How to return pid of a last command in Linux / Unix

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I need to find and return the PID of the last command executed in Linux. How do I return the PID of command in Linux?

One can easily find the PID of the last executed command in shell script or bash. This page explains how to get the PID of a last executed app/program.

How to return pid of a last command in Linux

The syntax is as follows:

  1. Open the terminal application
  2. Run your command or app in the background. For example: firefox &
  3. To get the PID of the last executed command type: echo "$!"
  4. Store the pid of the last command in a variable named foo: foo=$!
  5. Print it, run: echo "$foo"

The PID of the last executed command is in the $! variable

The bash treats several variable specially. These vairable may only be referenced and assignment to them is not allowed. For example, the $! expands to the process ID (PID) of the command/program most recently placed into the background, whether executed as an asynchronous command or using the bg command/builtin.

How to get pid of just started process

Let us start sleep process, run:
sleep 20 &
echo $!

OR
firefox &
echo $!

Linux return pid of a last command command
The PID of the last executed command displayed using special shell variable

Shell script example

Let us create a shell script wrapper named my-app.sh:

#!/bin/bash
echo "Starting my Linux/Unix awesome-app..."
/path/to/your/awesome-app &
_pid=$! 
echo "$_pid" > /var/run/awesome-app.pid
echo "Pid $_pid stored in /var/run/awesome-app.pid"

Using jobs internal command

The jobs command show status of jobs such as:
sleep 20 &
To lists process IDs only pass the -p option to the jobs:
my-app &
jobs -p

Sample outputs:

19967

The jobs command works with ksh and other shells. Typically, I put something as follows in my shell script wrappers:

#!/bin/bash
_name="/path/to/my-java-app"
_pid="/path/to/my-java-app.pid"
((${_name}) & jobs -p >${_pid})

It is also possible to get get the command's PID even before it runs using the following syntax:
bash -c 'echo $$; exec /path/to/my-command'
However, I recommend to stick with $_ or jobs -p method to follow KISS (keep it simple stupid) principle.

Conclusion

You learned how to find and display the process ID of last executed application or programe in Linux and Unix-like system using variuous methods. For more information see bash man page by typing the following man command:
man bash
help jobs

Sample outputs:

jobs: jobs [-lnprs] [jobspec ...] or jobs -x command [args]
    Display status of jobs.
 
    Lists the active jobs.  JOBSPEC restricts output to that job.
    Without options, the status of all active jobs is displayed.
 
    Options:
      -l	lists process IDs in addition to the normal information
      -n	lists only processes that have changed status since the last
    		notification
      -p	lists process IDs only
      -r	restrict output to running jobs
      -s	restrict output to stopped jobs
 
    If -x is supplied, COMMAND is run after all job specifications that
    appear in ARGS have been replaced with the process ID of that job's
    process group leader.
 
    Exit Status:
    Returns success unless an invalid option is given or an error occurs.
    If -x is used, returns the exit status of COMMAND.

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin, DevOps engineer, and a trainer for the Linux operating system/Unix shell scripting. Get the latest tutorials on SysAdmin, Linux/Unix and open source topics via RSS/XML feed or weekly email newsletter.

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