POSIX::Spawn

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.

Constants

OFLAGS

Mapping of string open modes to integer oflag versions.

VERSION

Public Instance Methods

`(cmd) click to toggle source

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.

# File lib/posix/spawn.rb, line 268
def `(cmd)
  r, w = IO.pipe
  pid = spawn(['/bin/sh', '/bin/sh'], '-c', cmd, :out => w, r => :close)

  if pid > 0
    w.close
    out = r.read
    ::Process.waitpid(pid)
    out
  else
    ''
  end
ensure
  [r, w].each{ |io| io.close rescue nil }
end
fspawn(*args) click to toggle source

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.

# File lib/posix/spawn.rb, line 193
def fspawn(*args)
  env, argv, options = extract_process_spawn_arguments(*args)
  valid_options = [:chdir, :unsetenv_others, :pgroup]

  if badopt = options.find{ |key,val| !fd?(key) && !valid_options.include?(key) }
    raise ArgumentError, "Invalid option: #{badopt[0].inspect}"
  elsif !argv.is_a?(Array) || !argv[0].is_a?(Array) || argv[0].size != 2
    raise ArgumentError, "Invalid command name"
  end

  fork do
    begin
      # handle FD => {FD, :close, [file,mode,perms]} options
      options.each do |key, val|
        if fd?(key)
          key = fd_to_io(key)

          if fd?(val)
            val = fd_to_io(val)
            key.reopen(val)
          elsif val == :close
            if key.respond_to?(:close_on_exec=)
              key.close_on_exec = true
            else
              key.close
            end
          elsif val.is_a?(Array)
            file, mode_string, perms = *val
            key.reopen(File.open(file, mode_string, perms))
          end
        end
      end

      # setup child environment
      ENV.replace({}) if options[:unsetenv_others] == true
      env.each { |k, v| ENV[k] = v }

      # { :chdir => '/' } in options means change into that dir
      ::Dir.chdir(options[:chdir]) if options[:chdir]

      # { :pgroup => pgid } options
      pgroup = options[:pgroup]
      pgroup = 0 if pgroup == true
      Process::setpgid(0, pgroup) if pgroup

      # do the deed
      ::Kernel::exec(*argv)
    ensure
      exit!(127)
    end
  end
end
popen4(*argv) click to toggle source

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.

# File lib/posix/spawn.rb, line 293
def popen4(*argv)
  # create some pipes (see pipe(2) manual -- the ruby docs suck)
  ird, iwr = IO.pipe
  ord, owr = IO.pipe
  erd, ewr = IO.pipe

  # spawn the child process with either end of pipes hooked together
  opts =
    ((argv.pop if argv[-1].is_a?(Hash)) || {}).merge(
      # redirect fds        # close other sides
      :in  => ird,          iwr  => :close,
      :out => owr,          ord  => :close,
      :err => ewr,          erd  => :close
    )
  pid = spawn(*(argv + [opts]))

  [pid, iwr, ord, erd]
ensure
  # we're in the parent, close child-side fds
  [ird, owr, ewr].each { |fd| fd.close }
end
pspawn(*args) click to toggle source

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.

# File lib/posix/spawn.rb, line 174
def pspawn(*args)
  env, argv, options = extract_process_spawn_arguments(*args)
  raise NotImplementedError unless respond_to?(:_pspawn)

  if defined? JRUBY_VERSION
    # On the JVM, changes made to the environment are not propagated down
    # to C via get/setenv, so we have to fake it here.
    unless options[:unsetenv_others] == true
      env = ENV.merge(env)
      options[:unsetenv_others] = true
    end
  end

  _pspawn(env, argv, options)
end
spawn(*args) click to toggle source

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

spawn(, 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.

# File lib/posix/spawn.rb, line 158
def spawn(*args)
  if respond_to?(:_pspawn)
    pspawn(*args)
  elsif ::Process.respond_to?(:spawn)
    ::Process::spawn(*args)
  else
    fspawn(*args)
  end
end
system(*args) click to toggle source

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.

# File lib/posix/spawn.rb, line 254
def system(*args)
  pid = spawn(*args)
  return false if pid <= 0
  ::Process.waitpid(pid)
  $?.exitstatus == 0
rescue Errno::ENOENT
  false
end

[Validate]

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