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Linux Process Management - The Easy Way

#devops #linux #processmanagement #sysadmin

A Linux process is an instance of a program in execution. When you open an application or run a command on your Linux system, a process is created to carry out that specific task. Each process operates independently with its own set of resources, including memory, CPU time, and open files.

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  • Each process has a process id also called as PID
  • Every process has a parent process called as PPID

Child process is often started by the parent process

init process (runit,systemd,openrc,s6,dinit) have PID as 1, i.e it is the first process that boot your Linux system. If you don't know about what init in your system it's systemd

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When you want a process to die you can kill it.

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Process that starts at system startup and keeps on running forever are called daemon. The daemon never dies

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When a process is killed but it is still showing up in the system then that process is known as zombie. You can't kill zombie coz they are already dead Zombie processes never occupy the resources like CPU or RAM, only an entry remains in the process but these process are already killed.

Syntax

ps [options]
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options we will check in a while. But what if we just type

ps
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We will can see two processes one is current shell and another one is the ps command itself that we just entered.

  • PID is the process ID (every process has unique id)
  • TTY is the terminal type of user logged into.
  • TIME is amount of CPU in min and sec that process has been running.
  • CMD is name of the command that launch the process

To check the process id of your shell

prints the PID of your shell

echo $$
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prints PID with the process name

ps -C bash
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Print the process id of the parent 

echo $PPID
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Not only print the process id of current shell, but the process id of it's parent as well.

echo $$ $PPID
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first id is current and second id is parent id

Check the level of the shell 

echo $SHLVL
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3 is the level of the shell

The parent will always have less number as PID, coz parent came first. (parent will start the child process, so always the PID of parent is less in number.)

pidof command

Find the process ID of a running program

pidof bash
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Remember we discussed about init process that PID is 1 as it is the first process executes when we power on the machine, in my case i am running runit as my init , in your case it will be systemd

pidof runit
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When a process starts another process in two phases First the process create a fork of itself then a identical copy, then the fork process executes and exec to replace fork process with the target child process.

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IK, lol we will see what that even mean with that following example

In bash shell

echo $$
echo $$ $PPID
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Switch to zsh or any other shell

echo $$
echo $$ $PPID
exec bash
echo $$ $PPID
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To see all the processes in system 

ps fx
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  • PID : Process ID
  • TTY : Terminal from where the process has started
  • STAT : State and Signals (High/Low priorities,Stop/Idle conditions)
  • TIME : Time
  • COMMAND : The command for which that process has started

To check a particular process 

ps fx | grep bash
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With some options 

ps -ef
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We will see output in different format

Get the process id

Directly grep the process

pgrep bash
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We can even see the processes in tree format.

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To check the process in tree format

Process there child along with how many processes in tree format

pstree
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Now, let see with p option

pstree -p
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option -p will show the PID with tree format

To check the process with respect to user

For which user,which process is running

pstree -p -u username
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  • -p option means the PID
  • -u option means the user

Check the details of particular process

sleep process will run on background

sleep 200 &
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-p option means PID, -s option means process and the number is the PID of sleep which is running on background

pstree -p -s 8379
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ps -C sleep
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To kill the process

kill 8379
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Now, if we check the state of that process

ps fx | grep 8379
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  • D : uninterruptible sleep (usually IO)
  • R : running or runnable (on run queue)
  • S : interruption sleep (warning for an event to complete
  • T : stopped, either by a job control signal or because it is being traced
  • W : paging (not valid since the 2.6.X.X kernal)
  • X : dead (should never be seen)
  • Z : defunct (zombie) process, terminated but not repead by it parent
  • I : idle state

Here are the different values that the s, stat and state output specifies (header "STAT" or "S") will display to describe the state of process

For BSD formats & when the state keyword is used,additional characters may display

  • < high priority (not nice to other user)
  • N low priority (nice to other users)
  • L has pages locked into memory (for real-time & custom IO)
  • s is a session leader
  • l is multi-threaded (using CLONE_THREAD, like NPTL thread)
  • is in the foreground process group

Kill process

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sleep 80 &
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jobs
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ps -C sleep
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kill 9439
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jobs
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Signals in kill command

There are many signals in kill, to list all

kill -l
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By default kill means kill -15

Let, us see the most used signals one by one

1 SIGHUP : The process should re-read it's configuration file.

kill -1 1
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This command will re-read init (runit in my case) conf. file

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15 SIGTERM : When we run kill command that means kill -15 (standard kill)

sleep 100 &
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kill -15 10386
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or kill 10386

ps -C sleep
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When we kill some process normally, few process did't got killed. So, we kill them from kernel itself.

9 SIGKILL : To kill the process from kernel (sure kill)
The kernel will shoot down the process and as a developer you have no means to intercept a kill -9 signal

sleep 120 &
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kill -9 10572
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ps -C sleep
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In TTY we can see Killed , Terminate and Killed are not same

Now, it's not so much in use but in case you have to see the system calls

It, may not installed by default in some distros

strace 

sleep 120 &
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strace -p 7430
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kill -9 7430
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strace -p 7430
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18 SIGCONT : To start any process
19 SIGSTOP : To stop the process (we can resume it later)

sleep 280 &
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process stopped

kill -19 10744
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process started

kill -18 10744
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ps -C sleep
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Kill a process it's name (pkill) 

sleep 160 &
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sleep 280 &
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pkill sleep
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jobs
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ps -C sleep
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No sleep processes running all sleep process are killed

Kill Multiple processes (killall) 

sleep 120 &
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sleep 200 &
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In new terminal

top
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killall sleep top
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That's all you need to manage Linux processes :)

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