want)
+		}
+	}
+}
+
+func TestExpandPrompt_Errors(t *testing.T) {
+	os.Unsetenv("GOSH_PS_UNSET")
+	if _, err := expandPrompt(`$GOSH_PS_UNSET`, testInfo); err == nil {
+		t.Error("expected an error for an unset variable")
+	}
+	if got, err := expandPrompt(`${GOSH_PS_UNSET:-none}`, testInfo); err != nil || got != "none" {
+		t.Errorf("got %q, %v", got, err)
+	}
+}
+
+func TestExpandPrompt_KeepsStatus(t *testing.T) {
+	lastStatus, substStatus = 3, 0
+	defer func() { lastStatus = 0 }()
+	got, err := expandPrompt(`$(exit 7)$?`, testInfo)
+	if err != nil || got != "3" {
+		t.Errorf("got %q, %v", got, err)
+	}
+	if substStatus != 0 {
+		t.Errorf("substStatus = %d, want 0", substStatus)
+	}
+}
+
+func TestExpandPrompt_Sanitizes(t *testing.T) {
+	t.Setenv("GOSH_PS_EVIL", "a\x1b[2Jb")
+	info := testInfo
+	info.wd = "/tmp/evil\x1b]0;x\x07"
+	got, err := expandPrompt(`\w $GOSH_PS_EVIL $(printf 'c\033[2Jd')`, info)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if strings.ContainsAny(got, "\x1b\x07") {
+		t.Errorf("prompt contains control characters: %q", got)
+	}
+}
+
+func TestSplitPrompt(t *testing.T) {
+	tests := []struct{ prompt, prefix, line string }{
+		{"a $ ", "", "a $ "},
+		{"top\n\x1b[1mbold\x1b[0m $ ", "top\n", "\x1b[1mbold\x1b[0m $ "},
+		{"l1\nl2\n$ ", "l1\nl2\n", "$ "},
+		{"\x1b]0;title\x07$ ", "\x1b]0;title\x07", "$ "},
+		{"\x1b]0;t\x1b\\\x1b[2K$ \a", "\x1b]0;t\x1b\\\x1b[2K\a", "$ "},
+		{"x\x1b", "\x1b", "x"},
+		{"x\x1b[", "\x1b[", "x"},
+	}
+	for _, tt := range tests {
+		prefix, line := splitPrompt(tt.prompt)
+		if prefix != tt.prefix || line != tt.line {
+			t.Errorf("splitPrompt(%q) = %q, %q, want %q, %q", tt.prompt, prefix, line, tt.prefix, tt.line)
+		}
+	}
+}
+
+func TestGetPrompt_PS1(t *testing.T) {
+	t.Setenv("PS1", `[\s] \$ `)
+	want := "[gosh] $ "
+	if os.Geteuid() == 0 {
+		want = "[gosh] # "
+	}
+	if got := getPrompt(); got != want {
+		t.Errorf("getPrompt() = %q, want %q", got, want)
+	}
+	// An invalid PS1 falls back to the default prompt
+	t.Setenv("PS1", `$GOSH_PS_UNSET`)
+	os.Unsetenv("GOSH_PS_UNSET")
+	if got := getPrompt(); !strings.Contains(got, "@") {
+		t.Errorf("expected default prompt, got %q", got)
+	}
+}