Special Variables in Shell Scriptinh

𝗨𝗻𝗱𝗲𝗿𝘀𝘁𝗮𝗻𝗱𝗶𝗻𝗴 𝗦𝗽𝗲𝗰𝗶𝗮𝗹 𝗩𝗮𝗿𝗶𝗮𝗯𝗹𝗲𝘀 𝗶𝗻 𝗦𝗵𝗲𝗹𝗹 𝗦𝗰𝗿𝗶𝗽𝘁𝗶𝗻𝗴

While learning Shell Scripting, I came across something very useful: 𝗦𝗽𝗲𝗰𝗶𝗮𝗹 𝗩𝗮𝗿𝗶𝗮𝗯𝗹𝗲𝘀.

These variables are automatically provided by the shell and are used to access script arguments, process information, user details, and command execution results.

They play an important role in making Shell Scripts dynamic by handling inputs, processes, users, and execution results.

They help us:

• Read script arguments

• Identify who is running the script

• Track process information

• Monitor command execution status

• Access system and environment details

Let's look at some commonly used special variables.

1️⃣ $𝟭, $𝟮, ..., $𝗡 – 𝗣𝗼𝘀𝗶𝘁𝗶𝗼𝗻𝗮𝗹 𝗣𝗮𝗿𝗮𝗺𝗲𝘁𝗲𝗿𝘀

These represent arguments passed to a script. $1 is the first argument passed to the script, $2 is the second, and so on.. If you pass parameters while running the script, these will automatically capture them.

Example:

./script.sh DevOps Linux

Inside the script:

echo "$1"

echo "$2"

Output:

DevOps Linux

2️⃣ **$@ 𝗮𝗻𝗱 $* – 𝗔𝗹𝗹 𝗔𝗿𝗴𝘂𝗺𝗲𝗻𝘁𝘀**

Both represent all arguments passed to the script.

Difference:

$@ treats arguments individually
$* treats all arguments as a single string.

Example

./script.sh DevOps Linux Cloud

echo "$@"

echo "$*"

Output:

DevOps Linux Cloud

3️⃣$# – 𝗡𝘂𝗺𝗯𝗲𝗿 𝗼𝗳 𝗔𝗿𝗴𝘂𝗺𝗲𝗻𝘁𝘀

This special variable returns the total number of arguments passed to the script.

Example

./script.sh a b c

Output: 3

Useful for input validation and argument checking.

4️⃣$𝟬 – 𝗦𝗰𝗿𝗶𝗽𝘁 𝗡𝗮𝗺𝗲

Stores the name of the script being executed.

Useful for displaying usage information, logging, and debugging.

Example

# shell.sh (file name)

#!/bin/bash
echo "$0"

Execute: sh shell.sh
Output: shell.sh
Returns the name of the script being executed.

5️⃣ $𝗣𝗪𝗗 – 𝗣𝗿𝗲𝘀𝗲𝗻𝘁 𝗪𝗼𝗿𝗸𝗶𝗻𝗴 𝗗𝗶𝗿𝗲𝗰𝘁𝗼𝗿𝘆

Returns the current working directory path where the script is running.

Example: echo "$PWD"

Output: /home/ec2-user
Useful when working with files and directories.

6️⃣ $𝗛𝗢𝗠𝗘 – 𝗨𝘀𝗲𝗿'𝘀 𝗛𝗼𝗺𝗲 𝗗𝗶𝗿𝗲𝗰𝘁𝗼𝗿𝘆

Returns the home directory of the current user.

Example: echo "$HOME"

Output: /home/ec2-user

7️⃣$𝗨𝗦𝗘𝗥 – 𝗪𝗵𝗼 𝗶𝘀 𝗥𝘂𝗻𝗻𝗶𝗻𝗴 𝘁𝗵𝗲 𝗦𝗰𝗿𝗶𝗽𝘁? (𝗖𝘂𝗿𝗿𝗲𝗻𝘁 𝘂𝘀𝗲𝗿)

This variable returns the username of the person running the script.

Example: echo $USER

Output: /ec2-user

8️⃣ $$ – 𝗖𝘂𝗿𝗿𝗲𝗻𝘁 𝗦𝗰𝗿𝗶𝗽𝘁 𝗣𝗜𝗗

• Returns the Process ID (PID) of the running script.

• PID (Process ID) uniquely identifies a running process in Linux.

• PIDs are commonly used for process tracking, logging, creating unique temporary files, and managing running processes.

9️⃣$! – 𝗟𝗮𝘀𝘁 𝗕𝗮𝗰𝗸𝗴𝗿𝗼𝘂𝗻𝗱 𝗣𝗿𝗼𝗰𝗲𝘀𝘀 𝗣𝗜𝗗

Returns the PID of the most recently executed background process.

Example:

When you run a command in the background using &, this variable gives you its PID.

sleep 100 & → starts the process in the background.

$! → stores the PID of the last background process (12345).

It is commonly used to monitor, track, or terminate background jobs.

If you want to stop the background process, you can use its PID with the kill command.

sleep 100 &

PID=$!

kill $PID

🔟 $? – Exit Status of the Last Command

Stores the exit code of the last executed command.

• 0 = Success ✅

• Non-zero = Failure ❌

Widely used for error handling in Shell Scripts.

Example:

dnf install nginx -y

if [ $? -eq 0 ]; then

echo "Installation Successful"

else

echo "Installation Failed"

⸻

For reference, I have included screenshots of both the script and its output.

Script:

Output:

💡𝗞𝗲𝘆 𝗧𝗮𝗸𝗲𝗮𝘄𝗮𝘆

Special variables make Shell Scripts more dynamic by helping scripts handle inputs, identify users, track processes, and respond to command execution results.

Special Variables in Shell Scripting: Small Symbols, Powerful Impact

𝗨𝗻𝗱𝗲𝗿𝘀𝘁𝗮𝗻𝗱𝗶𝗻𝗴 𝗦𝗽𝗲𝗰𝗶𝗮𝗹 𝗩𝗮𝗿𝗶𝗮𝗯𝗹𝗲𝘀 𝗶𝗻 𝗦𝗵𝗲𝗹𝗹 𝗦𝗰𝗿𝗶𝗽𝘁𝗶𝗻𝗴