103.5. Create, monitor and kill processes
Weight: 4
Description: Candidates should be able to perform basic process management.
Key Knowledge Areas:
Run jobs in the foreground and background
Signal a program to continue running after logout
Monitor active processes
Select and sort processes for display
Send signals to processes
Terms and Utilities:
&
bg
fg
jobs
kill
nohup
ps
top
free
uptime
pgrep
pkill
killall
screen
In this section we are talking about jobs and processes. What you should know is that, what exactly is a job?
If you run a command from the shell, that is job.
[email protected]:~# lsclient.conf files-backup openvpn.key unzip_6.0-20ubuntu1_amd64.debdead.letter jcal_0.4.1-2_amd64.deb test-space zip_3.0-11_amd64.deb
A job might be executed and exit immediately and runs for a short time only but some jobs run for a long time.
[email protected]:~# sleep 3600
that goes to sleep for 3600 seconds, that is an hour! So you can wait for an hour (considering that you can not use your terminal) or you can run a job in background.
A job is a process that the shell manages. Each job is assigned a sequential job ID. Because a job is a process, each job has an associated PID. There are two types of job statuses:
1. Foreground: When we enter a command in a terminal window, the command occupies that terminal window until it completes. This is a foreground job. 2. Background: When we enter an ampersand (&) symbol at the end of a command line, the command runs without occupying the terminal window. The shell prompt is displayed immediately after you press Return. This is an example of a background job.
[email protected]:~$ sleep 5 &[1] 68481[1]+ Done sleep 5
By default you get notified about terminated jobs when you hit enter ,Try set -b to get notified immediately.
Job Control Commands
Job control commands enable us to place jobs in the foreground or background, and to start or stop jobs. The table describes the job control commands.
Command | Description |
jobs | Lists all jobs |
bg %n | Places the current or specified job in the background, where n is the job ID |
fg %n | Brings the current or specified job into the foreground, where n is the job ID |
Control-Z | Stops the foreground job and places it in the background as a stopped job |
Control-C | Ctrl+C kills the process |
if no job id is mentioned bg and fg consider the most recent job
[email protected]:~# jobs[1]+ Running sleep 1000 &[email protected]:~# sleep 2000 &[2] 54832[email protected]:~# jobs[1]- Running sleep 1000 &[2]+ Running sleep 2000 &[email protected]:~# sleep 3000^Z[3]+ Stopped sleep 3000[email protected]:~# jobs[1] Running sleep 1000 &[2]- Running sleep 2000 &[3]+ Stopped sleep 3000
[email protected]:~# bg[3]+ sleep 3000 &[email protected]:~# jobs[1] Running sleep 1000 &[2]- Running sleep 2000 &[3]+ Running sleep 3000 &
we cloud use fg %3 command and it would have the same result.
[email protected]:~# jobs[1] Running sleep 1000 &[2]- Running sleep 2000 &[3]+ Running sleep 3000 &[email protected]:~# fg %2sleep 2000^C[email protected]:~# jobs[1]- Running sleep 1000 &[3]+ Running sleep 3000 &
We brought a job to foreground and use Ctrl + C to kill that job.
we can disable and enable shell job control feature with
set +m and set -mcommands.
jobs normally stick to the shell running it, so they are killed when we close the terminal or log out. Some times we need to make sure that the running job is not attached to the running shell.This is where nohup and disown come to play.
huponexit
jobs are killed only if the huponexit option is set!
runshopt huponexit to see if this is true.
If
huponexitis false, which is the default on at least some linuxes these days, then backgrounded jobs will not be killed on normal logout!If
huponexitis true, then we can usenohupordisownto dissociate the process from the shell so it does not get killed when you exit. Or, run things withscreen.
use shopt -s huponexit and shopt -u huponexit in order to set and unset it.
Nohup stands for no hangup, and that means even if the parent shell is diconnected the job just will continue, The output of the nohup command will write in nohup.out the file if any redirecting filename is not mentioned in nohup command.
nohup command [command-argument ...]
Using nohup with commands:
[email protected]:~/test-space# nohup ping 8.8.8.8 &[2] 55210[email protected]:~/test-space# nohup: ignoring input and appending output to 'nohup.out'[email protected]:~/test-space# lsnohup.out[email protected]:~/test-space# head -3 nohup.outPING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.64 bytes from 8.8.8.8: icmp_seq=1 ttl=128 time=97.2 ms64 bytes from 8.8.8.8: icmp_seq=2 ttl=128 time=74.4 ms
Now even if we close the terminal and open it again ping is still running.
[email protected]:~/test-space# jobs[email protected]:~/test-space# ps aux | grep ping | grep -v greproot 55210 0.0 0.0 6536 756 ? S 02:32 0:00 ping 8.8.8.8[email protected]:~/test-space#
Please notice that we can not use
fg,bgcommands on that particular job anymore.
