Shell Expansions (Bash Reference Manual)

3.5 Shell Expansions

Expansion is performed on the command line after it has been split into tokens. There are seven kinds of expansion performed:

brace expansion
tilde expansion
parameter and variable expansion
command substitution
arithmetic expansion
word splitting
filename expansion

• Brace Expansion		Expansion of expressions within braces.
• Tilde Expansion		Expansion of the ~ character.
• Shell Parameter Expansion		How Bash expands variables to their values.
• Command Substitution		Using the output of a command as an argument.
• Arithmetic Expansion		How to use arithmetic in shell expansions.
• Process Substitution		A way to write and read to and from a command.
• Word Splitting		How the results of expansion are split into separate arguments.
• Filename Expansion		A shorthand for specifying filenames matching patterns.
• Quote Removal		How and when quote characters are removed from words.

The order of expansions is: brace expansion; tilde expansion, parameter and variable expansion, arithmetic expansion, and command substitution (done in a left-to-right fashion); word splitting; and filename expansion.

On systems that can support it, there is an additional expansion available: process substitution. This is performed at the same time as tilde, parameter, variable, and arithmetic expansion and command substitution.

After these expansions are performed, quote characters present in the original word are removed unless they have been quoted themselves (quote removal).

Only brace expansion, word splitting, and filename expansion can increase the number of words of the expansion; other expansions expand a single word to a single word. The only exceptions to this are the expansions of "$@" and $* (see Special Parameters), and "${name[@]}" and ${name[*]} (see Arrays).

After all expansions, quote removal (see Quote Removal) is performed.