Override a variable in a Bash script from the command line

You need to use parameter expansion for the variable(s) you want to override:

$ cat override.sh
#!/bin/bash

: ${var1:=foo} # var1 will take on the value "foo" if not overridden
var2=${var2:-foo} # same thing but more typing

echo "var1 is $var1 | var2 is $var2"

Without Override Values

$ ./override.sh
var1 is foo | var2 is foo

With Override Values

$ var1=bar var2=baz ./override.sh
var1 is bar | var2 is baz