Terrapin

Run shell commands safely, even with user-supplied values

Usage

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")
begin
  line.run
rescue Terrapin::ExitStatusError => e
  e.message # => "Command 'git commit' returned 1. Expected 0"
end

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])
begin
  line.run
rescue Terrapin::ExitStatusError => e
  # => You never get here!
end

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

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

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:

FileUtils.rm("/opt/bin/lolwut")
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

Security

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.

Runners

Terrapin will choose from among a couple different ways of running commands. The simplest is Process.spawn, which is also the default. Terrapin can also just use backticks, so if for some reason you'd prefer that, you can ask Terrapin to use that:

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.

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

Caveat

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 #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.

Feedback

Security concerns must be privately emailed to security@thoughtbot.com.

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

GitHub Issues For All!

Credits

Thank you to all the contributors!

License

About thoughtbot

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thoughtbot/terrapin