Special parameters

A number of special parameters, all starting with $, get expanded by Bash.

• $1, $2, $3, … are positional special characters,
• $@ is an array-like construct referring of all positional parameters,
• $# expands to the number of arguments,
• $$ pid of the current shell,
• $! expands to the PID of the most recent background command,
• $0 expands to the name of the current shell or script.

Example:

function arguments {
    echo First argument: $1
    echo Second argument: $2
    echo Third argument: $3
    echo Number of arguments: $#
    echo All arguments: $@
}

arguments one two three four five

First argument: one
Second argument: two
Third argument: three
Number of arguments: 5
All arguments: one two three four five

Additionally, !! is replaced by the previous command.