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mchid
Asked: 2019-10-06 16:46:31 +0800 CST

Pass a bash variable to python script

  • 772

I have this python script:

#!/usr/bin/env python
def getPermutation(s, prefix=''):
        if len(s) == 0:
                print prefix
        for i in range(len(s)):
                getPermutation(s[0:i]+s[i+1:len(s)],prefix+s[i] )



getPermutation('abcd','')

However, I want to be able to call this script using a variable for "abcd" so I can insert any combination of letters instead of "abcd" like "efgh" for example.

Normally, I can use a $@ or $1 instead of abcd on the last line in a bash script like this:

#!/usr/bin/env python
def getPermutation(s, prefix=''):
        if len(s) == 0:
                print prefix
        for i in range(len(s)):
                getPermutation(s[0:i]+s[i+1:len(s)],prefix+s[i] )



getPermutation("$1",'')

But when I run the script using something like ./scriptname.py efgh I get:

$1
1$

instead of the permutations for "efgh".

scripts command-line python bash

4 Answers

  1. Best Answer

    The python equivalent of the shell's positional parameter array $1, $2 etc. is sys.argv

    So:

    #!/usr/bin/env python

import sys

def getPermutation(s, prefix=''):
        if len(s) == 0:
                print prefix
        for i in range(len(s)):
                getPermutation(s[0:i]+s[i+1:len(s)],prefix+s[i] )



getPermutation(sys.argv[1],'')

    then

    $ ./foo.py abcd
abcd
abdc
acbd
acdb
adbc
adcb
bacd
badc
bcad
bcda
bdac
bdca
cabd
cadb
cbad
cbda
cdab
cdba
dabc
dacb
dbac
dbca
dcab
dcba
    • 24

  2. Lots of ways to parameterize python. positional args, env variables, and named args. Env variables:

    import os and use the getenv like:

    fw_main_width  =os.getenv('FW_MAIN_WIDTH',  fw_main_width)

    Where the second parameter is the default for the env variable not being set.

    Positional args:

    Use the sys.argc, sys.argv[n] after you import sys.

    Named parameters:

    Or for named parameters,(what you asked) 

     import argparse

    then describe the possible parameters: 

    parser = argparse.ArgumentParser(description = "Project", fromfile_prefix_chars='@')
parser.add_argument("-V", "--version", help="show program version", action="store_true")
parser.add_argument("-W", "--width", help="set main screen width")  
read arguments from the command line  

args = parser.parse_args()

    and use them as args.width etc.

    • 7

  3. Okay, I figured out a workaround while I was writing the question but I felt that this would useful to other users so here it is.

    For python (python2), we can use raw_input() instead of $1 but it works a bit differently. Instead of entering the input after the script name in bash, you are prompted to input the value after you run the script.

    Here is an example:

    #!/usr/bin/env python
def getPermutation(s, prefix=''):
        if len(s) == 0:
                print prefix
        for i in range(len(s)):
                getPermutation(s[0:i]+s[i+1:len(s)],prefix+s[i] )



getPermutation(raw_input("enter characters: "),'')

    Running the script will prompt the user to "enter characters:". After the user enters the characters and then presses ENTER, the permutations will print in the terminal.

    Here is the source which also explains how to do this for python3.

    • 2 
  4. import itertools, argparse

def main():
        parser = argparse.ArgumentParser()
        parser.add_argument('-l', '--letters', dest='letters',  type=str, help='Letters to iterate over')
        options = parser.parse_args()


        a = [''.join(i) for i in itertools.permutations(options.letters)]
        print(a)

if __name__ == '__main__': main()

    On the command line: FileName.py -l abcd

    • 1