About

This article is about expansion in Bash.

An expansion is the replacement of a special token in your code by the result of the expansion during code execution.

It's performed on the command line after it has been split into words. See Bash - Command Execution

There are seven kinds of expansion performed:

• pathname expansion (globbing) - replace a glob expression by file name
• brace expansion {} - generate strings
• tilde expansion ~, - replace the tilde by a path (user home, print working directory, …)
• parameter expansion - replace the $ expression by a parameter value
• variable expansion - replace the $ expression by a variable value,
• arithmetic expansion, - perform arithmetic operation
• word splitting, splits a string into an array of words (token)
• command substitution,
• process substitution

Order

The order of expansions is:

• brace expansion,
• tilde expansion,
• parameter expansion,
• variable expansion
• arithmetic expansion
• and command substitution (done in a left-to-right fashion),
• word splitting,
• and pathname expansion.

On systems that can support it, there is an additional expansion available:

• process substitution.

Word change

Expansions can be use to change the number of word (token) processed. See Shell - (Word|Token)