External Commands

To call other programs from a Python script

>>> import subprocess

simple use: same as one would do on the system command line, shell needs to be activated (not recommended)

>>> subprocess.call('ls -l',shell=True)

0 drwxrwsr-x 1 user group 0 Nov 27 18:44 .

4 drwxrwsr-x 1 user group 88 Nov 27 18:44 ..

better: command arguments as list which are passed directly to the program without using the system shell

>>> subprocess.call(['ls', '-l'])

0 drwxrwsr-x 1 user group 0 Nov 27 18:44 .

4 drwxrwsr-x 1 user group 88 Nov 27 18:44 ..

capturing the output

>>> cmd=['ls', '-l']

>>> output=subprocess.check_output(cmd).decode('utf-8').strip() # decode: python3 'bytes' into string

>>> print(output)

0 drwxrwsr-x 1 user group 0 Nov 27 18:44 .

4 drwxrwsr-x 1 user group 88 Nov 27 18:44 ..

capturing both output and command's error-messge (when running in a python script)

try:

output=subprocess.check_output('ls -l; ls x', \

stderr=subprocess.STDOUT,shell=True).decode('utf-8')

except subprocess.CalledProcessError as e:

output = e.output.decode('utf-8') # stdout and stderr

# print(e.cmd)

# print(e.returncode)

print(output)

0 drwxrwsr-x 1 user group 0 Nov 27 18:44 .

4 drwxrwsr-x 1 user group 88 Nov 27 18:44 ..

ls: cannot access x: No such file or directory

try:

cmd='ls x'

output = check_output(cmd,stderr=subprocess.STDOUT,shell=True).decode('utf-8');