On to parameter expansion! As we've hinted, Bash allows us to do many cool things directly with variables. We'll start with the seemingly simple example of defining a default value for your variables.

When dealing with user input, this makes both your life and the script user's life much easier: as long as there is a reasonable default value, we can make sure that we use that instead of throwing an error when the user does not supply the information we want.

We will reuse one of our earliest scripts, interactive.sh, from Chapter 8, Variables and User Input. It was a very simple script that did not verify user input, and was thus prone to all sorts of problems. Let's get it up-to-date and include our new default values for our parameters, as follows:

reader@ubuntu:~/scripts/chapter_16$ cp ../chapter_08/interactive-arguments.sh default-interactive-arguments.sh
reader@ubuntu:~/scripts/chapter_16$ vim default-interactive-arguments.sh 
reader@ubuntu:~/scripts/chapter_16$ cat default-interactive-arguments.sh 
#!/bin/bash

#####################################
# Author: Sebastiaan Tammer
# Version: v1.0.0
# Date: 2018-12-16
# Description: Interactive script with default variables.
# Usage: ./interactive-arguments.sh <name> <location> <food>
#####################################

# Initialize the variables from passed arguments.
character_name=${1:-Sebastiaan}
location=${2:-Utrecht}
food=${3:-frikandellen}

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

Instead of just grabbing user input with $1, $2 and $3, we will now create a more complicated syntax, defined by man bash, as follows:

Again, you should read the word parameter as variable in this context (even though, when user-supplied, it is actually an argument to a parameter, but it could very well be a constant as well). With this syntax, if the variable is either not set or null (empty), the value supplied after the dash (called word in the man page) will be inserted instead.

We've done this for all three parameters, so let's check out how this works in practice:

reader@ubuntu:~/scripts/chapter_16$ bash default-interactive-arguments.sh 
Recently, Sebastiaan was seen in Utrecht eating frikandellen!
reader@ubuntu:~/scripts/chapter_16$ bash default-interactive-arguments.sh '' Amsterdam ''
Recently, Sebastiaan was seen in Amsterdam eating frikandellen!

If we do not supply any values to the script, all the defaults are inserted. If we supply three arguments, of which two are just empty strings (''), we can see that Bash will still substitute the defaults for us for the empty string. However, the actual string, Amsterdam, is correctly entered into the text, instead of Utrecht.

While dealing with empty strings in this manner is often desirable behavior, you can also write your scripts to allow for empty strings as a variable default. That looks like the following:

reader@ubuntu:~/scripts/chapter_16$ cat /tmp/default-interactive-arguments.sh 
<SNIPPED>
character_name=${1-Sebastiaan}
location=${2-Utrecht}
food=${3-frikandellen}
<SNIPPED>

reader@ubuntu:~/scripts/chapter_16$ bash /tmp/default-interactive-arguments.sh '' Amsterdam
Recently,  was seen in Amsterdam eating frikandellen!

Here, we created a temporary copy to illustrate this functionality. When you remove the colon from the default declaration (${1-word} instead of ${1:-word}), it no longer inserts the default for empty strings. It does, however, for values that are not set at all, as can be seen when we call it with '' Amsterdam instead of '' Amsterdam ''.

In our experience, in most cases, the default should ignore empty strings, so the syntax as presented in the man page is more desirable. If you have a niche case, though, you are now aware of this possibility!

For some of your scripts, you might find that just substituting a default is not enough: you'd rather have the variable set to a value that can then be evaluated with more granularity. This is also possible with parameter expansion, as follows:

We've never seen the need to use this function, especially since it is not compatible with positional parameters (and as such, we only mention it here and do not go into detail). But, as with all things, it is good to be aware of the possibilities parameter expansion provides in this area.