Variable expansion is a powerful and important Bash feature. It allows for more advanced, convenient manipulation of text without invoking another process. Some expansion types will only work for newer versions of Bash. These limitations are noted where relevant.

:!: **Note:** Each type of expansion is described in the order that Bash interprets it.

===== Brace Expansion =====

  * Asterisks (*) **do not work**! They don't expand until Path Name Expansion, AKA globbing.
  * Braces must have a comma somewhere to count as an expansion. ''foo{bar,}'' -> ''"foobar foo"'' and ''foo{,bar}'' -> ''"foo foobar"''
  * Supports simple ranges. ''{a..m}'' -> ''a b c d e f g h i j k l m''
  * Nests. ''%%{{a..z},{A..Z}}%%'' -> ''a b c d e f g h i j k l m n o p q r s t u v w x y z A B C D E F G H I J K L M N O P Q R S T U V W X Y Z''

==== Bash 4+ ====

  * Numeric ranges can be prepended with zeros. ''{001..010}'' -> ''001 002 003 004 005 006 007 008 009 010''
  * Iteration step can be defined. ''{1..10..2}'' -> ''1 3 5 7 9''
    * Works on letters, too. ''{a..m..3}'' -> ''a d g j m''

===== Tilde Expansion =====

===== "PPAC" Operations =====

These operations all occur at the same time, in left-to-right order:

==== Parameter Expansion ====

==== Process Substitution ====

Compatibility depends on OS. It's used to avoid subshells, or use the filesystem as an API.

==== Arithmetic Expansion ====

==== Command Substitution ====

===== Word Splitting =====

===== Path Name Expansion =====

===== References =====

[[bhw>syntax/expansion/intro|Bash Hacker's Wiki – Expansion Intro]]