How to call shell commands from Ruby

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

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

-
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}`

-
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

-
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 )

-
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

Stack Overflow Q#2232, score: 1478