${Parameter Expansion}

Basic reference.

${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}

Expansion	Description
${var:-default}	Returns default if var is unset or empty (var is not modified).
${var:=default}	Assigns default to var and returns it if var is unset or empty.
${var:?msg}	Prints an error message and exits the script if var is unset or empty.
${var:+alt}	Returns alt if var is set. Returns an empty string if var is unset.
${#var}	Returns the length of the variable's string value.
${var#pat}	Removes the shortest prefix of var that matches pat.
${var##pat}	Removes the longest prefix of var that matches pat.
${var%pat}	Removes the shortest suffix of var that matches pat.
${var%%pat}	Removes the longest suffix of var that matches pat.
${var/old/new}	Replaces the first occurrence of old with new.
${var//old/new}	Replaces all occurrences of old with new.
${var:offset:len}	Extracts len characters starting at position offset.
${var^^} / ${var,,}	Converts all characters to uppercase / lowercase (Bash 4 and later).

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

default.sh

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

bash default.sh
Guest

Use ${var:=default} to assign a default value while also returning it.

assign_default.sh

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

bash assign_default.sh
/etc/myapp.conf

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

strlen.sh

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

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.

path_parse.sh

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

bash path_parse.sh
bash
/usr/local/bin

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

replace.sh

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

bash replace.sh
Hello Bash World
Hello Bash Bash

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

substring.sh

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

bash substring.sh
Bash

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

case_convert.sh

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

bash case_convert.sh
HELLO
world

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.