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Python subprocess wrapper

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Octocat-spinner-32 LICENSE.txt

PBS is a unique subprocess wrapper that maps your system programs to Python functions dynamically. PBS helps you write shell scripts in Python by giving you the good features of Bash (easy command calling, easy piping) with all the power and flexibility of Python.

from pbs import ifconfig
print ifconfig("eth0")

PBS is not a collection of system commands implemented in Python.


If you're writing a shell-style script, import the following way:

from pbs import *

This will make all of your system programs available to the script. Note that this does not actually import every system program, but provides a dynamic lookup mechanism.

Or if you just want to import a few system programs:

from pbs import ifconfig, supervisorctl, ffmpeg

You can also try out PBS through an interactive REPL:

$> python


Executing Commands

Commands work like you'd expect. Just call your program's name like a function:

# print the contents of this directory 
print ls("-l")

# get the longest line of this file
longest_line = wc(__file__, "-L")

# get interface information
print ifconfig("eth0")

Note that these aren't Python functions, these are running the binary commands on your system dynamically by resolving your PATH, much like Bash does. In this way, all the programs on your system are easily available in Python.

Keyword Arguments

Keyword arguments also work like you'd expect: they get replaced with the long-form and short-form commandline option:

# resolves to "curl -o page.html --silent"
curl("", o="page.html", silent=True)

# or if you prefer not to use keyword arguments, these do the same thing:
curl("", "-o page.html", "--silent")
curl("", "-o", "page.html", "--silent")

# resolves to "adduser amoffat --system --shell=/bin/bash --no-create-home"
adduser("amoffat", system=True, shell="/bin/bash", no_create_home=True)

# or
adduser("amoffat", "--system", "--shell /bin/bash", "--no-create-home")


Piping has become function composition:

# sort this directory by biggest file
print sort(du("*", "-sb"), "-rn")

# print the number of folders and files in /etc
print wc(ls("/etc", "-1"), "-l")

Sudo and With Contexts

Commands can be run within a "with" context. Popular commands using this might be "sudo" or "fakeroot":

with sudo:
    print ls("/root")

If you need to run a command in a with context AND call it, for example, specifying a -p prompt with sudo, you need to use the "_with" keyword argument. This let's the command know that it's being run from a with context so it can behave correctly.

with sudo(p=">", _with=True):
    print ls("/root")

Finding Commands

"Which" finds the full path of a program, or returns None if it doesn't exist. This command is one of the few commands implemented as a Python function, and therefore doesn't rely on the "which" program actually existing.

print which("python") # "/usr/bin/python"
print which("ls") # "/bin/ls"
print which("some_command") # None

if not which("supervisorctl"): apt_get("install", "supervisor", "-y")

Environment Variables

Environment variables are available much like they are in Bash:

print HOME
print SHELL
print PS1

You can set enviroment variables the usual way, through the os.environ mapping:

import os
os.environ["TEST"] = "123"

Now any new subprocess commands called from the script will be able to access that environment variable.


Exceptions are dynamically generated based on the return code of the command. This lets you catch a specific return code, or catch all error return codes through the base class ErrorReturnCode:

try: print ls("/some/non-existant/folder")
except ErrorReturnCode_2:
    print "folder doesn't exist!"
except ErrorReturnCode:
    print "unknown error"

Commandline Arguments

You can access commandline arguments similar to Bash's $1, $2, etc by using ARG1, ARG2, etc:

print ARG1, ARG2

# if an argument isn't defined, it's set to None
if ARG10 is None: do_something()

You can access the entire argparse/optparse-friendly list of commandline arguments through "ARGV". This is recommended for flexibility:

import argparse
parser = argparse.ArgumentParser(prog="PROG")
parser.add_argument("-x", default=3, type=int)
ns = parser.parse_args(ARGV)
print ns.x

Background Processes

Commands can be run in the background with the special _bg=True keyword argument:

# blocks
print "...3 seconds later"

# doesn't block
p = sleep(3, _bg=True)
print "prints immediately!"
print "...and 3 seconds later"

You can also pipe together background processes!

p = wc(curl("", silent=True, _bg=True), "--bytes")
print "prints immediately!"
print "byte count of github: %d" % int(p) # lazily completes

This lets you start long-running commands at the beginning of your script (like a file download) and continue performing other commands in the foreground.

Weirdly-named commands

PBS automatically handles underscore-dash conversions. For example, if you want to call apt-get:

apt_get("install", "mplayer", y=True)

PBS looks for "apt_get", but if it doesn't find it, replaces all underscores with dashes and searches again. If the command still isn't found, a CommandNotFound exception is raised.

Commands with other, more uncommonly-named symbols in them must be accessed directly through the "Command" class wrapper. The Command class takes the full path to the program as a string:

p27 = Command(which("python2.7"))
print p27("-h")

The Command wrapper is also useful for commands that are not in your standard PATH:

script = Command("/tmp/")
print script()
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