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Bash Parameter Expansion

Parameter expansion is a way to modify variables in Bash.

Parameter expansions are used in braces (${VAR...}).

Table of Contents

Parameter Expansion (Slicing/Substitution)

Parameter expansion is a way to modify values in-place.
Generally speaking, it uses the syntax: 

${variable<operator><args>}

The operator sometimes requires arguments, sometimes it doesn't.

Replace strings and variables in place with parameter expansion.

Slicing

name="John"  
echo "${name}"  
echo "${name/J/j}"    #=> "john" (substitution)  
echo "${name:0:2}"    #=> "Jo" (slicing)  
echo "${name::2}"     #=> "Jo" (slicing)  
echo "${name::-1}"    #=> "Joh" (slicing)  
echo "${name:(-1)}"   #=> "n" (slicing from right)  
echo "${name:(-2):1}" #=> "h" (slicing from right)  
echo "${food:-Cake}"  #=> $food or "Cake"  
length=2
echo "${name:0:length}"  #=> "Jo"  
# Cutting out the suffix  
str="/path/to/foo.cpp"  
echo "${str%.cpp}"    # /path/to/foo  
echo "${str%.cpp}.o"  # /path/to/foo.o  
echo "${str%/*}"      # /path/to  
# Cutting out the prefix  
echo "${str##*.}"     # cpp (extension)  
echo "${str##*/}"     # foo.cpp (basepath)  
# Cutting out the path, leaving the filename  
echo "${str#*/}"      # path/to/foo.cpp  
echo "${str##*/}"     # foo.cpp  

echo "${str/foo/bar}" # /path/to/bar.cpp  

str="Hello world"  
echo "${str:6:5}"   # "world"  
echo "${str: -5:5}"  # "world"

Substitution

${foo%suffix}       # Remove suffix  
${foo#prefix}       # Remove prefix  

${foo%%suffix}      # Remove long suffix  
${foo/%suffix}      # Remove long suffix  

${foo##prefix}      # Remove long prefix  
${foo/#prefix}      # Remove long prefix  

${foo/from/to}      # Replace first match  
${foo//from/to}     # Replace all  

${foo/%from/to}     # Replace suffix  
${foo/#from/to}     # Replace prefix  

src="/path/to/foo.cpp"  
base=${src##*/}   #=> "foo.cpp" (basepath)  
dir=${src%$base}  #=> "/path/to/" (dirpath)

Substrings

${foo:0:3}      # Substring (position, length)  
${foo:(-3):3}   # Substring from the right

Getting the Length of a String/Variable

${#foo}        # Length of $foo

Parameter Transformation

man://bash /^\s*Parameter\s*transformation Parameter transformation is a subset of parameter expansion.
It's not generally compatible with POSIX shell.
Parameter transformation is a way to perform a transformation on a value before it is used.

Syntax:

${parameter@operator}
Each operator is a single letter.

Parameter Transformation Operators

  • U: Converts all lowercase letters in the value to uppercase.
  • u: Capitalizes only the first letter of the value.
  • L: Converts all uppercase letters in the value to lowercase.
  • Q: Quotes the value, making it safe to reuse as input.
  • E: Expands escape sequences in the value (like $'...' syntax).
  • P: Expands the value as a prompt string.
  • A: Generates an assignment statement to recreate the variable with its attributes.
  • K: Produces a quoted version of the value, displaying arrays as key-value pairs.
  • a: Returns the variable's attribute flags (like readonly, exported).

Parameter Transformation on Arrays

Just like using $@ or $*, you can use parameter transformation on multiple positional parameters or arguments at once by using ${@} or ${*}.

  • ${@} (like $@): Treats each positional parameter as a separate item and applies transformations to each individually.
    • When you use ${@}, Bash treats each positional parameter (e.g., command-line arguments, $1, $2, etc) as separate arguments.
    • They're processed one by one and the transformation is applied to each.
    • The output is a list with each item transformed according to the specified operator.
  • ${*} (like $*): Also applies the transformations to each item separately, but returns a single string.
    • With ${*}, it applies the transformations to each element, then combines all postitional arguments into a space-separated string.
    • The output is a single string with the transformation applied to each element.

Ex:

all_args_lowercase_arr=("${@@L}") # Output will be an array
all_args_lowercase_str="${*@L}"   # Output will be a single string

If you use ${array[@]} or ${array[*]}, Bash applies the transformation to each element of the array, one by one.

The result is either an array (${arr[@]}) with each array item transformed individually, or a single string (${arr[*]}) with each array item transformed individually.

If enabled, the final transformed output might go through word splitting (separating by spaces) and pathname expansion (turning wildcard characters like * into matching filenames), so the result could expand further into multiple words or paths.

Special Characters for Parameter Transformation

Bash supports the use of ^ and , for making values uppercase/lowercase.

  • ^: Uppercase
  • ,: Lowercase
  • ~: Toggle/invert case
    • Lowercase becomes uppercase, and uppercase becomes lowercase.
    • Like using ~ in vim.

These are called "case modifications" in the Bash man page. 

${@^} # Capitalize the first character of each value in the array
${@,} # Lowercase the first character of each value in the array
${@^^} # Capitalize all characters of each value in the array
${@,,} # Lowercase all characters of each value in the array
${@~}  # Toggle the case of the first char of each value
${@~~} # Toggle the case of the all chars of each value

${*^} # Capitalize the first character of each value in the array
${*,} # Lowercase the first character of each value in the array
${*^^} # Capitalize all characters of each value in the array
${*,,} # Lowercase all characters of each value in the array
${*~}  # Toggle the case of the first char of each value in the array
${*~~} # Toggle the case of the all chars of each value in the array

When using *, the return value will be a single string, but the transformation is being applied to each element individually.

