Special Parameters

Parameters: $?#

The exit code of the previous command or script, 0 for success, non-0 for failure.

ls 404

echo $?

Will output 2, indicating failure. The return code can be specified in scripts using exit.

Parameters: $##

Indicates the number of arguments passed to the script or function.

#!/usr/bin/env bash
echo $#

./foo p1 p2

2

Parameters: $*#

All arguments passed to the script, separated by spaces by default.

#!/usr/bin/env bash
echo $*

./foo p1 p2p3

p1 p2p3

The separator can be modified through the IFS variable.

Parameters: $@#

All arguments passed to the script, separated by spaces by default.

#!/usr/bin/env bash
echo $@

./foo p1 p2p3

p1 p2p3

The separator can be modified through the IFS variable.

Difference: $* vs $@#

The main difference lies in handling arguments with spaces.

• $*: Represents all arguments as a single string, a whole.
• $@:
  • Without double quotes: Same as $*.
  • With double quotes: "$@", represents an array of arguments, each argument is independent.

# Assuming the arguments passed are "arg1" "arg2 with space" "arg3"
for arg in "$@"; do
    echo "Argument: $arg"
done

# Passing all arguments to another script
another_script "$@"

Parameters: $$#

The process ID of the current script.

echo $$

Parameters: $!#

The process ID of the most recently backgrounded process.

echo $!

Parameters: $0#

The name of the current script.

echo $0

-bash

Parameters: $n#

The nth argument.

#!/usr/bin/env bash
echo $1

./foo a b c

a

Parameters: $-#

The options of the script.

echo $-

himBHs

Above are the startup options of the current Shell.