> ## Documentation Index
> Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt
> Use this file to discover all available pages before exploring further.
# pid
> This article explains the use of special shell variables `$$` and `$!` for managing process IDs in Bash and POSIX-compatible shells.
In Bash and other POSIX-compatible shells, special variables like `$$` and `$!` help you manage process IDs (PIDs) for debugging, automation, and scripting tasks. While they might not pop up every day, knowing how to use them can streamline background jobs, service management, and process monitoring.
Before diving into these variables, let’s clarify how TTYs, shells, and PIDs relate:
1. When you open a terminal, it’s assigned a TTY (teletype) name.
2. A shell (e.g., Bash) runs on that TTY and acts as the parent process.
3. Any command or script executed in that shell becomes a child process.
4. Every process—from parent shells to background jobs—has a PID and a lifecycle (start → run → exit).
***
## Table of Special PID Variables
| Variable | Description | Typical Use Case |
| -------- | ---------------------------------------------------- | --------------------------------------- |
| `$!` | PID of the most recently launched **background** job | Tracking & controlling background tasks |
| `$$` | PID of the **current** shell or shell-script process | Self-awareness in scripts |
***
## Using `$!` to Capture Background Job PIDs
The `$!` variable returns the PID of the last job you sent to the background.
```bash theme={null}
$ sleep 5 &
[1] 93506
$ echo $!
93506
```
Even if you run foreground commands afterward, `$!` holds the PID until you start another background job:
```bash theme={null}
$ echo "First background PID is $!"
First background PID is 93506
$ echo "Hello" &
[1] 93761
Hello
[1] + done echo "Hello"
$ echo $!
93761
```
### Storing `$!` in a Bash Script
A common pattern is launching a service in the background, capturing its PID, and later terminating it. For example, starting an Apache JMeter server:
```bash theme={null}
#!/usr/bin/env bash
jmeter_pid=""
start_jmeter() {
echo "Starting JMeter server..."
jmeter-server &
jmeter_pid=$!
}
start_jmeter
# Other workflow steps...
echo "Now running additional setup tasks..."
Always ensure you `kill` the correct PID. Accidentally terminating the wrong process can disrupt critical services.
echo "Stopping JMeter server (PID: $jmeter_pid)"
kill -SIGTERM "$jmeter_pid"
```
***
## Using `$$` to Identify Your Shell or Script
The `$$` variable prints the PID of the current shell or script process:
```bash theme={null}
$ echo $$
79315
$ ps -p $$ -o pid,cmd
PID CMD
79315 -bash
```
Opening a new terminal tab or window yields a different `$$` value:
```bash theme={null}
$ echo $$
91933
```
### Inspecting `$$` Inside a Script
Create `print_pid.sh`:
```bash theme={null}
#!/usr/bin/env bash
echo "This script’s PID is $$"
sleep 60
```
Run it in the background:
```bash theme={null}
$ ./print_pid.sh &
[1] 94479
94479
$ ps --pid 94479,94481 -o pid,ppid,cmd
PID PPID CMD
94479 79315 /bin/bash ./print_pid.sh
94481 94479 sleep 60
```
* **PID 94479** corresponds to the script itself (`$$`).
* **PID 94481** is the child `sleep` process.
***
## `$$` and Subshell Behavior
Subshells inherit the parent shell’s PID, so `$$` stays constant:
```bash theme={null}
#!/usr/bin/env bash
echo "Parent shell PID: $$"
(
echo "Inside subshell PID: $$"
)
```
Output:
```bash theme={null}
$ ./subshell_pid.sh
Parent shell PID: 54344
Inside subshell PID: 54344
```
All references to `$$` show the same PID because subshells share the parent’s process ID.
***
## Key Takeaways
* **`$!`** returns the PID of the last command run **in the background**.
* **`$$`** returns the PID of **your current shell** or the running script.
Understanding these variables will help you write more robust scripts, automate process control, and debug complex workflows with confidence.
***
## Links and References
* [Bash Reference Manual](https://www.gnu.org/software/bash/manual/bash.html)
* [Linux Process Management](https://tldp.org/LDP/abs/html/processes.html)
* [Apache JMeter](https://jmeter.apache.org/)