"subprocess.Popen" - checking for success and errors
I want to check if a subprocess has finished execution successfully or failed. Currently I have come up with a solution but I am not sure if it is correct and reliable. Is it guaranteed that every process outputs its errors only to stderr respectfully to stdout
:
Note: I am not interested in just redirecting/printing out the output. That I know already how to do.
pipe = subprocess.Popen(command,
stdout=subprocess.PIPE,
stderr=subprocess.PIPE,
universal_newlines=True)
if "" == pipe.stdout.readline():
print("Success")
self.isCommandExectutionSuccessful = True
if not "" == pipe.stderr.readline():
print("Error")
self.isCommandExectutionSuccessful = True
alternatively:
if "" == pipe.stdout.readline():
print("Success")
self.isCommandExectutionSuccessful = True
else:
print("Error")
self.isCommandExectutionSuccessful = False
and:
if not "" == pipe.stderr.readline():
print("Success")
self.isCommandExectutionSuccessful = True
else:
print("Error")
self.isCommandExectutionSuccessful = False
Do you need to do anything with the output of the process?
The check_call
method might be useful here. See the python docs here: https://docs.python.org/2/library/subprocess.html#subprocess.check_call
You can then use this as follows:
try:
subprocess.check_call(command)
except subprocess.CalledProcessError:
# There was an error - command exited with non-zero code
However, this relies on command
returning an exit code of 0 for succesful completion and a non-zero value for an error.
If you need to capture the output as well, then the check_output
method may be more appropriate. It is still possible to redirect the standard error if you need this as well.
try:
proc = subprocess.check_output(command, stderr=subprocess.STDOUT)
# do something with output
except subprocess.CalledProcessError:
# There was an error - command exited with non-zero code
See the docs here: https://docs.python.org/2/library/subprocess.html#subprocess.check_output
Complete solution with check on return code, stdout and stderr:
import subprocess as sp
# ok
pipe = sp.Popen( 'ls /bin', shell=True, stdout=sp.PIPE, stderr=sp.PIPE )
# res = tuple (stdout, stderr)
res = pipe.communicate()
print("retcode =", pipe.returncode)
print("res =", res)
print("stderr =", res[1])
for line in res[0].decode(encoding='utf-8').split('\n'):
print(line)
# with error
pipe = sp.Popen( 'ls /bing', shell=True, stdout=sp.PIPE, stderr=sp.PIPE )
res = pipe.communicate()
print("retcode =", pipe.returncode)
print("res =", res)
print("stderr =", res[1])
Prints:
retcode = 0
res = (b'bash\nbunzip2\nbusybox\nbzcat\n...zmore\nznew\n', b'')
stderr = b''
bash
bunzip2
busybox
bzcat
...
zmore
znew
retcode = 2
res = (b'', b"ls: cannot access '/bing': No such file or directory\n")
stderr = b"ls: cannot access '/bing': No such file or directory\n"
output,error=pipe.communicate()
This will wait for command to finish and give you output or error depending on the state of command.
You can check return code of the process using check_call() method. In case if process returned non-zero value CalledProcessError will be raised.