add aliases

- alias name=value defines aliases, alias lists or shows them in a form
  that can be read back, unalias removes them (unalias -a: all)
- the first word of every command (at the start and after ;, &, &&, ||
  and |) is replaced, the value is expanded again except for aliases
  that are already being expanded (alias ls='ls --color' works)
- a value ending with a blank makes the next word a command name too
- quoted or escaped words (\ll, 'll') skip the alias
- aliases are only expanded in interactive shells and their startup
  files, like in bash
- tab completion includes aliases

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

main.go

@@ -702,7 +702,7 @@ type shellCompleter struct {
 
 func newShellCompleter() *shellCompleter {
 	return &shellCompleter{
-		commands: []string{"cd", "pwd", "echo", "exit", "quit", "set", "export", "source", "jobs", "fg", "bg", "wait", "help"},
+		commands: []string{"cd", "pwd", "echo", "exit", "quit", "set", "export", "source", "jobs", "fg", "bg", "wait", "alias", "unalias", "help"},
 	}
 }
 
@@ -726,7 +726,8 @@ 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):])+" "))
@@ -951,6 +952,8 @@ 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)
 	}
@@ -1327,6 +1330,9 @@ 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)
@@ -1636,19 +1642,21 @@ 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},
-	"pwd":    {},
-	"exit":   {changesState: true},
-	"quit":   {changesState: true},
-	"set":    {changesState: true},
-	"export": {changesState: true},
-	"help":   {},
-	"source": {changesState: true},
-	".":      {changesState: true},
-	"jobs":   {},
-	"fg":     {changesState: true},
-	"bg":     {changesState: true},
-	"wait":   {changesState: true},
+	"cd":      {changesState: true},
+	"pwd":     {},
+	"exit":    {changesState: true},
+	"quit":    {changesState: true},
+	"set":     {changesState: true},
+	"export":  {changesState: true},
+	"help":    {},
+	"source":  {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.
@@ -1728,6 +1736,10 @@ 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":
@@ -1758,6 +1770,8 @@ Built-in commands:
   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:
@@ -1784,6 +1798,8 @@ Features:
     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