How to store output of os.system() in a variable

Question

I wrote a small code:

import os
os.system('users')
os.system('w')

This prints

ubuntu
 09:27:25 up 9 days, 21:23,  1 user,  load average: 0.00, 0.00, 0.00
USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
ubuntu   pts/0    42.99.164.66     09:06    5.00s  0.10s  0.00s sh -c w

But when i try :

import os
from pyslack import SlackClient

user_name = os.system('users')
login_details = os.system('w')

print user_name
print login_details

It has the following output:

ubuntu
 09:28:32 up 9 days, 21:24,  1 user,  load average: 0.00, 0.00, 0.00
USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
ubuntu   pts/0    42.99.164.66     09:06    0.00s  0.11s  0.00s w
0
0

Now i am not sure why i am not able to store the result in the varible , i.e why is it printing 0 ? And what should be the correct way to get rid of it?

Solution 1

The os.system return the exit code of the command.

To capture the output of the command, you can use subprocess.check_output

output = subprocess.check_output('users', shell=True)

Solution 2

The value returned by the os.system is identical to the return value of the command you launched. Since most calls, like 'users' are written in C, they return 0 when the code is executed successfully ( they have a return 0; at the end of the main() ).

If you want to save their output, you can redirect their output path (by default stdout) to a text file, then read the text file.

user_name = os.system('users > users.txt')
login_details = os.system('w > w.txt')

with open("users.txt", "r") as f:
    for line in f:
        print line
with open("w.txt", "r") as f:
    for line in f:
        print line

os.system("rm users.txt")
os.system("rm w.txt")

I bow to the subprocess.check_output solution

Solution 3

From the os.system(command).

os.system just execute the command (a string) in a subshell.

USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
ubuntu   pts/0    42.99.164.66     09:06    5.00s  0.10s  0.00s sh -c w

Which means the above data is the output written to stdout by calling the Standard C function system() not the return value.

On Unix, the return value is the exit status of the process encoded in the format specified for wait(). Note that POSIX does not specify the meaning of the return value of the C system() function, so the return value of the Python function is system-dependent.

On Windows, the return value is that returned by the system shell after running command, given by the Windows environment variable COMSPEC: on command.com systems (Windows 95, 98 and ME) this is always 0; on cmd.exe systems (Windows NT, 2000 and XP) this is the exit status of the command run; on systems using a non-native shell, consult your shell documentation.

So if the exit status is success, user_name and login_details will get a zero.

As a matter of fact, you can try this:;

import subprocess
user = subprocess.check_output(['users'])
details = subprocess.check_output(['w'])

print(user)
print(details)

Comments

[Nishant Singh]

If i have to get the specific fields out of details , lets say FROM , i,e the IP it was logged in , how can we achieve that

[McGrady]

@NishantSingh You need awk try to run this command w | awk '{print $3}' in terminal or you can split the login_details string.

[Daniyal Warraich]

If the return code is not 0, then check_output returns an error.