In this article, we will look at different terms w.r.t process and understand them.
We all should know how to manage process(es) in linux.
This post is very useful for our day to day activities which revolves around the processes.
This article explains almost all terms related to a process in Linux.
Let us have a look at these terms point wise to have a better understanding.
1. What is a process ?
A process is responsible to execute a job or task(some lines of code to perform a specific function).
CPU performs the job of process execution. A process can have following states(can be seen in man page of ps command):
D Uninterruptible sleep (usually IO) R Running or runnable (on run queue) S Interruptible sleep (waiting 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.xx kernel) X dead (should never be seen) Z Defunct ("zombie") process, terminated but not reaped by its parent.
2. What is child process ?
A process forked from parent process is a child process.
Child process uses code and data of the parent process.
3. How to see all process running in machine?
[root@ngelinux ~]# ps -ef | wc -l 147 [root@ngelinux ~]# ps -A | wc -l 147
4. How to check if a particular process is running or not ?
### Lets see the process with name bash [root@ngelinux ~]# ps -A | grep bash 4509 pts/0 00:00:00 bash ### We can also use pgrep command. [root@ngelinux ~]# pgrep bash 4509 [root@ngelinux ~]#
5. How to get details of a particular process ?
One of the most useful command to see all process details.
[root@ngelinux ~]# ps -FC bash UID PID PPID C SZ RSS PSR STIME TTY TIME CMD root 4509 4505 0 1316 1744 0 21:19 pts/0 00:00:00 -bash [root@ngelinux ~]#
6. How to kill all processes of specific name using single command ?
### Usin pkill command [root@ngelinux ~]# pkill -9 bash Connection closing...Socket close. Connection closed by foreign host. ### Using killall command [root@ngelinux ~]# killall -9 bash Connection closing...Socket close. Connection closed by foreign host.
7. How to kill one process using pid ?
### Get current shell process id [root@ngelinux ~]# echo $$ 4736 ### Kill the process using PID [root@ngelinux ~]# kill -9 4736 Connection closing...Socket close. Connection closed by foreign host.
8. How to start process in background ?
### Get current tty [root@ngelinux ~]# tty /dev/pts/0 [root@ngelinux ~]# ### echo hello every 5 seconds on your tty in background [root@ngelinux ~]# while true; do echo "hello" > /dev/pts/0; sleep 5; done &  4851 [root@ngelinux ~]# hello [root@ngelinux ~]# hello
9. How to list all background processes ?
[root@ngelinux ~]# jobs -l + 4851 Running while true; do echo "hello" > /dev/pts/0; sleep 5; done & [root@ngelinux ~]#
 is the job number of the current shell.
We can specify the job number instead of process IDs to kill the process to to bring it in foreground.
10. How to kill background process ?
We can send various signals including termination/stop or restart to various jobs of the current shell using job numbers instead of process numbers.
### Job numbers are presented with % like below. ### Lets send SIGTERM to job number 1 of this shell [root@ngelinux ~]# kill -9 %1 [root@ngelinux ~]# jobs + Killed while true; do echo "hello" > /dev/pts/0; sleep 5; done [root@ngelinux ~]#
11. What is foreground process ?
By default any process started on the bash shell runs in the foreground.
12. How to move a process from background to foreground ?
### Lets start two new background jobs. [root@ngelinux ~]# yes > /dev/null &  5087 [root@ngelinux ~]# yes > /dev/null &  5088 ### Check out two background jobs initiated. [root@ngelinux ~]# jobs - Running yes > /dev/null & + Running yes > /dev/null & [root@ngelinux ~]# ### Bring second job to foreground [root@ngelinux ~]# fg %+2 yes > /dev/null ### Bring first job to foreground. [root@ngelinux ~]# fg %-1 yes > /dev/null
In the above scenario, we can simply refer the jobs as fg %1 and fg %2.
13. How to move foreground process to background process ?
To move the process to background, we need to stop it and then send it to background.
### Stop a running process using Ctrl+Z [root@ngelinux ~]# yes > /dev/null ^Z + Stopped yes > /dev/null ### Send the process in background now and it will be started automatically. [root@ngelinux ~]# bg %1 + yes > /dev/null & [root@ngelinux ~]# jobs + Running yes > /dev/null & [root@ngelinux ~]#
14. What happens after we close the current shell session ?
When we close the terminal/shell, the shell sends SIGHUP signal to all foreground process and background processes. Hence in ideal case, all running foreground and background processes get killed by SIGHUP signal until and unless started using nohup command in bakground or shell options are modified.
15. How to retain process(continuously running) after closed terminal ?
We can use nohup command and an run the process in background to keep it running in background.
