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>

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)
+	}
+}