bash Cookbook

Giving an Error Message for Unset Parameters

Problem

Those shorthands for giving a default value are cool, but maybe you need to force the users to give you a value, otherwise you don’t want to proceed. Perhaps if they left off a parameter, they don’t really understand how to invoke your script. You want to leave nothing to guesswork. Is there anything shorter than lots of if statements to check each of your several parameters?

Solution

Use the ${:?} syntax when referring to the parameter. bash will print an error message and then exit if the parameter is unset or null.

#!/usr/bin/env bash
# cookbook filename: check_unset_parms
#
USAGE="usage: myscript scratchdir sourcefile conversion"
FILEDIR=${1:?"Error. You must supply a scratch directory."}
FILESRC=${2:?"Error. You must supply a source file."}
CVTTYPE=${3:?"Error. ${USAGE}"}

Here’s what happens when we run that script with insufficient arguments:

$ ./myscript /tmp /dev/null
./myscript: line 5: 3: Error. usage: myscript scracthdir sourcefile conversion
$

Discussion

The check is made to see if the first parameter is set (or null) and if not, it will print an error message and exit.

The third variable uses another shell variable in its message. You can even run another command inside it:

CVTTYPE=${3:?"Error. $USAGE. $(rm $SCRATCHFILE)"}

If parameter three is not set, then the error message will contain the phrase “Error.”, along with the value of the variable named $USAGE and then any output from the command which removes the filename named ...