pexpect-cli

Use the pexpect_start, pexpect_exec, pexpect_stop, and pexpect_list tools for all pexpect operations. The pexpect_exec tool description contains the full API reference — this skill teaches the workflow for effective interactive automation.

Workflow

1. Start a named session

Always name sessions for clarity:

code

pexpect_start({ name: "ssh-prod" })

Save the returned session ID — you need it for every subsequent call.

2. Spawn the target process first

The first pexpect_exec call should spawn the process and wait for the initial prompt:

code

pexpect_exec({
  session_id: "abc12345",
  code: "child = pexpect.spawn('ssh user@host')\nchild.expect('password:', timeout=30)\nchild.sendline('mypassword')\nchild.expect('[#$]', timeout=30)\nprint('Connected')"
})

The child variable persists across calls — subsequent pexpect_exec calls can use it directly.

3. Execute commands incrementally

Send one logical operation per pexpect_exec call. This makes debugging easier:

code

pexpect_exec({
  session_id: "abc12345",
  code: "child.sendline('uptime')\nchild.expect('[#$]', timeout=10)\nprint(child.before.decode())"
})

Don't try to do everything in a single code block.

4. Always handle errors

Wrap expect() calls in try/except. The two failure modes are:

•pexpect.TIMEOUT — the expected pattern never appeared
•pexpect.EOF — the process exited unexpectedly

If you get a timeout, the session is still alive. Check what's on screen with print(child.before.decode()) and adjust the expect pattern.

5. Clean up

Always stop sessions when done:

code

pexpect_stop({ session_id: "abc12345" })

Use pexpect_list() to check for leaked sessions.

Key Principles

•Always set timeouts on expect(): child.expect('prompt', timeout=30). Without a timeout, a missing prompt hangs the entire execution forever.
•Use generic prompt patterns: '[#$]' or '\\$' instead of 'user@host:~$'. Prompts vary between systems.
•Use expect() with a list for branching: index = child.expect(['password:', 'yes/no', pexpect.TIMEOUT]) returns the index of the matched pattern. Use this for multi-path flows (password prompts, SSH key confirmation, errors).
•Output requires print(): Only text passed to print() is captured and returned. child.before.decode() gives you what appeared between the last two matches.
•One session per interactive process: Don't try to multiplex. Start separate sessions for separate processes.
•After exit, expect EOF: When a process is done (sendline('exit')), wait for pexpect.EOF, not a shell prompt.