Module: PTY

Relationships & Source Files
Namespace Children
Defined in: ext/pty/pty.c


Creates and manages pseudo terminals (PTYs). See also en.wikipedia.org/wiki/Pseudo_terminal

PTY allows you to allocate new terminals using .open or .spawn a new terminal with a specific command.


In this example we will change the buffering type in the factor command, assuming that factor uses stdio for stdout buffering.

If IO.pipe is used instead of .open, this code deadlocks because factor’s stdout is fully buffered.

# start by requiring the standard library PTY
require 'pty'

master, slave = PTY.open
read, write = IO.pipe
pid = spawn("factor", :in=>read, :out=>slave)
read.close     # we dont need the read
slave.close    # or the slave

# pipe "42" to the factor command
write.puts "42"
# output the response from factor
p master.gets #=> "42: 2 3 7\n"

# pipe "144" to factor and print out the response
write.puts "144"
p master.gets #=> "144: 2 2 2 2 3 3\n"
write.close # close the pipe

# The result of read operation when pty slave is closed is platform
# dependent.
ret = begin
        master.gets     # FreeBSD returns nil.
      rescue Errno::EIO # GNU/Linux raises EIO.
p ret #=> nil


© Copyright 1998 by Akinori Ito.

This software may be redistributed freely for this purpose, in full or in part, provided that this entire copyright notice is included on any copies of this software and applications and derivations thereof.

This software is provided on an “as is” basis, without warranty of any kind, either expressed or implied, as to any matter including, but not limited to warranty of fitness of purpose, or merchantability, or results obtained from use of this software.

Class Method Summary

Class Method Details

.check(pid, raise = false) ⇒ Process::Status? .check(pid, true) ⇒ nil, PTY

Checks the status of the child process specified by pid. Returns nil if the process is still alive.

If the process is not alive, and raise was true, a ::PTY::ChildExited exception will be raised. Otherwise it will return a Process::Status instance.


The process id of the process to check


If true and the process identified by pid is no longer alive a PTY::ChildExited is raised.

[ GitHub ]

# File 'ext/pty/pty.c', line 659

static VALUE
pty_check(int argc, VALUE *argv, VALUE self)
    VALUE pid, exc;
    rb_pid_t cpid;
    int status;
    const int flag =
#ifdef WNOHANG

    rb_scan_args(argc, argv, "11", &pid, &exc);
    cpid = rb_waitpid(NUM2PIDT(pid), &status, flag);
    if (cpid == -1 || cpid == 0) return Qnil;

    if (!RTEST(exc)) return rb_last_status_get();
    raise_from_check(cpid, status);


.spawn([env,] command_line) {|r, w, pid| ... } (mod_func) .spawn([env,] command_line) ⇒ Array, ... .spawn([env,] command, arguments, ...) {|r, w, pid| ... } .spawn([env,] command, arguments, ...) ⇒ Array, ...

Alias for .spawn.

.openArray, slave_file .open {|(master_io, slave_file)| ... } ⇒ block value

Allocates a pty (pseudo-terminal).

In the block form, yields an array of two elements (master_io, slave_file) and the value of the block is returned from open.

The IO and File are both closed after the block completes if they haven’t been already closed.

PTY.open {|master, slave|
  p master      #=> #<IO:masterpty:/dev/pts/1>
  p slave      #=> #<File:/dev/pts/1>
  p slave.path #=> "/dev/pts/1"

In the non-block form, returns a two element array, [master_io, slave_file].

master, slave = PTY.open
# do something with master for IO, or the slave file

The arguments in both forms are:


the master of the pty, as an ::IO.


the slave of the pty, as a File. The path to the terminal device is available via slave_file.path

IO#raw! is usable to disable newline conversions:

require 'io/console'
PTY.open {|m, s|
  # ...
[ GitHub ]

# File 'ext/pty/pty.c', line 497

static VALUE
pty_open(VALUE klass)
    int master_fd, slave_fd;
    char slavename[DEVICELEN];
    VALUE master_io, slave_file;
    rb_io_t *master_fptr, *slave_fptr;
    VALUE assoc;

    getDevice(&master_fd, &slave_fd, slavename, 1);

    master_io = rb_obj_alloc(rb_cIO);
    MakeOpenFile(master_io, master_fptr);
    master_fptr->mode = FMODE_READWRITE | FMODE_SYNC | FMODE_DUPLEX;
    master_fptr->fd = master_fd;
    master_fptr->pathv = rb_obj_freeze(rb_sprintf("masterpty:%s", slavename));

    slave_file = rb_obj_alloc(rb_cFile);
    MakeOpenFile(slave_file, slave_fptr);
    slave_fptr->fd = slave_fd;
    slave_fptr->pathv = rb_obj_freeze(rb_str_new_cstr(slavename));

    assoc = rb_assoc_new(master_io, slave_file);
    if (rb_block_given_p()) {
        return rb_ensure(rb_yield, assoc, pty_close_pty, assoc);
    return assoc;

.spawn([env,] command_line) {|r, w, pid| ... } (mod_func) .spawn([env,] command_line) ⇒ Array, ... .spawn([env,] command, arguments, ...) {|r, w, pid| ... } .spawn([env,] command, arguments, ...) ⇒ Array, ...
Also known as: .getpty

Spawns the specified command on a newly allocated pty. You can also use the alias .getpty.

The command’s controlling tty is set to the slave device of the pty and its standard input/output/error is redirected to the slave device.

env is an optional hash that provides additional environment variables to the spawned pty.

# sets FOO to "bar"
PTY.spawn({"FOO"=>"bar"}, "printenv", "FOO") { |r,w,pid| p r.read } #=> "bar\r\n"
# unsets FOO
PTY.spawn({"FOO"=>nil}, "printenv", "FOO") { |r,w,pid| p r.read } #=> ""

command and command_line are the full commands to run, given a String. Any additional arguments will be passed to the command.

Return values

In the non-block form this returns an array of size three, [r, w, pid].

In the block form these same values will be yielded to the block:


A readable ::IO that contains the command’s standard output and standard error


A writable ::IO that is the command’s standard input


The process identifier for the command.

[ GitHub ]

# File 'ext/pty/pty.c', line 575

static VALUE
pty_getpty(int argc, VALUE *argv, VALUE self)
    VALUE res;
    struct pty_info info;
    rb_io_t *wfptr,*rfptr;
    VALUE rport = rb_obj_alloc(rb_cFile);
    VALUE wport = rb_obj_alloc(rb_cFile);
    char SlaveName[DEVICELEN];

    MakeOpenFile(rport, rfptr);
    MakeOpenFile(wport, wfptr);

    establishShell(argc, argv, &info, SlaveName);

    rfptr->mode = rb_io_modestr_fmode("r");
    rfptr->fd = info.fd;
    rfptr->pathv = rb_obj_freeze(rb_str_new_cstr(SlaveName));

    wfptr->mode = rb_io_modestr_fmode("w") | FMODE_SYNC;
    wfptr->fd = rb_cloexec_dup(info.fd);
    if (wfptr->fd == -1)
    wfptr->pathv = rfptr->pathv;

    res = rb_ary_new2(3);

    if (rb_block_given_p()) {
        rb_ensure(rb_yield, res, pty_detach_process, (VALUE)&info);
        return Qnil;
    return res;