Bash Expansions

variable="value"
echo ${variable} # → value
echo $variable # → value

Curly braces are used to delimit the variable.

echo ${variable}_ # → value_
echo $variable_ # → (empty)
 
set -o nounset
echo $variable_ # exits with error "unbound variable"

Variable indirection

variable="HOME"
echo ${!variable} # /usr/alpha

Uppercase

variable="value"
 
echo ${variable} # → value
echo ${variable^} # → Value
echo ${variable^^} # → VALUE

Lowercase

variable="VALUE"
echo ${variable} # → VALUE
echo ${variable,} # → vALUE
echo ${variable,,} # → value

Pattern matching

variable="VALUE"
echo ${variable,,[VAL]} # → valUE
echo ${variable,,[LUE]} # → VAlue
 
variable="value"
echo ${variable^^[va]} # → VAlue

Arrays

variables=(one two three)
echo ${variables[@]} # → one two three
echo ${variables[@]^} # → One Two Three
echo ${variables[@]^^} # → ONE TWO THREE
echo ${variables[@]^^[oe]} # → OnE twO thrEE
echo ${variables[*]^^[oe]} # → OnE twO thrEE
 
echo ${variables[2]^^} # → THREE

Substring removal

From begginning of string

variable="value1 value2"
echo ${variable#va} # → lue1 value2
echo ${variable#val} # → ue1 value2
echo ${variable#*l} # → ue1 value2
echo ${variable##val} # → ue1 value2
echo ${variable##*l} # → ue2

From end of string

variable="value1 value2"
echo ${variable%lue2} # → value1 va
echo ${variable%al*} # → value1 v
echo ${variable%*l*} # → value1 va

String replacement

echo ${variable/value/example} # → example1 value2
echo ${variable//#value/example} # → example1 example2
echo ${variable/#value/example} # → example1 value2
echo ${variable/value*/example} # → example
echo ${variable/%value2/example} # → value1 example

Substring

echo ${variable:4} # → e1 value2
echo ${variable:2:3} # → lue
# The space before the - sign is required
echo ${variable: -3} # → ue2
echo ${variable:1: -1} # → alue1 value
echo ${variable: -3: -1} # → ue

Length

echo ${#variable} # → 13
echo ${#variables[@]} # → 3
echo ${#variables[2]} # → 5