Bash supports a concept called "Positional Parameters". These positional parameters represent arguments that are specified on the command line when a Bash script is invoked.
Positional parameters are referred to by the names $0, $1, $2 ... and so on. $0 is the name of the script itself, $1 is the first argument to the script, $2 the second, etc. $* represents all of the positional parameters, except for $0 (i.e. starting with $1).
An example:
#!/bin/bash
FILE="$1"
externalprogram "$FILE"
