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Python's subprocess module is a powerful tool for spawning new processes, connecting to their input/output/error pipes, and obtaining their return codes. It allows Python scripts to execute shell commands or other programs and handle their outputs and errors. The primary function in this module, subprocess.run(), runs a specified command and returns a CompletedProcess instance containing the command's exit status and any output. For more complex tasks, functions like subprocess.Popen() allow advanced interactions with a process, including streaming output in real-time, sending input, or managing multiple processes simultaneously. By setting options like stdout=subprocess.PIPE, the module can capture output directly within Python, making it versatile for tasks like automation, command-line interfacing, and scripting across different systems. This module is a preferred, secure alternative to older methods like os.system() and is generally used in conjunction with error-handling to manage command failures gracefully.

Code:

Python3 – Subprocess Overview
One-liner (Same Task)
• print(subprocess.check_output(['ls', '-1']).decode('utf-8'))
• print(''.join(map(chr, subprocess.check_output(['ls', '-1']))))
Waits (No Threading)
subprocess.call(), subprocess.check_output(),
subprocess.getoutput(),
subprocess.Popen().stdout.readlines()
Threading
subprocess.Popen()
#Windows only (threading)
DETACHED_PROCESS = 0x00000008
subprocess.Popen([sys.executable, 'ls'],
creationflags=DETACHED_PROCESS).pid
#BSD only (threading)
pid = subprocess.Popen([sys.executable, 'ls'],
stdout=subprocess.PIPE, stderr=subprocess.PIPE,
stdin=subprocess.PIPE)
Open Program or Execute Code
subprocess.call(['EXCUTABLE']) # no threading
Get PID
pid = subprocess.Popen(['ls', '-1'], shell=True,
stdout=subprocess.PIPE,
stderr=subprocess.STDOUT).stdout.read()
print(pid) # <subprocess.Popen object at 0x7f32405e80b8>
type(pid) # subprocess.Popen
# .readline() reads one stdout line at a time
# .readlines() reads all stdout at once
Usage
x = subprocess.check_output(['cmd', 'arg1'])
# x = b'Desktop\nDocuments\nDownloads\n'
z = x.decode('utf-8')) # plain text (str)
subprocess.getoutput('ls -l') # one str param
# Output = 'String\nString\nString'
Tricks
shlex.split('ls -l') # output: ['ls', '-l']
subprocess.check_output(shlex.split('ls -l'))
Shell2Python
Shell: output=`dmesg | grep hda`
Python: from subprocess import *
p1 = Popen(['dmesg'], stdout=PIPE)
p2 = Popen(['grep', 'hda'], stdin=p1.stdout, stdout=PIPE)
p1.stdout.close()
# allow p1 to receive a SIGPIPE if p2 exits
output = p2.communicate()[0]
Python:
output = check_output('dmesg | grep hda', shell=True)
Migrate os to subprocess
• os: output = os.spawnlp(os.P_WAIT, 'cmd', “arg')
• subprocess: output = call(['cmd', 'arg'])
• os: pid = os.spawnlp(os.P_NOWAIT, 'cmd', 'arg')
• subprocess: pid = Popen(['cmd', 'arg']).pid
Environment Variables
proc = subprocess.Popen(['echo',
os.environ['MY_ENV_VAR']])
proc = subprocess.Popen('echo '$MY_ENV_VAR'',
env=environ, shell=True)
proc = subprocess.Popen(['echo',
os.path.expandvars('$MY_ENV_VAR')])
• newenv = os.environ.copy()
• newenv['MY_ENV_VAR'] = 'value'
Example
import subprocess
proc = subprocess.Popen('mousepad')
print(proc) # <subprocess.Popen object at
0x7f3240609d30>
pid = proc.pid # 13731
print(proc.poll()) # None
proc.kill()
print(proc.poll()) # 0
proc = subprocess.Popen('mousepad')
pid = proc.pid
try:
outs, errs = proc.communicate(timeout=15)
except TimeoutExpired: # when timer expires
proc.kill() # close mousepad
outs, errs = proc.communicate()
# Example Lines
• proc.communicate(input='', timeout=int)
• Return Code: p_status = proc.wait()
• Send a Kill Signal: proc.send_signal(SIG)
• Stop the Process: proc.terminate()
• Wait for the Process: proc.wait()
Scripted Example
#!/usr/bin/env python3
import subprocess, sys
cmd = 'netstat -p --tcp'
p = subprocess.Popen(cmd, shell=True,
stderr=subprocess.PIPE)
while True: # displaying output immediately
out = p.stderr.read(1)
if out == '' and p.poll() != None:
break # exit after netstat closes
if out != '': # release output
sys.stdout.write(out)
sys.stdout.flush()