If you want to save the output into an array, use @ instead of *: 

declare -a arr=('one' 'two' 'three')
all_upper=("${arr[@]^^}")
echo "${all_upper[@]@Q}"
# 'ONE' 'TWO' 'THREE'

Table of Examples using Special Characters

Syntax Description Example Output (for "hello" "world" "bash")
${@^} Capitalizes each parameter Hello World Bash
${*^} Capitalizes each parameter Hello World Bash
${@^^} Uppercases each parameter completely HELLO WORLD BASH
${*^^} Uppercases each parameter completely HELLO WORLD BASH
${@,} Lowercases the first character of each parameter hello world bash
${*,} Lowercases the first character of each parameter hello world bash
${@~} Toggle the first character of each parameter Hello World Bash
${@~~} Toggle/invert all characters of each parameter HELLO WORLD BASH
${*~} Toggle/invert the first character of each parameter Hello world bash
${*~~} Toggle/invert all characters of each parameter HELLO WORLD BASH
${@@Q} Quotes each parameter individually 'hello' 'world' 'bash'
${*@Q} Quotes each parameter individually 'hello' 'world' 'bash'

  • Typically, * will combine the parameters into one string, whereas @ will split the parameters into an array.

    • However, in the case of these parameter expansions, * and @ behave the same way.

  • Both ${@^^} and ${*^^} make everything uppercase, applying the modification to each element of the array.

  • ${@^} applies the uppercase transformation to each argument individually.
  • ${*^} does the same thing as ${@^}.

Parameter Transformation Examples

Parameter transformation on variables, arrays, and associative arrays:

# Example variable for demonstration  
var="hello world"  
array_var=("one" "two" "three")  
declare -A assoc_array_var=([key1]="value1" [key2]="value2")  

# U: Convert all lowercase letters to uppercase  
echo "${var@U}"        # Output: HELLO WORLD  

# u: Capitalize only the first letter  
echo "${var@u}"        # Output: Hello world  

# L: Convert all uppercase letters to lowercase  
var="HELLO WORLD"  
echo "${var@L}"        # Output: hello world  

# Q: Quote the value, safe for reuse as input  
echo "${var@Q}"        # Output: 'HELLO WORLD' (or "HELLO WORLD" depending on context)  

# E: Expand escape sequences (e.g., newline, tab)  
esc_var=$'hello\nworld'  
echo "${esc_var@E}"    # Output: hello  
                       #         world  

# P: Expand as a prompt string (useful for prompt formatting)  
PS1="[\u@\h \W]\$ "    # Set the prompt  
echo "${PS1@P}"        # Output: [user@host directory]$  

# A: Generate an assignment statement that recreates the variable  
echo "${var@A}"        # Output: declare -- var="HELLO WORLD"  

# K: Quoted version of the value, with arrays as key-value pairs  
echo "${array_var@K}"          # Output: 'one' 'two' 'three'  
echo "${assoc_array_var@K}"    # Output: [key1]="value1" [key2]="value2"  

# a: Display attributes of the variable (flags)  
declare -r readonly_var="test"  
echo "${readonly_var@a}"       # Output: r (indicates readonly)

Examples of using parameter transformation with positional parameters and arrays
using the ^ and @ operators:

# Positional parameters example  
set -- "hello" "world"  
echo "${@^}"   # Each positional parameter capitalized: "Hello World"  

# Array example  
array=("one" "two" "three")  
echo "${array[@]^}"   # Capitalize each array element: "One Two Three"  

# Word splitting and pathname expansion example  
files=("file1.txt" "*.sh")  
echo "${files[@]^}"   # Expands "*.sh" to match any shell scripts in the directory  

# Using ${@} with U  
echo "${@^}"       # Capitalizes each argument: "Hello World Bash"  

# Using ${*} with U  
echo "${*^}"       # Capitalizes each argument: "Hello World Bash"  

# Using ${@} with U for full uppercase  
echo "${@^^}"      # Outputs: "HELLO WORLD BASH"  

# Using ${*} with U for full uppercase  
echo "${*^^}"      # Outputs: "HELLO WORLD BASH"  

# Using ${@} with L  
echo "${@,}"       # Converts first character of each argument to lowercase  

# Using ${*} with L  
echo "${*,}"       # Converts first character of each argument to lowercase  

# Using ${@} with Q  
echo "${@@Q}"       # Quotes each argument individually, e.g., "'hello' 'world' 'bash'"  

# Using ${*} with Q  
echo "${*@Q}"       # Quotes each argument individually, e.g., "'hello' 'world' 'bash'"

Using Default Values

You can use default values for variables that are either null or unset.

The operators used to do this:

  • :- Only uses the default value for the current string.
  • := Uses the default value given for the current string and assigns the variable to that value.

POSIX-Compliant Parameter Expansions

POSIX shell does support some parameter expansions.

  • ${var:-Default Value}
  • ${var:=Default Value}
  • ${var:?Error message}
  • ${var:+Alternative value}

The colons in these parameter expansions are saying "When this variable is either unset or null."

The colon (:) affects how the shell interprets the parameter's state:

  • With :

    • checks if variable is unset OR null (i.e., empty string)

  • Without :

    • checks if variable is unset only

The table shows what is used when the variable is set and not null, set and null, or unset.

Expression var is set & not null var is set & null ("") var is unset
${var:-word} value of var word word
${var-word} value of var "" word
${var:=word} value of var sets var=word sets var=word
${var=word} value of var "" sets var=word
${var:?word} value of var prints word and exits prints word and exits
${var?word} value of var word prints word and exits
${var:+word} word "" ""
${var+word} word word ""

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