subprocess.run Link to heading

python 3.5 provided new interface to replace older os.system to call programs and shell commands. doc

run() is wrapper around the lower interface Popen which provides finer and more complicated control. Also, run waits until process is done but popen will continue execution and user needs to check for process termination.

subprocess.run(args, *, stdin=None, input=None, stdout=None, stderr=None, capture_output=False, shell=False, cwd=None, timeout=None, check=False, encoding=None, errors=None, text=None, env=None, universal_newlines=None, **other_popen_kwargs)

as docs mentions these are the most important options.

args Link to heading

This could be list of strings for the program and program arguments.

subproccess.run(["ls", "-l"])

if it’s one string, shell=True must be used to allow shell to handle the whole command

run ( "ls -l f*",shell=True)

stdin, stdout, stderr Link to heading

According to doc,

stdin, stdout and stderr specify the executed program’s standard input, standard output and standard error file handles, respectively. Valid values are PIPE, DEVNULL, an existing file descriptor (a positive integer), an existing file object, and None

import subprocess

print("########## default")
ls = subprocess.run(["ls"])

print((ls))


print("########## suppress output")
ls = subprocess.run(["ls"], stdout= subprocess.DEVNULL)

print("########## send to file")
fh = open("t","w")
ls = subprocess.run(["ls"], stdout= fh)

print("########## capture in process")
ls = subprocess.run(["ls"], stdout= subprocess.PIPE)
print(ls.stdout)
ls = subprocess.run(["ls"], stdout= subprocess.PIPE, text=True)

text Link to heading

by default, stdout, stdin, stderr are using byte encoding for output. with text=True, the output is string

In [10]: run ( "echo test",shell=True,stdout=subprocess.PIPE).stdout                                                                  
Out[10]: b'test\n'

In [11]: run ( "echo test",shell=True,stdout=subprocess.PIPE, text=True).stdout                                                       
Out[11]: 'test\n'

subprocess.Popen Link to heading

Popen starts the process and user needs to check when it terminated if needed

proc = subprocess.Popen("sleep 10", shell=True)
try:
        outs, errs = proc.communicate(timeout=3)
except subprocess.TimeoutExpired:
        proc.kill()
        outs, errs = proc.communicate()
        print("timeout")

There is also Popen.wait which makes program wait for child process which make Popen() same as run()

also there are several API to control the process, probably the most important would be

• Popen.send_signal(signal)
• Popen.kill()