Terrapin Build Status

Run shell commands safely, even with user-supplied values

API reference


The basic, normal stuff:

line = Terrapin::CommandLine.new("echo", "hello 'world'")
line.command # => "echo hello 'world'"
line.run # => "hello world\n"

Interpolated arguments:

line = Terrapin::CommandLine.new("convert", ":in -scale :resolution :out")
line.command(in: "omg.jpg",
             resolution: "32x32",
             out: "omg_thumb.jpg")
# => "convert 'omg.jpg' -scale '32x32' 'omg_thumb.jpg'"

It prevents attempts at being bad:

line = Terrapin::CommandLine.new("cat", ":file")
line.command(file: "haha`rm -rf /`.txt") # => "cat 'haha`rm -rf /`.txt'"

line = Terrapin::CommandLine.new("cat", ":file")
line.command(file: "ohyeah?'`rm -rf /`.ha!") # => "cat 'ohyeah?'\\''`rm -rf /`.ha!'"

NOTE: It only does that for arguments interpolated via run, NOT arguments passed into new (see 'Security' below):

line = Terrapin::CommandLine.new("echo", "haha`whoami`")
line.command # => "echo haha`whoami`"
line.run # => "hahawebserver\n"

This is the right way:

line = Terrapin::CommandLine.new("echo", "haha:whoami")
line.command(whoami: "`whoami`") # => "echo haha'`whoami`'"
line.run(whoami: "`whoami`") # => "haha`whoami`\n"

You can ignore the result:

line = Terrapin::CommandLine.new("noisy", "--extra-verbose", swallow_stderr: true)
line.command # => "noisy --extra-verbose 2>/dev/null"

# ... and on Windows...
line.command # => "noisy --extra-verbose 2>NUL"

If your command errors, you get an exception:

line = Terrapin::CommandLine.new("git", "commit")
rescue Terrapin::ExitStatusError => e
  e.message # => "Command 'git commit' returned 1. Expected 0"

If your command might return something non-zero, and you expect that, it's cool:

line = Terrapin::CommandLine.new("/usr/bin/false", "", expected_outcodes: [0, 1])
rescue Terrapin::ExitStatusError => e
  # => You never get here!

You don't have the command? You get an exception:

line = Terrapin::CommandLine.new("lolwut")
rescue Terrapin::CommandNotFoundError => e
  e # => the command isn't in the $PATH for this process.

But don't fear, you can specify where to look for the command:

Terrapin::CommandLine.path = "/opt/bin"
line = Terrapin::CommandLine.new("lolwut")
line.command # => "lolwut", but it looks in /opt/bin for it.

You can even give it a bunch of places to look:

    File.open('/usr/local/bin/lolwut') {|f| f.write('echo Hello') }
    Terrapin::CommandLine.path = ["/opt/bin", "/usr/local/bin"]
    line = Terrapin::CommandLine.new("lolwut")
    line.run # => prints 'Hello', because it searches the path

Or just put it in the command:

line = Terrapin::CommandLine.new("/opt/bin/lolwut")
line.command # => "/opt/bin/lolwut"

You can see what's getting run. The 'Command' part it logs is in green for visibility! (where applicable)

line = Terrapin::CommandLine.new("echo", ":var", logger: Logger.new(STDOUT))
line.run(var: "LOL!") # => Logs this with #info -> Command :: echo 'LOL!'

Or log every command:

Terrapin::CommandLine.logger = Logger.new(STDOUT)
Terrapin::CommandLine.new("date").run # => Logs this -> Command :: date


Short version: Only pass user-generated data into the run method and NOT new.

As shown in examples above, Terrapin will only shell-escape what is passed in as interpolations to the run method. It WILL NOT escape what is passed in to the second argument of new. Terrapin assumes that you will not be manually passing user-generated data to that argument and will be using it as a template for your command line's structure.


You can potentially increase performance by installing the posix-spawn gem. This gem can keep your application's heap from being copied when forking command line processes. For applications with large heaps the gain can be significant. To include posix-spawn, simply add it to your Gemfile or, if you don't use bundler, install the gem.


Terrapin will attempt to choose from among 3 different ways of running commands. The simplest is using backticks, and is the default in 1.8. In Ruby 1.9, it will attempt to use Process.spawn. And, as mentioned above, if the posix-spawn gem is installed, it will attempt to use that. If for some reason one of the .spawn runners don't work for you, you can override them manually by setting a new runner, like so:

Terrapin::CommandLine.runner = Terrapin::CommandLine::BackticksRunner.new

And if you really want to, you can define your own Runner, though I can't imagine why you would.

JRuby issues


If you get Error::ECHILD errors and are using JRuby, there is a very good chance that the error is actually in JRuby. This was brought to our attention in https://github.com/thoughtbot/terrapin/issues/24 and probably fixed in http://jira.codehaus.org/browse/JRUBY-6162. You will want to use the BackticksRunner if you are unable to update JRuby.

Spawn warning

If you get unsupported spawn option: out warning (like in issue 38), try to use PopenRunner:

Terrapin::CommandLine.runner = Terrapin::CommandLine::PopenRunner.new

Thread Safety

Terrapin should be thread safe. As discussed here, in this climate_control thread, climate_control, which modifies the environment under which commands are run for the BackticksRunner and PopenRunner, is thread-safe but not reentrant. Please let us know if you find this is ever not the case.


Security concerns must be privately emailed to [email protected].

Question? Idea? Problem? Bug? Comment? Concern? Like using question marks?

GitHub Issues For All!


Thank you to all the contributors!


Terrapin is maintained and funded by thoughtbot, inc

The names and logos for thoughtbot are trademarks of thoughtbot, inc.


Copyright 2011-2018 Jon Yurek and thoughtbot, inc. This is free software, and may be redistributed under the terms specified in the LICENSE file.