Understanding sys.argv

If you are writing a Python program to accept command line arguments, the arguments passed on the command line are accessed as sys.argv. Here's a simple example, save it as command.py:

#!/usr/bin/env python

if __name__ == '__main__':
    import sys
    print "Executable: %s"%sys.argv[0]
    for arg in sys.argv[1:]:
        print "Arg: %s"%arg

The if __name__ == '__main__': line ensures the code beneath it only gets run if the file is executed, not if it is imported. Make this file executable:

$ chmod 755 command.py

Here are some examples of this being run:

$ python command.py
Executable: command.py
$ python command.py arg1
Executable: command.py
Arg: arg1
$ python command.py arg1 arg2
Executable: command.py
Arg: arg1
Arg: arg2

Notice that sys.argv[0] always contains the name of the script which was executed, regardless of how Python was invoked. sys.argv[1] and onward are the command line arguments. You can also invoke the program using Python's -m switch to run it as a module import:

$ python -m command
Executable: /home/james/Desktop/command.py
$ python -m command arg1
Executable: /home/james/Desktop/command.py
Arg: arg1
$ python -m command arg1 arg2
Executable: /home/james/Desktop/command.py
Arg: arg1
Arg: arg2

Once again, Python works out that python -m command are all part of the invoking command so sys.argv[0] only contains the path of the Python script. Let's try executing it directly:

$ ./command.py
Executable: ./command.py
$ ./command.py arg1
Executable: ./command.py
Arg: arg1
$ ./command.py arg1 arg2
Executable: ./command.py
Arg: arg1
Arg: arg2

As you can see, sys.argv[0] still contains the Python script and sys.argv[1] and onwards represent each argument.