dollar

shell expansion operator for variables and commands

TLDR

Reference a variable

$ echo $[VARIABLE]

Command substitution

$ result=$(command)

Arithmetic expansion

$ echo $((1 + 2))

Parameter expansion with default

$ echo ${VAR:-[default]}

String length

$ echo ${#VAR}

SYNOPSIS

$name${name}$(command)$((expression))

$ is the shell's expansion operator for variables, commands, and arithmetic. It triggers substitution of values before command execution.Variable expansion $VAR or ${VAR} retrieves the variable's value. Braces allow modifiers and are required for array access and complex expansions.Command substitution $(command) executes the command and substitutes its output. This replaces the older backtick syntax.Arithmetic expansion $((expr)) evaluates mathematical expressions.

PARAMETERS

${var}

Value of variable.

${var:-default}

Use default if var is unset or empty.

${var:=default}

Assign default if var is unset or empty.

${var:+alternate}

Use alternate value if var is set.

${var:?error}

Exit with error message if var is unset.

${#var}

String length of var.

${var%pattern}

Remove shortest suffix match.

${var%%pattern}

Remove longest suffix match.

${var#pattern}

Remove shortest prefix match.

${var##pattern}

Remove longest prefix match.

${var/old/new}

Replace first occurrence of old with new.

${var//old/new}

Replace all occurrences of old with new.

${var:offset:length}

Substring extraction.

Exit status of last command.

PID of the current shell.

PID of last background job.

Script or shell name.

$1-$9

Positional parameters.

All positional parameters as separate words.

All positional parameters as a single word.

Number of positional parameters.

CAVEATS

Unquoted expansions undergo word splitting and glob expansion. Always quote "$var" unless you specifically want splitting.$* vs $@ behave differently in quotes: "$@" preserves argument boundaries, **"$*"** joins them.Command substitution strips trailing newlines from output.