Module POSIX::Spawn
In: lib/posix/spawn.rb
lib/posix/spawn/child.rb
lib/posix/spawn/version.rb

The POSIX::Spawn module implements a compatible subset of Ruby 1.9‘s Process::spawn and related methods using the IEEE Std 1003.1 posix_spawn(2) system interfaces where available, or a pure Ruby fork/exec based implementation when not.

In Ruby 1.9, a versatile new process spawning interface was added (Process::spawn) as the foundation for enhanced versions of existing process-related methods like Kernel#system, Kernel#`, and IO#popen. These methods are backward compatible with their Ruby 1.8 counterparts but support a large number of new options. The POSIX::Spawn module implements many of these methods with support for most of Ruby 1.9‘s features.

The argument signatures for all of these methods follow a new convention, making it possible to take advantage of Process::spawn features:

  spawn([env], command, [argv1, ...], [options])
  system([env], command, [argv1, ...], [options])
  popen([[env], command, [argv1, ...]], mode="r", [options])

The env, command, and options arguments are described below.

Environment

If a hash is given in the first argument (env), the child process‘s environment becomes a merge of the parent‘s and any modifications specified in the hash. When a value in env is nil, the variable is unset in the child:

    # set FOO as BAR and unset BAZ.
    spawn({"FOO" => "BAR", "BAZ" => nil}, 'echo', 'hello world')

Command

The command and optional argvN string arguments specify the command to execute and any program arguments. When only command is given and includes a space character, the command text is executed by the system shell interpreter, as if by:

    /bin/sh -c 'command'

When command does not include a space character, or one or more argvN arguments are given, the command is executed as if by execve(2) with each argument forming the new program‘s argv.

NOTE: Use of the shell variation is generally discouraged unless you indeed want to execute a shell program. Specifying an explicitly argv is typically more secure and less error prone in most cases.

Options

When a hash is given in the last argument (options), it specifies a current directory and zero or more fd redirects for the child process.

The :chdir option specifies the current directory:

    spawn(command, :chdir => "/var/tmp")

The :in, :out, :err, a Fixnum, an IO object or an Array option specify fd redirection. For example, stderr can be merged into stdout as follows:

    spawn(command, :err => :out)
    spawn(command, 2 => 1)
    spawn(command, STDERR => :out)
    spawn(command, STDERR => STDOUT)

The key is a fd in the newly spawned child process (stderr in this case). The value is a fd in the parent process (stdout in this case).

You can also specify a filename for redirection instead of an fd:

    spawn(command, :in => "/dev/null")   # read mode
    spawn(command, :out => "/dev/null")  # write mode
    spawn(command, :err => "log")        # write mode
    spawn(command, 3 => "/dev/null")     # read mode

When redirecting to stdout or stderr, the files are opened in write mode; otherwise, read mode is used.

It‘s also possible to control the open flags and file permissions directly by passing an array value:

    spawn(command, :in=>["file"])       # read mode assumed
    spawn(command, :in=>["file", "r"])  # explicit read mode
    spawn(command, :out=>["log", "w"])  # explicit write mode, 0644 assumed
    spawn(command, :out=>["log", "w", 0600])
    spawn(command, :out=>["log", File::APPEND | File::CREAT, 0600])

The array is a [filename, open_mode, perms] tuple. open_mode can be a string or an integer. When open_mode is omitted or nil, File::RDONLY is assumed. The perms element should be an integer. When perms is omitted or nil, 0644 is assumed.

The :close It‘s possible to direct an fd be closed in the child process. This is important for implementing `popen`-style logic and other forms of IPC between processes using `IO.pipe`:

    rd, wr = IO.pipe
    pid = spawn('echo', 'hello world', rd => :close, :stdout => wr)
    wr.close
    output = rd.read
    Process.wait(pid)

Spawn Implementation

The POSIX::Spawn#spawn method uses the best available implementation given the current platform and Ruby version. In order of preference, they are:

 1. The posix_spawn based C extension method (pspawn).
 2. Process::spawn when available (Ruby 1.9 only).
 3. A simple pure-Ruby fork/exec based spawn implementation compatible
    with Ruby >= 1.8.7.

Methods

`   fspawn   popen4   pspawn   spawn   system  

Classes and Modules

Class POSIX::Spawn::Child
Class POSIX::Spawn::MaximumOutputExceeded
Class POSIX::Spawn::TimeoutExceeded

Constants

OFLAGS = { "r" => File::RDONLY, "r+" => File::RDWR | File::CREAT, "w" => File::WRONLY | File::CREAT | File::TRUNC, "w+" => File::RDWR | File::CREAT | File::TRUNC, "a" => File::WRONLY | File::APPEND | File::CREAT, "a+" => File::RDWR | File::APPEND | File::CREAT   Mapping of string open modes to integer oflag versions.
VERSION = '0.3.6'

Public Instance methods

Executes a command in a subshell using the system‘s shell interpreter and returns anything written to the new process‘s stdout. This method is compatible with Kernel#`.

Returns the String output of the command.

Spawn a child process with a variety of options using a pure Ruby fork + exec. Supports the standard spawn interface as described in the POSIX::Spawn module documentation.

Spawn a child process with all standard IO streams piped in and out of the spawning process. Supports the standard spawn interface as described in the POSIX::Spawn module documentation.

Returns a [pid, stdin, stderr, stdout] tuple, where pid is the new process‘s pid, stdin is a writeable IO object, and stdout / stderr are readable IO objects. The caller should take care to close all IO objects when finished and the child process‘s status must be collected by a call to Process::waitpid or equivalent.

Spawn a child process with a variety of options using the posix_spawn(2) systems interfaces. Supports the standard spawn interface as described in the POSIX::Spawn module documentation.

Raises NotImplementedError when the posix_spawn_ext module could not be loaded due to lack of platform support.

Spawn a child process with a variety of options using the best available implementation for the current platform and Ruby version.

spawn([env], command, [argv1, …], [options])

env - Optional hash specifying the new process‘s environment. command - A string command name, or shell program, used to determine the

          program to execute.

argvN - Zero or more string program arguments (argv). options - Optional hash of operations to perform before executing the

          new child process.

Returns the integer pid of the newly spawned process.

Raises any number of Errno:exceptions on failure.

Executes a command and waits for it to complete. The command‘s exit status is available as $?. Supports the standard spawn interface as described in the POSIX::Spawn module documentation.

This method is compatible with Kernel#system.

Returns true if the command returns a zero exit status, or false for non-zero exit.

[Validate]