Parameter expansion on Linux

Parameter expansion on a Linux system is a feature in the bash shell that allows you to manipulate the values of variables, but you need to use a specific syntax. They are often used in shell scripts, but they can also be used on the command line for tasks such as defaulting values and manipulating strings.

Here is a very simple example that returns “Hello” along with the value of the $name variable:

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

The following example uses a default value (Guest) if $name is not set or if it’s null. Note the use of braces – not parentheses – in these commands.

$ echo "${name:-Guest}"  # Uses "name" if set, otherwise "Guest"
Guest

This command displays the variable $MYID and, if not set, defaults to “anonymous”.

$ echo ${MYID-anonymous}
anonymous

The following command displays a default value if a variable if it is not set.

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

The example below assigns “anonymous” to the “user” variable if it has no assigned value.

$ echo "${user:=anonymous}"
anonymous

The command below will print a “not set” message and exits if the $hostname variable is not set or if it’s null.

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

You can display the length of a variable’s value with a command like this:

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

In the example below, the number of characters in the variable $name will be displayed.

$ name="sandra"
$ echo ${#name)
6

The two commands below display substrings for the specified variable.

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

In the example below, a substring of the $text variable is displayed. The 6 and 5 represent the offset and length of the text to be displayed.

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

Note that the 7^th character of the string is a blank. Think of the string as starting with character 0.

$ string="0123456"
$ echo ${string:2:5}
23456

To remove specified text from the beginning or end of a variable’s value, use a command like one of these:

$ file="archive.tar.gz"
$ echo ${file%.gz}	# removes .gz from the displayed file name (shortest match)
archive.tar
$ echo ${file%%.*}	# removes everything after the first string 
archive

Remove pattern match from the beginning of a variable:

$ echo ${text#You}
can be whatever you want to be
$ echo ${text#Y}
ou can be whatever you want to be

In the command shown below, the first and then all instances of the specified text are replaced.

$ text="You can be whatever you want to be"
$ echo ${text/be/do}	# change first instance
You can do whatever you want to be
$ echo ${text//be/do}	# change al instances
You can do whatever you want to do

The example below uses indirect expansion. A variable is created that points to another variable.

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

Wrap-up

Parameter expansion requires a bit of practice, but it can be very useful once you’ve become familiar with its many applications.

Source:: Network World