command.md

Command literal

A command literal is a string delimited by backticks ` or a %x percent literal. It will be substituted at runtime by the captured output from executing the string in a subshell.

The same escaping and interpolation rules apply as for regular strings.

Similar to percent string literals, valid delimters fo %x are parenthesis (), square brackets [], curly braces {}, angles <> and pipes ||. Except for the pipes, all delimiters can be nested meaning a start delimiter inside the string escapes the next end delimiter.

The special variable $? holds the exit status of the command as a Process::Status. It is only available in the same scope as the command literal.

`echo foo`  # => "foo"
$?.success? # => true

Internally, the compiler rewrites command literals to calls to the top-level method `() with a string literal containing the command as argument: `echo #{argument}` and %x(echo #{argument}) are rewritten to `("echo #{argument}").

Security concerns

While command literals may prove useful for simple script-like tools, special caution is advised when interpolating user input because it may easily lead to command injection.

user_input = "hello; rm -rf *"
`echo #{user_input}`

This command will write hello and subsequently delete all files and folders in the current working directory.

To avoid this, command literals should generally not be used with interpolated user input. Process from the standard library offers a safe way to provide user input as command arguments:

user_input = "hello; rm -rf *"
process = Process.new("echo", [user_input], output: Process::Redirect::Pipe)
process.output.gets_to_end # => "hello; rm -rf *"
process.wait.success?      # => true