Question
How do I call console/bash commands from inside of a Ruby Program? Also, how do I get output from these commands back into my program?
Answer
This explanation is based on this commented Ruby script from a friend of mine. If you want to improve the script, feel free to update it at the link.
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 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 of the shell command, just like the backticks
Docs: http://www.ruby-doc.org/docs/ProgrammingRuby/html/language.html
value = %x( echo 'hi' )
value = %x[ #{cmd} ]
3. Kernel#system
Executes the given command in a subshell,
Return: true if the command was found and ran 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.
Return: 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 beacuse of the line above
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 %{}.
You can then access the exitstatus and pid properties
$?.exitstatus
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