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Execution of shell code in Ruby scripts

There are several ways to execute shell code in Ruby. Spawn is the method behing exec, system and %x[…], don't use it, but check the documentation below.

%x{ } – quick and easy

Returns the standard output of running the given command in a subshell. This is an alias for `…`, and you can use string interpolation.


name = 'ls' result = `which #{name}`

It does not escape anything you inject in the string.

system – when you want to know how it went

Similar to exec, but executes the given command in a subshell. Your script will go on. Returns:

  • true if the command gives zero exit status
  • false for non zero exit status
  • nil if command execution fails, $? holds an error status.


result = system 'cp', '/full/path/to/my_file', '/target/directory' if result.nil? puts "Error was #{$?}" elsif result puts "You made it!" end

exec – the last thing you do

Replaces the current process. Take care: when an exec command exits, it exits your script as well; i.e. exec will be the last statement executed in your Ruby script. Output is written to stdout.

Raises SystemCallError if the command couldn‘t execute.


puts "Creating directory..." exec 'mkdir', 'new_directory' # the code here and below will never be read

spawn – gives you a good level of control

Takes the same parameters as exec and system: [env,] command [,options]

Optional hash with :name => environment_name, e.g. spawn(:name => 'development', 'echo hello world')
command, either:
  • A single string which is passed to the standard shell (with shell expansion), e.g. exec('cat * | grep denied')
  • The command name and one or more plain arguments (without shell expansion), e.g. exec *[ 'git', 'add', 'Gemfile' ]

If the first command is a two-element array, the first element is taken as the command to be executed, and the second argument is used as the argv[0] value, which may show up in process listings. E.g. exec ['ls', 'foo'], 'foo.bar', 'baz.bar'

see the Ruby docs

The shell used is /bin/sh on Unix-like systems, ENV["RUBYSHELL"] or ENV["COMSPEC"] on Windows NT series, and similar.

Spawn calls IO.popen.

open3 – for those who want to control everything

"Open3 grants you access to stdin, stdout, stderr and a thread to wait the child process when running another program. You can specify various attributes, redirections, current directory, etc., of the program as Process.spawn." Quoted from the docs

Use open3 when you have to explicitly manage all input, output, and errors.

Author of this card:

Dominik Schoeler
command, line, parameters, arguments, cli
About this deck:
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Posted by Dominik Schoeler to makandropedia