Introduction

One core functionality of Bash is to manage parameters. A parameter is an entity that stores values and is referenced by a name, a number or a special symbol.

parameters referenced by a name are called variables (this also applies to arrays)
parameters referenced by a number are called positional parameters and reflect the arguments given to a shell
parameters referenced by a special symbol are auto-set parameters that have different special meanings and uses

Parameter expansion is the procedure to get the value from the referenced entity, like expanding a variable to print its value. On expansion time you can do very nasty things with the parameter or its value. These things are described here.

If you saw some parameter expansion syntax somewhere, and need to check what it can be, try the overview section below!

Arrays can be special cases for parameter expansion, every applicable description mentions arrays below. Please also see the article about arrays.

For a more technical view what a parameter is and which types exist, see the dictionary entry for "parameter".