Override a variable in a Bash script from the command line

Question

How do you override a variable in your Bash script from the command line?

I know how to pass variables in, but I just want something like ./myscript.sh -Dvar=v

Answer 1

Bash isn't like Make or Ant. Those two programs won't allow you to reset the value of a macro/property once it is set on the command line. Instead in Bash, you'll have to write your scripts in such a way that allows you to set these values from the command line and not override them inside your scripts.

You might want to look at the getopts command which is a Bash builtin. That gives you an easy, flexible way to parse command line arguments and set values from the command line. For example, I have four variables OPT_A, OPT_B, OPT_C, and OPT_D. If I don't pass the parameter, they get their default value. However, I can override that default value on the command line:

 USAGE="$0 [-a  -b  -c  -d ]"

 OPT_A="Default Value of A"
 OPT_B="Default Value of B"
 OPT_C="Default Value of C"
 OPT_D="Default Value of D"

 while getopts ':a:b:c:d:' opt
 do
     case $opt in
         a) OPT_A=$OPTARG;;
         b) OPT_B=$OPTARG;;
         c) OPT_C=$OPTARG;;
         d) OPT_D=$OPTARG;;
        \?) echo "ERROR: Invalid option: $USAGE"
            exit 1;;
     esac
done

---

You can also export your environment variables to allow your Bash scripts access to them. That way, you can set a variable and use that value.

In Bash, the ${parameter:=word} construct says that if $parameter is set, use the value of $parameter. However, if $parameter is null or unset, use word instead.

Now, imagine if you did this:

$ export COMMANDLINE_FOO="FUBAR"

Now the variable $COMMANDLINE_FOO is set and readable for your shell scripts.

Then, in your shell script, you can do this:

FOO=BARFU

[...]    #Somewhere later on in the program...

echo "I'm using '${COMMANDLINE_FOO:=$FOO}' as the value"

This will now print

I'm using 'FUBAR' as the value

instead of

I'm using 'BARFU' as the value