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)
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)