Bash - (Argument|Positional Parameter)

About

An argument is a parameter given to a command or to the bash shell and referenced by position.

To parse positional parameters, you use Bash - getopts - Argument Parser (Builtin command)

A positional parameter is a parameter denoted:

  • by one or more digits,
  • other than the single digit 0.

Positional parameters are assigned from the shell’s arguments when it is invoked, and may be reassigned using the set builtin command. Positional parameters may not be assigned to with assignment statements. The positional parameters are temporarily replaced when a shell function is executed. When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces.

Special Parameters

There is some special parameter that gives extra function around positional parameters.

$0

The script name.

echo $0
/bin/bash

Star (*) - Single Word

* Expands to the positional parameters, starting from one. When the expansion occurs within double quotes, it expands to a single word with the value of each parameter separated by the first character of the IFS special variable.

That is, $* is equivalent to $1c$2c…, where c is the first character of the value of the IFS variable. If IFS is unset, the parameters are separated by spaces. If IFS is null, the parameters are joined without intervening separators.

$*: All of the positional parameters, seen as a single word

“$*” must be quoted.

$1, $2, etc.

Positional parameters, passed:

At (@)

@ expands to the positional parameters, starting from one.

  • When the expansion occurs within double quotes, each parameter expands to a separate word. That is, "[email protected]" is equivalent to "$1", "$2"
  • If the double-quoted expansion occurs within a word, the expansion of the first parameter is joined with the beginning part of the original word, and the expansion of the last parameter is joined with the last part of the original word.
  • When there are no positional parameters,"[email protected]" and [email protected] expand to nothing (i.e., they are removed).

Number of positional parameters (# Hashtag)

$#:

echo $#
0

Management

Shift method

Test if set

Because an argument is a parameter, we can use the parameter expansion to test if it's set.

if [ -z ${1+x} ]; then echo "1 is unset"; else echo "1 is set to '$1'"; fi
# or
if !([ -z ${1+x} ]); then echo "1 is set to '$1'"; else echo "1 is unset "; fi

Passing

  • When passing a string as an argument to a function, if the string is not quoted it will be parsed according to the IFS separator

Bash - How to pass arguments that have space in their values ?

Parsing

If the script expect the below arguments:

./script.sh --server serviceName --silent

It can parse them like that:

while [[ $# -gt 0 ]]
do
    case "$1" in
        --service)
            # Run in service mode
            shift # past argument
            SERVICE_FILE=$1
            shift
        ;;
        --silent)
            SILENT_MODE=1
            shift
        ;;
        *)
            # unknown option
            shift # past argument with no value
        ;;
    esac
done

with the help of:

while [[ $# -gt 0 ]]
do
   echo $1
   shift
done

Documentation / Reference


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