Python: “subprocess.Popen” check for success and e

Question

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

Answer 1

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"

Answer 2

      output,error=pipe.communicate()

This will wait for command to finish and give you output or error depending on the state of command.

Answer 3

You can check return code of the process using check_call() method. In case if process returned non-zero value CalledProcessError will be raised.

Answer 4

This is how I did it finally:

    # Call a system process
    try:
        # universal_newlines - makes manual decoding of subprocess.stdout unnecessary
        output = subprocess.check_output(command,
                                         stderr=subprocess.STDOUT,
                                         universal_newlines=True)

        # Print out command's standard output (elegant)
        for currentLine in output:
            self.textEdit_CommandLineOutput.insertPlainText(currentLine)

        self.isCommandExecutionSuccessful = True

    except subprocess.CalledProcessError as error:
        self.isCommandExecutionSuccessful = False

        errorMessage = ">>> Error while executing:\n"\
                       + command\
                       + "\n>>> Returned with error:\n"\
                       + str(error.output)
        self.textEdit_CommandLineOutput.append(errorMessage)

        QMessageBox.critical(None,
                             "ERROR",
                             errorMessage)
        print("Error: " + errorMessage)

    except FileNotFoundError as error:
        errorMessage = error.strerror
        QMessageBox.critical(None,
                             "ERROR",
                             errorMessage)
        print("Error: ", errorMessage)

I hope it will be useful to someone else.

Answer 5

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