dollar

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.

PARAMETER EXPANSION

${var}

Value of variable

${var:-default}

Use default if var is unset/empty

${var:=default}

Assign default if var is unset/empty

${var:+alternate}

Use alternate if var is set

${var:?error}

Exit with error if var is unset

${#var}

String length

${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

${var//old/new}

Replace all occurrences

${var:offset:length}

Substring extraction

SPECIAL VARIABLES

Exit status of last command

Current shell PID

PID of last background job

Script name

$1-$9

Positional parameters

All positional parameters (separate words)

All positional parameters (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.