In a shell script, `x=$?` is used to capture the exit status of the last executed command into the variable `x`.

Let’s see a quick rundown of how it works:

• `$?` is a special variable in shell scripting that holds the exit status of the most recently executed command.

Exit status is a number that a command returns to indicate whether it succeeded or failed. Typically, an exit status of `0` means success, while any non-zero value indicates an error.

If you use `x=$?` immediately after a command, `x` will store the exit status of that command. For example:

#!/bin/sh

# Run a command
ls /some/directory

# Capture the exit status of the 'ls' command
x=$?

# Check if the 'ls' command was successful
if [ $x -eq 0 ]; then
echo "The command succeeded."
else
echo "The command failed with exit status $x."
fi

In this script:

1. `ls /some/directory` attempts to list the contents of a directory.
2. `x=$?` captures the exit status of the `ls` command.
3. The `if` statement checks if the exit status (`x`) is `0` (indicating success) and prints a corresponding message.

This practice is useful for error handling and controlling the flow of your script based on the success or failure of commands.

In shell scripting, `$@` is a special variable that represents all the positional parameters passed to a script or function. Each parameter is treated as a separate argument.

Let’s see a breakdown of how `$@` works:

Usage in Scripts

1. Script Arguments

When you pass arguments to a script, `$@` contains all those arguments, preserving their individual integrity. For example:

#!/bin/sh

echo "All arguments using \$@:"
for arg in "$@"; do
echo "$arg"
done

If you run this script with the command:

./script.sh arg1 "arg2 with spaces" arg3

The output will be:

All arguments using $@:
arg1
arg2 with spaces
arg3

Note how `”$@”` ensures that arguments with spaces are handled correctly.

2. Function Arguments

Similarly, `$@` can be used inside functions to access all arguments passed to the function:

my_function() {
echo "Function arguments:"
for arg in "$@"; do
echo "$arg"
done
}

my_function "arg1" "arg2 with spaces" "arg3"

Output:

Function arguments:
arg1
arg2 with spaces
arg3

Differences Between `$@` and `$*`

• `”$@”` When quoted, it expands to each argument as a separate quoted string, preserving spaces within arguments. This is particularly useful when you want to preserve the integrity of arguments, especially those containing spaces.
• `$*` When quoted, it expands to a single string with arguments separated by the first character of the `IFS` (Internal Field Separator) variable, which is usually a space. This can cause issues if any arguments contain spaces.

echo “$*”

This will output all arguments as a single string separated by spaces (or another separator depending on the `IFS` setting).

In short, `$@` is often preferred over `$*` because it preserves the integrity of each argument when quoted, which is crucial for handling arguments with spaces or special characters correctly.