Python try block does not catch os.system exceptions

Question

I have this python code:

import os
try:
  os.system('wrongcommand')
except:
  print("command does not work")

The code prints:

wrongcommand: command not found

Instead of command does not work. Does anyone know why it's not printing my error message?

score 66 · Accepted Answer · edited Feb 16 '21 at 15:35

66

If you want to have an exception thrown when the command doesn't exist, you should use subprocess:

import subprocess
try:
    subprocess.run(['wrongcommand'], check = True)
except subprocess.CalledProcessError:
    print ('wrongcommand does not exist')

Come to think of it, you should probably use subprocess instead of os.system anyway ...

edited Feb 16 '21 at 15:35

Tiago Martins Peres

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answered Sep 11 '12 at 15:56

mgilson

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My command is for running a .jar file. When I run the correct command with square braces the above snippet still showing the exception. When I run the wrong command without square braces the above snippet not showing the exception. How to handle this? – Shiva Krishna Chippa Aug 20 '19 at 08:44
What does your command look like? I suspect that you want something like: `subprocess.call(["java", "-jar", "yourjar.jar"])` – mgilson Aug 22 '19 at 03:13
`sikuli_jar = "C:\\sikulix.jar" sikuli_script = "C:\\Sikuli.sikuli" try: command = "java -jar {} -r {}".format(sikuli_jar, sikuli_script) subprocess.call([command]) except OSError: raise ("\"{}\" \"{}\" one of these is missing.".format(sikuli_jar, sikuli_script))` With the above I was facing problem. – Shiva Krishna Chippa Aug 23 '19 at 05:49
2

But fixed with below code. `try: command = "java -jar {} -r {}".format(sikuli_jar, sikuli_script) subprocess.check_call(command) except (OSError, subprocess.SubprocessError, subprocess.CalledProcessError): raise ("\"{}\" \"{}\" one of these is missing.".format(sikuli_jar, sikuli_script))` – Shiva Krishna Chippa Aug 23 '19 at 05:54
@ShivakrishnaChippa, can you add your answer as an `EDIT:` to your question! It would be easier to read instead of what's in the comment! – Anu Aug 09 '20 at 05:49
With SSH keys in place, when invoked with `subprocess.call()`, `rsync` asks for a password, but does not ask when invoked with `os.system()`. Did I miss something? – Eric Nelson Jun 03 '21 at 02:58

score 34 · Answer 2 · edited Aug 11 '19 at 04:28

Because os.system() indicates a failure through the exit code of the method

return value == 0 -> everything ok
return value != 0 -> some error

The exit code of the called command is directly passed back to Python.

There is documentation telling you that os.system() would raise an exeption in case of a failure. os.system() just calls the underlaying system() call of the OS and returns its return value.

Please read the os.system() documentation carefully.

Shouton Eulle · Answer 3 · 2021-06-26T13:15:52.100

18

Although subprocess might be your best friend. os.system is still useful in somewhere, especially to the programmer play C/C++ mode.

Hence, the code will be below.

import os

try:
  os_cmd = 'wrongcommand'
  if os.system(os_cmd) != 0:
      raise Exception('wrongcommand does not exist')
except:
  print("command does not work")

edited Jun 26 '21 at 13:15

answered Jun 24 '19 at 07:56

Shouton Eulle

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Dr. Jan-Philip Gehrcke · Answer 4 · 2012-09-11T16:08:28.890

There are two problems in your code snippet. First of all, never just do try: ... except:, always be specific about which exception you want to handle. Otherwise, your program simply swallows any kind of error, also those that you do not expect. In most cases, this will lead to unexpected behavior at some other point during runtime.

Furthermore, os.system() calls should most of the time be replaced by their counterparts from the subprocess module.

To see what goes wrong, leave out the try/except block and actually look at the traceback/exception. As others have pointed out, you will notice that there is no exception in your case which is why your custom string is not printed.

Bottom line: think about which specific exceptions can occur in your code block. Think hard about which of them you expect to happen for certain reasons and handle those appropriately. Do not handle those that you do not expect.

gefei · Answer 5 · 2012-09-11T16:09:26.667

1

wrongcommand: command not found is the output of the shell os.system is using to invoke the command. os.system did not throw an exception

EDIT: edited by copy-and-pasting part of mgilson's comment

edited Sep 11 '12 at 16:09

answered Sep 11 '12 at 15:54

gefei

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thanks, but how do you catch it in python? is it even possible? – Cinder Sep 11 '12 at 15:55
1

Yes, but not using `os.system()`. This is one reason why you should use the `subprocess` module, as outlined in @mgilson's answer. – Dr. Jan-Philip Gehrcke Sep 11 '12 at 16:02
More specifically, `wrongcommand: command not found` is the output of the *shell* that `os.system` is using to invoke the command. (a different shell with a different `PATH` might find the command). – mgilson Sep 11 '12 at 16:07

score 1 · Answer 6 · answered Dec 24 '19 at 10:17

1

There is one more easiest ways is:

import os

def dat():
        if os.system('date') == 0:
            print("Command successfully executed")
        else:
            print("Command failed to execute")

dat()

answered Dec 24 '19 at 10:17

Preetesh Sharma

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Python try block does not catch os.system exceptions

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