在python中的system（cmd）命令中形成subprocess.call（）语句

Question

I have a ssh command which I was using in a system() statement but I want to replace it with a subprocess.call() statement. My ssh command is:

cmd ="ssh -i pem-file.pem user@" + hostname + " 'cd /user/home/ && java -cp jar-file.jar com.packg.class -a opt1 -f text-file_" + ts + ".txt'"

system(cmd)

I want to replace above with a subprocess.call() statement as it is giving me some performance issues and I read that subprocess.call() is a much better option to use. I formulated this query but it is not executing:

result = subprocess.call(["ssh","-i", "pem-file.pem","user@" + hostname + " 'cd /user/home/ && java -cp jar-file.jar com.packg.class -a opt1 -f text-file_" + ts + ".txt'"])

What is the mistake I am doing and what is the correct syntax?

Answer 1

The function shlex.split() is useful for parsing command line arguments into the proper format. This should resolve your syntax error:

import shlex
import subprocess

cmd ="ssh -i pem-file.pem user@" + hostname + " 'cd /user/home/ && java -cp jar-file.jar com.packg.class -a opt1 -f text-file_" + ts + ".txt'"

result = subprocess.call(shlex.split(cmd))

If that doesn't fix your error, then you can pass subprocess.call the shell=True argument:

    import subprocess

cmd ="ssh -i pem-file.pem user@" + hostname + " 'cd /user/home/ && java -cp jar-file.jar com.packg.class -a opt1 -f text-file_" + ts + ".txt'"

result = subprocess.call(cmd, shell=True)

Using the shell argument will cause your command to be executed through a shell, rather than having the interpreter parse it. However, don't use the shell option if cmd can ever come from an untrusted source. Take at look at the warning in the Python docs .

One more note:

subprocess.system() is newer and more flexible than os.system() , but don't worry too much about "upgrading" to the new function. The advantages of subprocess.call() are in the more flexible options for communicating with your subprocess. If all you're doing is executing a single command and getting the return code, os.system() is probably fine. If you're finding that your command is being flaky and unreliable, switching to subprocess.call() probably isn't going to help much.

Answer 2

Assuming there are no shell meta-characters in hostname (likely), your command could look like this: each command-line argument is a separate list item:

#!/usr/bin/env python
import subprocess

cmd = ["ssh", "-i", "pem-file.pem",  "user@" + hostname,
       "cd /user/home/ && java -cp jar-file.jar com.packg.class -a opt1 "
       "-f text-file_" + ts + ".txt"]
subprocess.check_call(cmd)

Unlike os.system() ; it doesn't run the (local) shell.

You could get the argument list using shlex.split(your_original_system_command) ( mentioned by @skrrgwasme ) but shlex.split() can be fooled and therefore it is mostly useful as a hint on how the result should look like.