Python Execute Unix / Linux Command Examples

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", "sxi.io"], 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 sxi.io"
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 sxi.io (75.126.153.206) 56(84) bytes of data.
64 bytes from sxi.io (75.126.153.206): icmp_req=1 ttl=55 time=307 ms
64 bytes from sxi.io (75.126.153.206): icmp_req=2 ttl=55 time=307 ms
64 bytes from sxi.io (75.126.153.206): icmp_req=3 ttl=55 time=308 ms
64 bytes from sxi.io (75.126.153.206): icmp_req=4 ttl=55 time=307 ms
 
--- sxi.io 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:

References:

Posted by: SXI ADMIN

The author is the creator of SXI LLC and a seasoned sysadmin, DevOps engineer, and a trainer for the Linux operating system/Unix shell scripting. Get the latest tutorials on SysAdmin, Linux/Unix and open source topics via RSS/XML feed or weekly email newsletter.