34.8. pty — Pseudo-terminal utilities

Platforms: Linux

The pty module defines operations for handling the pseudo-terminal concept: starting another process and being able to write to and read from its controlling terminal programmatically.

Because pseudo-terminal handling is highly platform dependent, there is code to do it only for Linux. (The Linux code is supposed to work on other platforms, but hasn’t been tested yet.)

The pty module defines the following functions:

pty.fork()

Fork. Connect the child’s controlling terminal to a pseudo-terminal. Return value is (pid, fd). Note that the child gets pid 0, and the fd is invalid. The parent’s return value is the pid of the child, and fd is a file descriptor connected to the child’s controlling terminal (and also to the child’s standard input and output).

pty.openpty()

Open a new pseudo-terminal pair, using os.openpty() if possible, or emulation code for generic Unix systems. Return a pair of file descriptors (master, slave), for the master and the slave end, respectively.

pty.spawn(argv[, master_read[, stdin_read]])

Spawn a process, and connect its controlling terminal with the current process’s standard io. This is often used to baffle programs which insist on reading from the controlling terminal.

The functions master_read and stdin_read should be functions which read from a file descriptor. The defaults try to read 1024 bytes each time they are called.

34.8.1. Example

The following program acts like the Unix command script(1), using a pseudo-terminal to record all input and output of a terminal session in a “typescript”.

import sys, os, time, getopt
import pty

mode = 'wb'
shell = 'sh'
filename = 'typescript'
if 'SHELL' in os.environ:
    shell = os.environ['SHELL']

try:
    opts, args = getopt.getopt(sys.argv[1:], 'ap')
except getopt.error as msg:
    print('%s: %s' % (sys.argv[0], msg))
    sys.exit(2)

for opt, arg in opts:
    # option -a: append to typescript file
    if opt == '-a':
        mode = 'ab'
    # option -p: use a Python shell as the terminal command
    elif opt == '-p':
        shell = sys.executable
if args:
    filename = args[0]

script = open(filename, mode)

def read(fd):
    data = os.read(fd, 1024)
    script.write(data)
    return data

sys.stdout.write('Script started, file is %s\n' % filename)
script.write(('Script started on %s\n' % time.asctime()).encode())
pty.spawn(shell, read)
script.write(('Script done on %s\n' % time.asctime()).encode())
sys.stdout.write('Script done, file is %s\n' % filename)