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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.