Learn Linux Shell Scripting - Fundamentals of Bash 4.4

Closely related to setting default values with parameter expansion, we can also use parameter expansion to display an error if a variable is null or empty. Up until now, we've done this by implementing if-then logic within our scripts. While this is an excellent and flexible solution, it is a little verbose—especially if the only thing you're interested in is the user supplying the parameters.

Let's create a new version of our previous example: this one does not supply default values, but will alert the user if positional arguments are missing.

We'll use the following syntax:

${parameter:?word}
Display Error if Null or Unset. If parameter is null or unset, the expansion of word (or a message to that effect if word is not present) is written to the standard error and the shell, if it is not interactive, exits. Otherwise, the value of parameter is substituted.

When we use this in our script, it might look something like this:

reader@ubuntu:~/scripts/chapter_16$ cp default-interactive-arguments.sh check-arguments.sh
reader@ubuntu:~/scripts/chapter_16$ vim check-arguments.sh eader@ubuntu:~/scripts/chapter_16$ cat check-arguments.sh 
#!/bin/bash

#####################################
# Author: Sebastiaan Tammer
# Version: v1.0.0
# Date: 2018-12-16
# Description: Script with parameter expansion input checking.
# Usage: ./check-arguments.sh <name> <location> <food>
#####################################

# Initialize the variables from passed arguments.
character_name=${1:?Name not supplied!}
location=${2:?Location not supplied!}
food=${3:?Food not supplied!}

# Compose the story.
echo "Recently, ${character_name} was seen in ${location} eating ${food}!"

Note the colon again. In the same way the colon worked in the previous example, it also forces this parameter expansion to consider an empty string as a null/unset value.

When we run this script, we see the following:

reader@ubuntu:~/scripts/chapter_16$ bash check-arguments.sh 
check-arguments.sh: line 12: 1: Name not supplied!
reader@ubuntu:~/scripts/chapter_16$ bash check-arguments.sh Sanne
check-arguments.sh: line 13: 2: Location not supplied!
reader@ubuntu:~/scripts/chapter_16$ bash check-arguments.sh Sanne Alkmaar
check-arguments.sh: line 14: 3: Food not supplied!
reader@ubuntu:~/scripts/chapter_16$ bash check-arguments.sh Sanne Alkmaar gnocchi
Recently, Sanne was seen in Alkmaar eating gnocchi!
reader@ubuntu:~/scripts/chapter_16$ bash check-arguments.sh Sanne Alkmaar ''
check-arguments.sh: line 14: 3: Food not supplied!

While this works like a charm, it doesn't really look that great, does it? The script name and line numbers are printed, which seems like a bit too much in-depth information for a user of the script.

It is up to you to decide if you think these are acceptable looking feedback messages to your users; personally, we think a nice if-then is often better, but with regards to concise scripting, this cannot be beaten.

There is another parameter expansion closely related to these: ${parameter:+word}. This allows you to use word only if the parameter is NOT null or empty. In our experience, this isn't a common occurrence, but for your scripting needs it might be; look for the words Use Alternate Value in man bash to get more information.

Table of Contents for
Learn Linux Shell Scripting - Fundamentals of Bash 4.4

Input checking

Table of Contents for Learn Linux Shell Scripting - Fundamentals of Bash 4.4

Table of Contents for
Learn Linux Shell Scripting - Fundamentals of Bash 4.4