Protesilaos Stavrou
Philosopher. Polymath.

Parameter Expansion in BASH

In this screen cast I showcase some of the features of BASH for manipulating variables, aka “parameters”. Parameter expansion is a very powerful feature. Mastering it should greatly improve your shell scripting skills (see the GNU Bash manual).

Started applying this knowledge to the scripts that are distributed with my dotfiles, in preparation of the next beta version of “Prot’s Dots For Debian” (which I introduced in the previous entry).

Below I include the code samples shown in this screen cast.


First demo script:

#!/bin/bash

name=protesilaos

# Replace parameter with its value
echo "$name"

# Get parameter length
echo "${#name}"

# Extract substring ${parameter:offset:length}
echo "${name:0:4}"

# Capitalise first character
echo "${name^}"

# All majuscules
echo "${name^^}"

Second example:

#!/bin/bash

name=PROTESILAOS

# First character is lower case
echo "${name,}"

# All miniscules
echo "${name,,}"

# Use default value if parameter is unset/null
unset name
echo "$name"
echo "${name:-Protesilaos}"
echo "${name:-$USER}"

Third sample:

#!/bin/bash

freedom='GNU Not UNIX ; GNU plus Linux'

# Replace first occurence of pattern with string
echo "${freedom/GNU/Gahnoo}"

# Replace all matches of pattern with string
echo "${freedom//GNU/Gahnoo}"

# Remove first match of pattern
echo "${freedom/* ; /}"

freedom='GNU Not UNIX ; GNU is Not Unix ; Gahnoo slash Linaks ; GNU/Linux'

echo "$freedom"

# Remove all occurences of pattern
echo "${freedom//* ; /}"