Also the article for: variable, positional parameter, special parameter
In Bash, a parameter is simply an entity that stores values and can be referenced. Depending on the type, the parameters can be set directly, only indirectly, or only automatically by the shell.
Bash knows 3 types of parameters:
A shell variable is a parameter denoted by a variable name:
A value can be assigned to a variable, using the variable's name and an equal-sign:
Once a variable is set, it exists and can only be unset by the
unset builtin command.
The nullstring is a valid value:
A positional parameter is denoted by a number other than
Positional parameters reflect the shell's arguments that are not given to the shell itself (in practise, the script arguments, also the function arguments). You can't directly assign to the positional parameters, however, the set builtin command can be used to indirectly set them.
The first to ninth positional parameter is referenced by
$9. All following positional parameters (tenth and above) must be referenced by the number given in curly braces, i.e.,
Unlike popular belief,
$0 is not a positional parameter.
See also the scripting article about handling positional parameters.
There are a bunch of special parameters, which are set by the shell. Direct assignment to them is not possible. These parameter names are formed of one character.
Please see Special parameters and shell variables.