fork and exec combination in python

fork() - will fork a new process with separate memory block, open file descriptors from parent process.
exec*() - it will replace current process with the program it is going to execute.


Above combination can be used to create several process in background.  exec*() in detail :
There are various confusing forms in python.

1. execl(<program_path>,<arg1>,<arg2>,..,<argN>)
     where,
     program_path : either input absolute path of program file or set PATH variable and specify only program name
     arg1,arg2,..,argN : These are arguments to program. Always remember to set arg1 to program name because in python sys.argv[0] is name of program. Actual program arguments are starting from arg2.

2. execv(<program_path>,args)
    where,
     program_path : either input absolute path of program file or set PATH variable and specify only program name
     args : Create a tuple of arguments where first element is program name because in python sys.argv[0] is name of program. Actual program arguments are starting from second element in tuple.

3. execlpe(<program_path>,arg1,arg2,..,..,env)
    - everything is similar to execl. Only difference is 'p' means python will look into PATH if program path is not specified. 'e' means we can pass dictionary which contains environment variable name and its value which will be set before executing program.

5. execvpe(<program_path>,args,env)
    - everything is similar to execv. Only difference is 'p' means python will look into PATH if program path is not specified. 'e' means we can pass dictionary which contains environment variable name and its value which will be set before executing program.

Try below program:

[manas_khandeshe@static-61 ~/Documents/sandbox]cat test.sh
#!/bin/bash

script_name=$0
arg1=$1
current=`pwd`
echo "Script name : $script_name, $arg1"
echo "XYZ: "$XYZ
echo "PATH: "$PATH
echo "current directory: $current"


#!/usr/bin/env python3
import os
import sys

def _execl():
    arg1 = "test"
    arg2 = "abc"
    os.environ["XYZ"] = 'MyXYZ'
    os.execlp("/Users/manas_khandeshe/Documents/test.sh",arg1,arg2)
    return
 
def _execlpe():
    arg1 = "test"
    arg2 = "abc"
    env = {
        "XYZ" : 'MyXYZ'
    }
    os.execlpe("/Users/manas_khandeshe/Documents/sandbox/test.sh",arg1,arg2,env)
    return
 
def _execv():
    args = ("test","abc")
    os.environ["XYZ"] = 'MyXYZ'
    os.execv("/Users/manas_khandeshe/Documents/sandbox/test.sh",args)
    return
 
def _execvpe():
    env = {
        "PATH":"/Users/manas_khandeshe/Documents/sandbox",
        "XYZ":"BlaBla"
    }
    args = ("test","abc")
    os.execvpe("test.sh",args,env)
    return


def main():
    _execl()
    #_execlpe()
    #_execv()
    #_execvpe()
    return

main()
exit(0)

What does fork means in python

Technically, a forked process gets a copy of the original process’s global memory, including open file descriptors. Because of that, global objects like files start out with the same values in a child process, so all the processes here are tied to the same single stream. But it’s important to remember that global memory is copied, not shared; if a child process changes a global object, it changes only its own copy. (As we’ll see, this works differently in threads, the topic of the next section.)