What are the meanings of special variables $0, $1, $2, $@, $*, $#, $? in Shell scripts?

The meanings of special variables $0, $1, $2, $@, $*, $#, $? in Shell scripts are as follows:

Positional Parameter Variables

$0

• Meaning: The filename of the current script
• Usage: Get the script's own name or path
• Example:

bash
#!/bin/bash
echo "Script name: $0"
# Output: Script name: ./script.sh

$1, $2, $3, ..., ${10}, ${11}, ...

• Meaning: Arguments passed to the script
• $1: First argument
• $2: Second argument
• ${10}: Tenth argument (requires curly braces)
• Example:

bash
#!/bin/bash
echo "First argument: $1"
echo "Second argument: $2"
echo "Tenth argument: ${10}"
# Execute: ./script.sh arg1 arg2
# Output: First argument: arg1
#        Second argument: arg2

Special Parameter Variables

$#

• Meaning: Number of arguments passed to the script
• Usage: Check argument count or iterate through all arguments
• Example:

bash
#!/bin/bash
echo "Number of arguments: $#"
# Execute: ./script.sh arg1 arg2 arg3
# Output: Number of arguments: 3

$@

• Meaning: All positional parameters as separate strings
• Feature: Each parameter remains independent, won't merge when quoted
• Example:

bash
#!/bin/bash
for arg in "$@"; do
    echo "Argument: $arg"
done
# Execute: ./script.sh "hello world" "foo bar"
# Output: Argument: hello world
#        Argument: foo bar

$*

• Meaning: All positional parameters as a single string
• Feature: All parameters merged into one string, separated by the first character of IFS
• Example:

bash
#!/bin/bash
echo "All arguments: $*"
# Execute: ./script.sh arg1 arg2 arg3
# Output: All arguments: arg1 arg2 arg3

$?

• Meaning: Exit status code of the last command
• Usage: Check if a command executed successfully
• Return value: 0 indicates success, non-zero indicates failure
• Example:

bash
#!/bin/bash
ls /nonexistent
echo "Exit status: $?"
# Output: ls: /nonexistent: No such file or directory
#        Exit status: 1

$$

• Meaning: PID (Process ID) of the current Shell process
• Usage: Create unique temporary filenames or process management
• Example:

bash
#!/bin/bash
echo "Current PID: $$"
# Output: Current PID: 12345

$!

• Meaning: PID of the most recent background process
• Usage: Track background processes
• Example:

bash
#!/bin/bash
sleep 10 &
echo "Background PID: $!"

Other Special Variables

$-

• Meaning: Current Shell option flags
• Example:

bash
echo $-
# Output: himBH (indicates enabled options)

$_

• Meaning: Last argument of the previous command
• Example:

bash
ls /etc/passwd
echo $_
# Output: /etc/passwd

Practical Application Example

bash
#!/bin/bash
# Check argument count
if [ $# -lt 2 ]; then
    echo "Usage: $0 <arg1> <arg2>"
    exit 1
fi

echo "Script: $0"
echo "Total arguments: $#"
echo "All arguments (\$@):"
for arg in "$@"; do
    echo "  - $arg"
done

echo "All arguments (\$*): $*"
echo "First arg: $1"
echo "Second arg: $2"

# Execute command and check status
ls "$1"
if [ $? -eq 0 ]; then
    echo "Command succeeded"
else
    echo "Command failed"
fi

Summary of Differences Between $@ and $*

Feature	$@	$*
Quoted wrapping	Each parameter independent	All parameters merged
Space handling	Preserved	May be merged
Recommended use	Yes	No

Best Practice: Always use "$@" when iterating through arguments to correctly handle arguments containing spaces.