Python Execute Unix / Linux Command Examples

Posted on in Categories , , last updated December 30, 2013

How do I execute standard Unix or Linux shell commands using Python? Is there a command to invoke Unix commands using Python programs?


You can execute the command in a subshell using os.system(). This will call the Standard C function system(). This function will return the exit status of the process or command. This method is considered as old and not recommended, but presented here for historical reasons only. The subprocess module is recommended and it provides more powerful facilities for running command and retrieving their results.

os.system example (deprecated)

The syntax is:

import os
os.system("command")

In this example, execute the date command:

import os
os.system("date")

Sample outputs:

Sat Nov 10 00:49:23 IST 2012
0

In this example, execute the date command using os.popen() and store its output to the variable called now:

import os
f = os.popen('date')
now = f.read()
print "Today is ", now

Sample outputs:

Today is  Sat Nov 10 00:49:23 IST 2012

Say hello to subprocess

The os.system has many problems and subprocess is a much better way to executing unix command. The syntax is:

import subprocess
subprocess.call("command1")
subprocess.call(["command1", "arg1", "arg2"])

In this example, execute the date command:

import subprocess
subprocess.call("date")

Sample outputs:

Sat Nov 10 00:59:42 IST 2012
0

You can pass the argument using the following syntax i.e run ls -l /etc/resolv.conf command:

import subprocess
subprocess.call(["ls", "-l", "/etc/resolv.conf"])

Sample outputs:

<-rw-r--r-- 1 root root 157 Nov  7 15:06 /etc/resolv.conf
0

To store output to the output variable, run:

import subprocess
p = subprocess.Popen("date", stdout=subprocess.PIPE, shell=True)
(output, err) = p.communicate()
print "Today is", output

Sample outputs:

Today is Sat Nov 10 01:27:52 IST 2012

Another example (passing command line args):

import subprocess
p = subprocess.Popen(["ls", "-l", "/etc/resolv.conf"], stdout=subprocess.PIPE)
output, err = p.communicate()
print "*** Running ls -l command ***\n", output

Sample outputs:

*** Running ls -l command ***
-rw-r--r-- 1 root root 157 Nov  7 15:06 /etc/resolv.conf

In this example, run ping command and display back its output:

import subprocess
p = subprocess.Popen(["ping", "-c", "10", "www.cyberciti.biz"], stdout=subprocess.PIPE)
output, err = p.communicate()
print  output

The only problem with above code is that output, err = p.communicate() will block next statement till ping is completed i.e. you will not get real time output from the ping command. So you can use the following code to get real time output:

import subprocess
cmdping = "ping -c4 www.cyberciti.biz"
p = subprocess.Popen(cmdping, shell=True, stderr=subprocess.PIPE)
while True:
    out = p.stderr.read(1)
    if out == '' and p.poll() != None:
        break
    if out != '':
        sys.stdout.write(out)
        sys.stdout.flush()

Sample outputs:

PING www.cyberciti.biz (75.126.153.206) 56(84) bytes of data.
64 bytes from www.cyberciti.biz (75.126.153.206): icmp_req=1 ttl=55 time=307 ms
64 bytes from www.cyberciti.biz (75.126.153.206): icmp_req=2 ttl=55 time=307 ms
64 bytes from www.cyberciti.biz (75.126.153.206): icmp_req=3 ttl=55 time=308 ms
64 bytes from www.cyberciti.biz (75.126.153.206): icmp_req=4 ttl=55 time=307 ms
 
--- www.cyberciti.biz ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3001ms
rtt min/avg/max/mdev = 307.280/307.613/308.264/0.783 ms

Related media

A quick video demo of above python code:



HTML 5 Video 01: Python Run External Command And Get Output On Screen or In Variable

References:

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

13 comment

  1. Hi,

    First off, enjoy all your posts and youtube videos. Recently viewed your tutorial on installing freebsd. So thank you for sharing your knowledge.

    I have a query regarding launching an external bash script file (.sh) in freebsd.
    For linux I used:
    os.system(‘sh ‘ + filepath)
    For Mac:
    os.system(‘open ‘ + filepath)
    And for windows:
    os.startfile(filepath)

    I am unable to get any of these to work for freebsd. I know startfile is only for windows, however was wondering if there was an equivalent for freebsd without using subprocess. Or if not possible at all how to use subprocess to call a external script.

    Also, in freebsd, what would be the equivalent of say:
    sudo chown -R user:user file.bundle
    as both sudo and chown are not installed by default.

    Any help would be appreciated.

    Regards,
    iSh0w

  2. What if I want to create a variable in Python, then pass that variable to a bash command line?
    Something like this:
    ….
    celsius = sensor.read_temperature()
    import subprocess
    subprocess.call([“myscript.sh”, “-v”, “-t $celsius”])

    Is that possible?

  3. I use split so I dont have to write literal arrays. Works most of the time.

    usrToGroup = subprocess.call("getent -s sss group users".split(' '))
     
    pingGoogleDNS = subprocess.call("ping -c 4 8.8.8.8".split(' '))
    1. I also find `”my command goes here”.split()` easier to use and maintain. To get proper handling of quoted args, use shlex.split: `shlex.split(”’this is my “quoted string” argument”’)`, which gives `[‘this’, ‘is’, ‘my’, ‘quoted string’, ‘argument’]`

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