Calling 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 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 < , 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} ]
Kernel#system
Executes the given command in a subshell.
Returns 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 )
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:
The way I like to do this is using the %x literal, which makes it easy (and readable!) to use quotes in a command, like so:
directorylist = %x[find . -name '*test.rb' | sort]
Which, in this case, will populate file list with all test files under the current directory, which you can process as expected:
directorylist.each do |filename|
filename.chomp!
# work with file
end
Here's a flowchart based on this answer. See also, using script to emulate a terminal.
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