${Parameter Expansion}

Basic reference. ${var} makes the variable boundary explicit. While $varname treats the entire word as the variable name, writing ${var}name ensures only var is treated as the variable name.

${var}

Use a default value if var is unset or empty.

${var:-default}

Assign a default value if var is unset or empty.

${var:=default}

Print an error message and exit if var is unset or empty.

${var:?error message}

Return an alternative value only if var is set.

${var:+alternative}

String length.

${#var}

Remove shortest / longest prefix match.

${var#pattern}
${var##pattern}

Remove shortest / longest suffix match.

${var%pattern}
${var%%pattern}

Replace first / all occurrences.

${var/old/new}
${var//old/new}

Extract substring (offset:length).

${var:offset:length}

Use ${var:-default} to fall back to a default value when a variable is unset or empty.

sample_default.sh

name=""
echo "${name:-Guest}"

Run the following command:

bash default.sh
Guest

Use ${var:=default} to assign a default value while also returning it. The leading : is a no-op command used solely to trigger the assignment.

sample_assign_default.sh

: ${config:=/etc/myapp.conf}
echo "$config"

The following example demonstrates this:

bash assign_default.sh
/etc/myapp.conf

Use ${#var} to get the length of a string.

sample_strlen.sh

file="document.txt"
echo "${#file}"

Run the following command:

bash strlen.sh
12

Use ${var%.*} to strip the file extension (shortest suffix match).

file="document.txt"
echo "${file%.*}"
document

Use ${var##*/} to strip the directory portion (longest prefix match). This is a fast alternative to basename.

sample_path_parse.sh

path="/usr/local/bin/bash"
echo "${path##*/}"
echo "${path%/*}"

Run the following command:

bash path_parse.sh
bash
/usr/local/bin

Use ${var/old/new} to replace the first match. Use // to replace all matches.

sample_replace.sh

text="Hello World World"
echo "${text/World/Bash}"
echo "${text//World/Bash}"

Run the following command:

bash replace.sh
Hello Bash World
Hello Bash Bash

Use ${var:offset:length} to extract a substring.

sample_substring.sh

str="Hello, Bash!"
echo "${str:7:4}"

Run the following command:

bash substring.sh
Bash

Use ${var^^} to convert to uppercase and ${var,,} to convert to lowercase (Bash 4 and later).

sample_case_convert.sh

lower="hello"
echo "${lower^^}"
upper="WORLD"
echo "${upper,,}"

Run the following command:

bash case_convert.sh
HELLO
world

Common Mistake 1: Confusing ${var:-default} and ${var:=default}

${var:-default} returns the default value but does not assign it to the variable. ${var:=default} both returns and assigns the default.

unset name
echo "${name:-guest}"
guest
echo "$name"
（still empty — :- does not assign）

Use := if you want the variable itself to be set to the default.

echo "${name:=guest}"
guest
echo "$name"
guest

Common Mistake 2: Mixing up # (prefix) and % (suffix) trimming direction

# trims from the beginning (prefix); % trims from the end (suffix). Using the wrong one leaves the string unchanged.

file="report.tar.gz"
echo "${file%.gz}"
report.tar
echo "${file#*.}"
tar.gz

% strips the shortest match from the end; %% strips the longest. # strips the shortest match from the beginning; ## strips the longest.

Parameter expansion is a shell built-in feature, so it does not spawn a subprocess. File path operations such as stripping extensions and extracting directory names can be performed without calling sed or awk, making loops significantly faster.

Difference between :- and -: The colon form targets variables that are unset or empty; the form without a colon targets only unset variables. In most cases you want empty strings to be replaced with the default too, so use :-.

For basic variable definition and reference, see Variable Definition and Reference.