Parameter Expansion on Linux

Parameter expansion can seem a bit challenging at first, but once you become familiar with its syntax and applications, it can be incredibly useful for manipulating variable values in various ways, whether in shell scripts or directly on the command line.

Basic Usage

A simple instance of parameter expansion returns the greeting “Hello” combined with the value of a variable:

$ name="Bozo"$ echo "Hello, ${name}!"Hello, Bozo!

You can also set default values if a variable is not defined or is empty. For example:

$ echo "${name:-Guest}"  # If "name" isn't set, it will output "Guest"Guest

This command displays the value of the variable MYID, defaulting to “anonymous” if MYID isn’t set:

$ echo ${MYID-anonymous}anonymous

In different cases, using := will assign a value if it hasn’t been defined:

$ echo ${var:=NOT_SET}123456789$ echo ${var2:=NOT_SET}NOT_SET

Checking Values

You can print a message and exit if a variable is not set or is null using this syntax:

$ echo "${HOSTNAME:?HOSTNAME is not set}"fedora$ echo "${HOSTNAME2:?HOSTNAME2 is not set}"-bash: HOSTNAME2: HOSTNAME2 is not set

Measuring Length

To determine the length of a variable’s value, use:

$ var="bananas"$ echo ${#var}7

Extracting Substrings

You can also extract substrings using defined positions:

$ var="123456789"$ echo ${var:1:3}234$ echo ${var:0:3}123

In another example, extracting a substring:

$ text="I love Linux"$ echo ${text:7:5}Linux

Modifying Variables

To remove certain patterns from the variable:

$ file="archive.tar.gz"$ echo ${file%.gz}        # Removes the ".gz" extensionarchive.tar$ echo ${file%%.*}       # Removes everything after the first dotarchive

You can also manipulate strings to replace specific instances of text:

$text="You can be whatever you want to be"$ echo ${text/be/do}     # Changes the first instance onlyYou can do whatever you want to be$ echo ${text//be/do}    # Changes all instancesYou can do whatever you want to do

Indirect Expansion

Creating indirect expansions allows a variable to reference another variable:

name="user"user="Sandra"echo ${!name} Sandra

Conclusion

Parameter expansion may require some practice, but mastering it will greatly enhance your efficiency and versatility when working in a Linux environment.

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