### Lets run another process using nohup command. [root@ngelinux ~]# nohup yes > /dev/null &  5469 [root@ngelinux ~]# nohup: ignoring input and redirecting stderr to stdout ### Now we can see two commands, one with nohup. [root@ngelinux ~]# jobs - Running yes > /dev/null & + Running nohup yes > /dev/null & ### Logout from current shell [root@ngelinux ~]# logout Connection closing...Socket close. Connection closed by foreign host. ### Now prgep with yes command, only one process is running with same process id of nohup. [root@ngelinux ~]# pgrep yes 5469
nohup marks the process to ignore NOHUP signal and keeps it running in backgroud.
After shell logout, “yes” background process is adopted by init process.
And yes process becomes an orphan process.
16. What is orphan process?
If parent process is completed/dead, and child process are still running.
These child processes without parents are known as orphaned process.
These orphaned child processes are adopted by init process and hence their PPID is reassigned to 1.
To see PPID i.e. parent PID, we can use ps command.
[root@ngelinux ~]# ps -FC yes UID PID PPID C SZ RSS PSR STIME TTY TIME CMD root 5469 1 33 1017 488 0 22:04 ? 00:01:38 yes
17. How to list all available process signals in linux ?
[root@ngelinux ~]# kill -l 1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL 5) SIGTRAP 6) SIGABRT 7) SIGBUS 8) SIGFPE 9) SIGKILL 10) SIGUSR1 11) SIGSEGV 12) SIGUSR2 13) SIGPIPE 14) SIGALRM 15) SIGTERM 16) SIGSTKFLT 17) SIGCHLD 18) SIGCONT 19) SIGSTOP 20) SIGTSTP 21) SIGTTIN 22) SIGTTOU 23) SIGURG 24) SIGXCPU 25) SIGXFSZ 26) SIGVTALRM 27) SIGPROF 28) SIGWINCH 29) SIGIO 30) SIGPWR 31) SIGSYS 34) SIGRTMIN 35) SIGRTMIN+1 36) SIGRTMIN+2 37) SIGRTMIN+3 38) SIGRTMIN+4 39) SIGRTMIN+5 40) SIGRTMIN+6 41) SIGRTMIN+7 42) SIGRTMIN+8 43) SIGRTMIN+9 44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13 48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13 52) SIGRTMAX-12 53) SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9 56) SIGRTMAX-8 57) SIGRTMAX-7 58) SIGRTMAX-6 59) SIGRTMAX-5 60) SIGRTMAX-4 61) SIGRTMAX-3 62) SIGRTMAX-2 63) SIGRTMAX-1 64) SIGRTMAX [root@ngelinux ~]#
18. What are two different sleep states of process in Linux ?
1. Interruptible sleep
2. UnInterruptible sleep
What is Interruptible sleep and UnInterruptible sleep?
In Interruptible sleep, we can communicate process using signal like kill -9 or kill SIGTERM. That is why it is called as Interruptible sleep.
To find Interruptible sleep process:
[root@ngelinux ~]# ps -A -o pid,comm,stat | grep 'S' PID COMMAND STAT 1 init Ss 2 kthreadd S 3 migration/0 S 4 ksoftirqd/0 S 5 stopper/0 S 6 watchdog/0 S
In UnInterruptible sleep, we can not communicate with process using signal because kernel locks the process for security purpose. Hence It is called UnInterruptible sleep.
To find UnInterruptible sleep
[root@ngelinux ~]# ps -A -o pid,comm,stat | grep 'D' PID COMMAND STAT [root@ngelinux ~]#
My system does not have any uninterrupted sleep process. In case you system has too many frequent uninterrupted sleep processes, then you need to contact your admin support team for any bug assistance with kernel/or specific program.
19. How to kill UnInterruptible sleep?
We should not kill unInterruptible sleep process. Wait for the process to finish execution or then start it again.
20. How to find that what all processes are consuming more MEMORY ?
[root@ngelinux ~]# ps -A --sort -rsz -o pid,comm,pmem,rsz | head -5 PID COMMAND %MEM RSZ 2455 mysqld 2.9 30604 2734 Xorg 1.7 17944 2901 gdm-simple-gree 1.3 14260 2874 gnome-settings- 1.1 11552 [root@ngelinux ~]#
21. How to find which processes are consuming more CPU usage ?
[root@ngelinux ~]# ps -A --sort -pcpu -o pid,comm,time,pcpu | head -5 PID COMMAND TIME %CPU 5268 yes 00:10:22 48.4 5388 yes 00:06:24 36.6 5469 yes 00:04:28 33.1 5703 ps 00:00:00 1.0 [root@ngelinux ~]#
Using above ps command, we can get average cpu usage from starting of server.
I have tried to cover almost every term related to process, however there is a lot more and everything can’t be covered in a single post.
Do post your comments/suggestions below.