gabrielmoreira Persistent PTY sessions
PTY sessions are how you talk to interactive programs — programs that detect a terminal (isatty()) and behave differently when they don't see one. Use a session when:
- You need state to persist across calls (
cd, env vars, sourced scripts) - You're driving a REPL (
python -i,mysql,psql,node,irb) - A program demands an interactive prompt (
sudo,ssh,npm login,gh auth login)
For everything else, terminal_exec is simpler. Sessions cost more (per-session bash process, ring buffer, idle-reaping bookkeeping) and have a hard cap (TERMINAL_TOOLS_MAX_PTY, default 8).
Why PTY (and not subprocess pipes)
Subprocess pipes break on every interactive program. The moment a program calls isatty() and sees False, it disables prompts, color, line-editing, password masking, progress bars — sometimes refuses to start. PTY makes us look like a real terminal so these programs work the same as in your shell.
The cost: PTY output includes terminal escape codes (cursor moves, color codes). The session captures them as-is; if you need clean text, strip ANSI escapes in your processing layer.
Bash on macOS — by deliberate policy
terminal_pty_open always invokes /bin/bash, regardless of the user's $SHELL. macOS users: yes, even when zsh is your interactive default. This is the terminal-tools-foundations policy applied to PTYs.
Reasons:
- zsh has command/builtin classes (
zmodload,=cmdexpansion,zpty,ztcp) that bypass bash-shaped security checks - One shell behavior across platforms eliminates "works on Linux, breaks on macOS" surprises
- Bash is universal: any shell you've used will accept the bash subset
The bash invocation uses --norc --noprofile so user dotfiles don't leak in. PS1 is set to a unique sentinel for prompt detection. PS2 is empty. PROMPT_COMMAND is empty.
Three modes of terminal_pty_run
1. Default: send command, wait for prompt sentinel
terminal_pty_run(session_id, command="ls -la")
→ { output, prompt_after: True, ... }
The session writes ls -la\n, waits for the sentinel that its custom PS1 emits, returns the slice between submission and prompt. One in-flight call per session — a concurrent call returns a "session busy" error.
2. raw_send: send raw input, no waiting
terminal_pty_run(session_id, command="print('hi')\n", raw_send=True)
→ { bytes_sent: 12 }
For REPLs, vim keystrokes, password prompts. The session writes the bytes and returns immediately — it doesn't wait for a prompt (REPLs don't print bash's prompt; they print their own).
After a raw_send, you typically follow with:
3. read_only: drain currently-buffered output
terminal_pty_run(session_id, read_only=True, timeout_sec=2)
→ { output: "hi\n", more: False, ... }
Reads whatever the session has accumulated since the last drain, with a brief settle window. Use after raw_send to capture the REPL's response.
Custom prompt detection (expect)
When the command launches a program with its own prompt (Python REPL's >>> , mysql's mysql> , sudo's password prompt), the bash sentinel won't appear until the program exits. Override:
terminal_pty_run(session_id, command="python3", expect=r">>>\s*$", timeout_sec=10)
→ output up to and including ">>>", then control returns
For sudo:
terminal_pty_run(session_id, command="sudo -k && sudo whoami", expect=r"[Pp]assword:")
terminal_pty_run(session_id, command="<password>", raw_send=True, command="<password>\n")
terminal_pty_run(session_id, read_only=True, timeout_sec=5)
(Treat passwords carefully — they end up in the ring buffer.)
Always close
terminal_pty_close(session_id)
Leaked sessions count against TERMINAL_TOOLS_MAX_PTY (default 8). Idle reaping happens lazily on every _open call (sessions inactive longer than idle_timeout_sec, default 1800s, are dropped) — but don't rely on it. Close when you're done.
For unresponsive sessions, force=True skips the graceful "exit" attempt and goes straight to SIGTERM/SIGKILL.
Common patterns
Stateful navigation
sid = terminal_pty_open(cwd="/")
terminal_pty_run(sid, command="cd /var/log")
terminal_pty_run(sid, command="ls -la *.log | head")
terminal_pty_close(sid)
Python REPL
sid = terminal_pty_open()
terminal_pty_run(sid, command="python3", expect=r">>>\s*$")
terminal_pty_run(sid, command="x = 42", raw_send=True)
terminal_pty_run(sid, command="print(x*x)\n", raw_send=True)
result = terminal_pty_run(sid, read_only=True) # → "1764\n>>> "
terminal_pty_run(sid, command="exit()", raw_send=True)
terminal_pty_close(sid)
ssh with host-key prompt
sid = terminal_pty_open()
terminal_pty_run(sid, command="ssh user@new-host", expect=r"\(yes/no.*\)\?")
terminal_pty_run(sid, command="yes\n", raw_send=True)
terminal_pty_run(sid, read_only=True, timeout_sec=10) # password prompt or login