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>

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()
-	if home != "" && (wd == home || strings.HasPrefix(wd, home+"/")) {
-		wd = "~" + strings.TrimPrefix(wd, home)
-	}
+	wd = tildePath(wd, getHomeDir())
 	
 	var colors *promptColors
 	if useColor() {
@@ -1369,19 +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)
 			reportCwd()
-			l.SetPrompt(getPrompt())
+			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 {
@@ -2181,6 +2189,11 @@ 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