Bash command line arguments, replacing defaults for variables

Question

I have a script which has several input files, generally these are defaults stored in a standard place and called by the script.

However, sometimes it is necessary to run it with changed inputs.

In the script I currently have, say, three variables, $A $B, and $C. Now I want to run it with a non default $B, and tomorrow I may want to run it with a non default $A and $B.

I have had a look around at how to parse command line arguments:

How do I parse command line arguments in Bash?

How do I deal with having some set by command line arguments some of the time?

I don't have enough reputation points to answer my own question. However, I have a solution:

Override a variable in a Bash script from the command line

#!/bin/bash a=input1 b=input2 c=input3 while getopts  "a:b:c:" flag do     case $flag in         a) a=$OPTARG;;         b) b=$OPTARG;;         c) c=$OPTARG;;     esac done 

Answer 1

You can do it the following way. See Shell Parameter Expansion on the Bash man page.

#! /bin/bash  value=${1:-the default value} echo value=$value 

On the command line:

$ ./myscript.sh value=the default value $ ./myscript.sh foobar value=foobar