Skip to content


Commandlib is a dependencyless library for calling external UNIX commands (e.g. in build scripts) in a clean, readable way.

Using method chaining, you can build up Command objects that run in a specific directory, with specified environment variables and PATHs, etc.

For simplicity's sake, the library itself only runs commands in a blocking way (all commands run to completion before continuing), although it contains hooks to run non-blocking via either icommandlib or pexpect.

Pretend 'django/':

# Pretend django "" that just prints out arguments:
import sys ; sys.stdout.write(' '.join(sys.argv[1:]))

from commandlib import Command

# Create base command
python = Command("python")

# Create command "python" that runs in the django directory
manage = python("").in_dir("django")

# Build even more specific command
dev_manage = manage.with_trailing_args("--settings", "")
# Run combined command
dev_manage("runserver", "8080").run()

Will output:

runserver 8080 --settings


$ pip install commandlib



Commandlib avoids the tangle of messy code that you would get using the subprocess library directly (Popen, call, check_output(), .communicate(), etc.) and the confusion that results.

It's a heavily dogfooded library.

Is subprocess really that bad?

The code will likely be longer and messier. For example, from stack overflow:

import subprocess, os
previous_directory = os.getcwd()

my_env = os.environ.copy()
my_env["PATH"] = "/usr/sbin:/sbin:" + my_env["PATH"]
subprocess.Popen(my_command, env=my_env)


from commandlib import Command


Why not use Delegator instead (Kenneth Reitz's 'subprocesses for humans')?

Kenneth Reitz (author of requests "urllib2/3 for humans"), wrote a similarly inspired "subprocess for humans" called envoy. That is now deprecated and there is now a replacement called delegator, which is a very thin wrapper around subprocess.

Features delegator has which commandlib does not:

  • Delegator can chain commands, much like bash does (delegator.chain('fortune | cowsay')). Commandlib doesn't do that because while dogfooding the library I never encountered a use case where I found this to be necessary. You can, however, easily get the output of one command using .output() as a string and feed it into another using piped.from_string(string).

  • Delegator runs subprocesses in both a blocking and nonblocking way (using pexpect). commandlib only does blocking by itself but if you pip install pexpect or icommandlib it can run via either one of them.

  • Runs on windows

Features which both have:

  • Ability to set environment variables.
  • Ability to run pexpect process from command object.

Features which only commandlib has:

  • Ability to set PATH easily.
  • Ability call code from within the current virtualenv easily.
  • Ability to pipe in strings or files and easily pipe out to strings or file (or file handles).
  • Hook to easily run commands in from the current virtualenv.

Why not use other tools?

  • os.system(*) - only capable of running very simple bash commands.

  • sh - uses a lot of magic. Attempts to make python more like shell rather than making running commands more pythonic.

  • plumbum - similar to amoffat's sh, tries to make a sort of "bash inside python". Also has a weird way of building commands from dict syntax (grep["-v", "\.py"]).