Python Execute Unix / Linux Command Examples

http://www.cyberciti.biz/faq/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-- 0="" 157="" 15:06="" 1="" 7="" etc="" nov="" pre="" resolv.conf="" root="">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:

Python 2.x:  subprocess documentation.
Python 3.x:  subprocess documentation.