How to call shell commands from Ruby

Question

How do I call shell commands from inside of a Ruby program? How do I then get output from these commands back into Ruby?

Answer 1

If you have a more complex case than the common case that can not be handled with ``, then check out Kernel.spawn(). This seems to be the most generic/full-featured provided by stock Ruby to execute external commands.

You can use it to:

• create process groups (Windows).
• redirect in, out, error to files/each-other.
• set env vars, umask.
• change the directory before executing a command.
• set resource limits for CPU/data/etc.
• Do everything that can be done with other options in other answers, but with more code.

The Ruby documentation has good enough examples:

env: hash
  name => val : set the environment variable
  name => nil : unset the environment variable
command...:
  commandline                 : command line string which is passed to the standard shell
  cmdname, arg1, ...          : command name and one or more arguments (no shell)
  [cmdname, argv0], arg1, ... : command name, argv[0] and zero or more arguments (no shell)
options: hash
  clearing environment variables:
    :unsetenv_others => true   : clear environment variables except specified by env
    :unsetenv_others => false  : dont clear (default)
  process group:
    :pgroup => true or 0 : make a new process group
    :pgroup => pgid      : join to specified process group
    :pgroup => nil       : dont change the process group (default)
  create new process group: Windows only
    :new_pgroup => true  : the new process is the root process of a new process group
    :new_pgroup => false : dont create a new process group (default)
  resource limit: resourcename is core, cpu, data, etc.  See Process.setrlimit.
    :rlimit_resourcename => limit
    :rlimit_resourcename => [cur_limit, max_limit]
  current directory:
    :chdir => str
  umask:
    :umask => int
  redirection:
    key:
      FD              : single file descriptor in child process
      [FD, FD, ...]   : multiple file descriptor in child process
    value:
      FD                        : redirect to the file descriptor in parent process
      string                    : redirect to file with open(string, "r" or "w")
      [string]                  : redirect to file with open(string, File::RDONLY)
      [string, open_mode]       : redirect to file with open(string, open_mode, 0644)
      [string, open_mode, perm] : redirect to file with open(string, open_mode, perm)
      [:child, FD]              : redirect to the redirected file descriptor
      :close                    : close the file descriptor in child process
    FD is one of follows
      :in     : the file descriptor 0 which is the standard input
      :out    : the file descriptor 1 which is the standard output
      :err    : the file descriptor 2 which is the standard error
      integer : the file descriptor of specified the integer
      io      : the file descriptor specified as io.fileno
  file descriptor inheritance: close non-redirected non-standard fds (3, 4, 5, ...) or not
    :close_others => false : inherit fds (default for system and exec)
    :close_others => true  : dont inherit (default for spawn and IO.popen)

Answer 2

Here's a cool one that I use in a ruby script on OS X (so that I can start a script and get an update even after toggling away from the window):

cmd = %Q|osascript -e 'display notification "Server was reset" with title "Posted Update"'|
system ( cmd )

Answer 3

This explanation is based on a commented Ruby script from a friend of mine. If you want to improve the script, feel free to update it at the link.

First, note that when Ruby calls out to a shell, it typically calls /bin/sh, not Bash. Some Bash syntax is not supported by /bin/sh on all systems.

Here are ways to execute a shell script:

cmd = "echo 'hi'" # Sample string that can be used

1. Kernel#` , commonly called backticks – `cmd`

   This is like many other languages, including Bash, PHP, and Perl.

   Returns the result (i.e. standard output) of the shell command.

   Docs: http://ruby-doc.org/core/Kernel.html#method-i-60

   value = `echo 'hi'`
   value = `#{cmd}`
   

2. Built-in syntax, %x( cmd )

   Following the x character is a delimiter, which can be any character. If the delimiter is one of the characters (, [, {, or <, the literal consists of the characters up to the matching closing delimiter, taking account of nested delimiter pairs. For all other delimiters, the literal comprises the characters up to the next occurrence of the delimiter character. String interpolation #{ ... } is allowed.

   Returns the result (i.e. standard output) of the shell command, just like the backticks.

   Docs: https://docs.ruby-lang.org/en/master/syntax/literals_rdoc.html#label-Percent+Strings

   value = %x( echo 'hi' )
   value = %x[ #{cmd} ]
   

3. Kernel#system

   Executes the given command in a subshell.

   Returns true if the command was found and run successfully, false otherwise.

   Docs: http://ruby-doc.org/core/Kernel.html#method-i-system

   wasGood = system( "echo 'hi'" )
   wasGood = system( cmd )
   

4. Kernel#exec

   Replaces the current process by running the given external command.

   Returns none, the current process is replaced and never continues.

   Docs: http://ruby-doc.org/core/Kernel.html#method-i-exec

   exec( "echo 'hi'" )
   exec( cmd ) # Note: this will never be reached because of the line above
   

Here's some extra advice: $?, which is the same as $CHILD_STATUS, accesses the status of the last system executed command if you use the backticks, system() or %x{}. You can then access the exitstatus and pid properties:

$?.exitstatus

For more reading see:

• http://www.elctech.com/blog/i-m-in-ur-commandline-executin-ma-commands
• http://blog.jayfields.com/2006/06/ruby-kernel-system-exec-and-x.html
• http://tech.natemurray.com/2007/03/ruby-shell-commands.html

Answer 4

The backticks (`) method is the easiest one to call shell commands from Ruby. It returns the result of the shell command:

     url_request = 'http://google.com'
     result_of_shell_command = `curl #{url_request}`

Answer 5

Here's a flowchart based on "When to use each method of launching a subprocess in Ruby". See also, "Trick an application into thinking its stdout is a terminal, not a pipe".

Answer 6

Using the answers here and linked in Mihai's answer, I put together a function that meets these requirements:

1. Neatly captures STDOUT and STDERR so they don't "leak" when my script is run from the console.
2. Allows arguments to be passed to the shell as an array, so there's no need to worry about escaping.
3. Captures the exit status of the command so it is clear when an error has occurred.

As a bonus, this one will also return STDOUT in cases where the shell command exits successfully (0) and puts anything on STDOUT. In this manner, it differs from system, which simply returns true in such cases.

Code follows. The specific function is system_quietly:

require 'open3'

class ShellError < StandardError; end

#actual function:
def system_quietly(*cmd)
  exit_status=nil
  err=nil
  out=nil
  Open3.popen3(*cmd) do |stdin, stdout, stderr, wait_thread|
    err = stderr.gets(nil)
    out = stdout.gets(nil)
    [stdin, stdout, stderr].each{|stream| stream.send('close')}
    exit_status = wait_thread.value
  end
  if exit_status.to_i > 0
    err = err.chomp if err
    raise ShellError, err
  elsif out
    return out.chomp
  else
    return true
  end
end

#calling it:
begin
  puts system_quietly('which', 'ruby')
rescue ShellError
  abort "Looks like you don't have the `ruby` command. Odd."
end

#output: => "/Users/me/.rvm/rubies/ruby-1.9.2-p136/bin/ruby"