Linux in a Nutshell, 6th Edition

Variable Substitution

No spaces should be used in the following expressions. The colon (:) is optional; if it’s included, var must be nonnull, as well as set.

Variable expression	Description
var= value ...	Set each variable var to a value.
${ var}	Use value of var; braces are optional if var is separated from the following text. They are required for array variables.
${ var:- value}	Use var if set; otherwise, use value.
${ var:= value}	Use var if set; otherwise, use value and assign value to var.
${ var:? value}	Use var if set; otherwise, print value and exit (if not interactive). If value isn’t supplied, print the phrase “parameter null or not set.”
${ var:+ value}	Use value if var is set; otherwise, use nothing.
${# var}	Use the length of var.
${#*}, ${#@}	Use the number of positional parameters.
${ var# pattern}	Use value of var after removing pattern from the left. Remove the shortest matching piece.
${ var## pattern}	Same as # pattern, but remove the longest matching piece.
${ var% pattern}	Use value of var after removing pattern from the right. Remove the shortest matching piece.
${ var%% pattern}	Same as % pattern, but remove the longest matching piece.
${! prefix*}, ${! prefix@}	List of variables whose names begin with prefix.
${ var: pos}, ${ var: pos: len}	Starting at position pos (0-based) in variable var, extract len characters, or rest of string if no len. pos and len may be arithmetic expressions.
${ var/ pat/ repl}	Use value of var, with first match of pat replaced with repl.
${ var/ pat}	Use value ...