1:mod:`pty` --- Pseudo-terminal utilities
2========================================
3
4.. module:: pty
5   :platform: Unix
6   :synopsis: Pseudo-Terminal Handling for Unix.
7
8.. moduleauthor:: Steen Lumholt
9.. sectionauthor:: Moshe Zadka <moshez@zadka.site.co.il>
10
11**Source code:** :source:`Lib/pty.py`
12
13--------------
14
15The :mod:`pty` module defines operations for handling the pseudo-terminal
16concept: starting another process and being able to write to and read from its
17controlling terminal programmatically.
18
19Pseudo-terminal handling is highly platform dependent. This code is mainly
20tested on Linux, FreeBSD, and macOS (it is supposed to work on other POSIX
21platforms but it's not been thoroughly tested).
22
23The :mod:`pty` module defines the following functions:
24
25
26.. function:: fork()
27
28   Fork. Connect the child's controlling terminal to a pseudo-terminal. Return
29   value is ``(pid, fd)``. Note that the child  gets *pid* 0, and the *fd* is
30   *invalid*. The parent's return value is the *pid* of the child, and *fd* is a
31   file descriptor connected to the child's controlling terminal (and also to the
32   child's standard input and output).
33
34
35.. function:: openpty()
36
37   Open a new pseudo-terminal pair, using :func:`os.openpty` if possible, or
38   emulation code for generic Unix systems. Return a pair of file descriptors
39   ``(master, slave)``, for the master and the slave end, respectively.
40
41
42.. function:: spawn(argv[, master_read[, stdin_read]])
43
44   Spawn a process, and connect its controlling terminal with the current
45   process's standard io. This is often used to baffle programs which insist on
46   reading from the controlling terminal. It is expected that the process
47   spawned behind the pty will eventually terminate, and when it does *spawn*
48   will return.
49
50   A loop copies STDIN of the current process to the child and data received
51   from the child to STDOUT of the current process. It is not signaled to the
52   child if STDIN of the current process closes down.
53
54   The functions *master_read* and *stdin_read* are passed a file descriptor
55   which they should read from, and they should always return a byte string. In
56   order to force spawn to return before the child process exits an
57   empty byte array should be returned to signal end of file.
58
59   The default implementation for both functions will read and return up to 1024
60   bytes each time the function is called. The *master_read* callback is passed
61   the pseudoterminal’s master file descriptor to read output from the child
62   process, and *stdin_read* is passed file descriptor 0, to read from the
63   parent process's standard input.
64
65   Returning an empty byte string from either callback is interpreted as an
66   end-of-file (EOF) condition, and that callback will not be called after
67   that. If *stdin_read* signals EOF the controlling terminal can no longer
68   communicate with the parent process OR the child process. Unless the child
69   process will quit without any input, *spawn* will then loop forever. If
70   *master_read* signals EOF the same behavior results (on linux at least).
71
72   Return the exit status value from :func:`os.waitpid` on the child process.
73
74   :func:`waitstatus_to_exitcode` can be used to convert the exit status into
75   an exit code.
76
77   .. audit-event:: pty.spawn argv pty.spawn
78
79   .. versionchanged:: 3.4
80      :func:`spawn` now returns the status value from :func:`os.waitpid`
81      on the child process.
82
83Example
84-------
85
86.. sectionauthor:: Steen Lumholt
87
88The following program acts like the Unix command :manpage:`script(1)`, using a
89pseudo-terminal to record all input and output of a terminal session in a
90"typescript". ::
91
92    import argparse
93    import os
94    import pty
95    import sys
96    import time
97
98    parser = argparse.ArgumentParser()
99    parser.add_argument('-a', dest='append', action='store_true')
100    parser.add_argument('-p', dest='use_python', action='store_true')
101    parser.add_argument('filename', nargs='?', default='typescript')
102    options = parser.parse_args()
103
104    shell = sys.executable if options.use_python else os.environ.get('SHELL', 'sh')
105    filename = options.filename
106    mode = 'ab' if options.append else 'wb'
107
108    with open(filename, mode) as script:
109        def read(fd):
110            data = os.read(fd, 1024)
111            script.write(data)
112            return data
113
114        print('Script started, file is', filename)
115        script.write(('Script started on %s\n' % time.asctime()).encode())
116
117        pty.spawn(shell, read)
118
119        script.write(('Script done on %s\n' % time.asctime()).encode())
120        print('Script done, file is', filename)
121