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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.

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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()