Bash - (Argument|Positional Parameter)

About

An argument is a parameter given:

to a command
to a function
or to the bash shell

They are referenced by position.

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.

How to

How to loop or parse them?

To parse positional parameters, you can use the getopts command

but you can also loop over them:

with for:

for arg in "$@"; do
  echo "$arg"
done

With while

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

How to extract a command as first argument

# Assign the first argument to a string
COMMAND=${1:-}
if [ "$COMMAND" = "" ]; then
    echo "A command is mandatory"
    exit 1
fi
shift  # Remove the first argument from the argument list
# "$@" is then the rest of the arguments

case $COMMAND in
  "hello")
    hello_function "$@"
    ;;
  *)
    echo  "Command $COMMAND is unknown"
    exit 1
esac

How to normalize options ?

getopt is a helper that:

handle different options format such as --arg=option or --arg option
and normalize them

args=$(getopt -l "no-header:,help" -o "s:h" -- "$@")
# eval set to set the positional arguments back to $args
eval set -- "$args"

where:

l specifies a set of long option value separated by a colon. ie
- no-header: means that we expect a –no-header option with a value
- help means that we expect a –help option without a value
o specifies a set of short option value separated by a colon
- s: means that we expect a -s option with a value
- h means that we expect a -h option without a value

How to parse arguments from stdin?

You can generate arguments with Xargs

Example:

echo argument1 argument2 argument3 | xargs bash -c 'echo This is third arg : $2'

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:

from command line to script,
passed to a function,
or set to a variable

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, "$@" is equivalent to "$1", "$2" …
If the double-quoted expansion occurs within a word, the expansion of the first parameter is joined with the beginning part of the original word, and the expansion of the last parameter is joined with the last part of the original word.
When there are no positional parameters,"$@" and $@ expand to nothing (i.e., they are removed).

Number of positional parameters (# Hashtag)

$#:

Number of command-line arguments or positional parameters
expands to the number of positional parameters in decimal.

echo $#

Explicit Null

The explicit null parameters are:

""
# or
''

Management

Shift method

Bash - Shift

Test if set

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

with the z conditional operator true if the variable/argument is not 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

with the n conditional operator true if the variable/argument is set

if [ -n ${1+x} ]; then echo "1 is set to '$1'"; else echo "1 is unset"; fi
# or
if !([ -n ${1+x} ]); then echo "1 is no set"; else echo "1 is set to '$1'"; 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:

How to pass a generated file with Process substitution

If a script expect only a path such as openssl storeutl, you can fake it with Process_substitution

Example: If the cli script expect only a path, you can still give it generated content like that:

script <(echo "Content")

Documentation / Reference

Positional Parameters