🐚 Bash Special Variables – $?, $#, $@, $0, $1 Explained with Examples
🧲 Introduction to Bash Special Variables – Understand Script Arguments & Exit Status
Bash includes a powerful set of special variables that provide insight into script arguments, command success, process IDs, and more. These built-in variables begin with a dollar sign ($) and serve different roles when writing or debugging scripts.
Whether you’re automating tasks or handling input parameters, knowing how to use Bash’s special variables like $?, $#, $@, $0, and $1 is essential.
🎯 In this article, you’ll learn:
- What Bash special variables are and how to use them
- The purpose of
$0,$1,$@,$#, and$? - Real examples of how they behave in scripts
- How to debug scripts using these variables
📌 List of Common Bash Special Variables
| Variable | Description |
|---|---|
$0 | Name of the script itself |
$1 – $9 | Positional parameters (first to ninth argument) |
$@ | All arguments as separate words |
$* | All arguments as a single word |
$# | Number of arguments passed |
$? | Exit status of the last command |
$$ | Process ID (PID) of the current shell |
$! | PID of the last background command |
🧪 Example – Script Using Special Variables
📝 args-demo.sh
#!/bin/bash
echo "Script name: $0"
echo "1st argument: $1"
echo "2nd argument: $2"
echo "All arguments (\$@): $@"
echo "Number of arguments: $#"
ls /nonexistentdir
echo "Exit status of last command: $?"
▶️ Run the Script:
chmod +x args-demo.sh
./args-demo.sh one two
✅ Output:
Script name: ./args-demo.sh
1st argument: one
2nd argument: two
All arguments ($@): one two
Number of arguments: 2
ls: cannot access '/nonexistentdir': No such file or directory
Exit status of last command: 2
🔍 Detailed Explanation of Key Variables
🔸 $0 – Script Name
Returns the name of the script as it was called.
echo "Running $0"
# Output: Running ./myscript.sh
🔸 $1, $2, …, $9 – Positional Parameters
Access individual arguments passed to the script.
echo "First input: $1"
🔸 $@ – All Arguments (Preserves Quoting)
Treats each parameter as a separate word.
for arg in "$@"; do
echo "$arg"
done
🔸 $* – All Arguments (Single String)
Treats all arguments as a single string.
echo "$*"
🔸 $# – Argument Count
Returns the number of arguments passed.
echo "There are $# arguments"
🔸 $? – Exit Status of Last Command
Returns the exit code of the last command.
0means success- Any other number indicates failure
mkdir myfolder
echo "Status: $?" # Output: 0 (if success)
🔸 $$ – Current Shell PID
Useful for creating unique filenames in temp scripts.
echo "Current PID: $$"
🔸 $! – Last Background Process ID
When a command runs in the background (&), $! stores its process ID.
sleep 10 &
echo "Sleep PID: $!"
📌 Summary: Bash Special Variables
Bash special variables unlock dynamic scripting and make shell scripts more robust, interactive, and debuggable. Use them to handle arguments, track errors, and control background processes effectively.
🔍 Key Takeaways:
$0,$1–$9handle script names and arguments$@and$*differ in how they preserve input$#tells how many arguments were passed$?helps in tracking command success/failure
⚙️ Real-world Uses:
- Writing parameterized automation scripts
- Checking command execution results in cron jobs
- Capturing process IDs for cleanup or logs
❓ FAQ – Bash Special Variables
❓ What is the difference between $@ and $* in Bash?
✅ $@ treats each argument as a separate quoted string, while $* merges them into a single string. Example:
for arg in "$@"; do echo "$arg"; done # each argument separately
echo "$*" # all arguments as one
❓ How do I know how many arguments a script received?
✅ Use $#:
echo "Argument count: $#"
❓ What is $? used for in Bash?
✅ $? gives the exit status of the last executed command. 0 means success, non-zero means failure.
❓ What does $$ mean in Bash?
✅ It holds the process ID (PID) of the currently running shell.
❓ How do I use $! in a script?
✅ Use it to capture the PID of a command run in the background:
command &
echo "PID: $!"
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