When we run nohup command without ‘&’ then it returns to shell command prompt immediately after running that particular command in the background.
We useually use nohup with output redirection
nohup bash script.sh > myresult.txt 2>&1
disown
We can also use disown command, it is used after the a process has been launched and put in the background, it’s work is to remove a shell job from the shell’s active list jobs, therefore we can not use fg, bg commands on that particular job anymore but that process keeps running even after closing current shell.
[email protected]:~# jobs[email protected]:~# sleep 111 &[1] 6394[email protected]:~# jobs -l[1]+ Running sleep 111 &[email protected]:~# disown %1[email protected]:~# jobs[email protected]:~# pgrep sleep6394
disown -h %1 lets us to have normal job control mechanisms to continue controlling the process until closing the terminal.
Option | Description |
-a | Delete all jobs if jobID is not supplied. |
-r | Delete only running jobs. |
There is a problem with nohup and disown commands. There is no way to bring back that job to forground and work interactivly with that. So we need a different solution, screen.
screen command in Linux provides the ability to launch and use multiple shell sessions from a single ssh session.
When a process is started with ‘screen’, the process can be detached from session and then can reattach the session at a later time. When the session is detached, the process that was originally started from the screen is still running and managed by the screen itself. The process can then re-attach the session at a later time, and the terminals are still there, the way it was left. (you might need to install it)
simple use screen command :
[email protected]:~# screen
Screen version 4.03.01 (GNU) 28-Jun-15Copyright (c) 2010 Juergen Weigert, Sadrul Habib ChowdhuryCopyright (c) 2008, 2009 Juergen Weigert, Michael Schroeder, Micah Cowan,Sadrul Habib ChowdhuryCopyright (c) 1993-2002, 2003, 2005, 2006, 2007 Juergen Weigert, MichaelSchroederCopyright (c) 1987 Oliver LaumannThis program is free software; you can redistribute it and/or modify it underthe terms of the GNU General Public License as published by the Free SoftwareFoundation; either version 3, or (at your option) any later version.This program is distributed in the hope that it will be useful, but WITHOUTANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESSFOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.You should have received a copy of the GNU General Public License along withthis program (see the file COPYING); if not, see http://www.gnu.org/licenses/,or contact Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,[Press Space for next page; Return to end.]
use screen -s Session_Name to start a named session.now lets run a command inside screen:
[email protected]:~# sleep 1111
Inorder to create a new screen inside the current screen (nested screen) use just press Ctrl-a +c
One of the advantages of screen that is you can detach it. Then, you can restore it without losing anything you have done on the screen. use Ctrl-a + d to detach:
[email protected]:~# sleep 1111
[email protected]:~# screen[detached from 56798.pts-4.ubuntu16-1]
-d is also used to detach a screen session so that it can be reattached in future.
List screens
screen -ls is used to display the currently opened screens including those running in the background:
[email protected]:~# screen -listThere is a screen on:56798.pts-4.ubuntu16-1 (10/14/2019 04:15:00 AM) (Detached)1 Socket in /var/run/screen/S-root.
Reattach to a screen
-r It is used to reattach a screen session which was detached in past:
[email protected]:~# screen -r 56798
[email protected]:~# sleep 1111
We usually use
screen -dr <Screen-ID>command.This means detach the specified screen first and then reattach it.
When we do nested screen, you can switch between screen using command Ctrl-a +n. It will be move to the next screen. When need to go to the previous screen, just press Ctrl-a +p.
Use “Ctrl-A” and “K” to kill the screen.
A program is a series of instructions that tell the computer what to do. When we run a program, those instructions are copied into memory and space is allocated for variables and other stuff required to manage its execution. This running instance of a program is called a process and it's processes which we manage
Each process got a PID. PID stands for process identifier and it is is a unique number that identifies each of the running processes in an operating system
processes can further be categorized into:
Parent processes – these are processes that create other processes during run-time.
Child processes – these processes are created by other processes during run-time.
therefore PPID stands for Parent Process ID. notes:
note1:Used up pid’s can be used in again for a newer process since all the possible combinations are used.
note2:At any point of time, no two processes with the same pid exist in the system because it is the pid that Unix uses to track each process.
If we use the
jobscommand with the-loption, it will also show process ID.
ps (Process status) can be used to see/list all the running processes and their PIDs along with some other information depends on different options.
ps [options]
ps reads the process information from the virtual files in /proc file-system. In it's simplest form, when used without any option, ps will print four columns of information for minimum two processes running in the current shell, the shell itself, and the processes that run in the shell when the command was invoked.
[email protected]:~# psPID TTY TIME CMD57301 pts/18 00:00:00 bash59076 pts/18 00:00:00 ps
Where, PID – the unique process ID TTY – terminal type that the user is logged into TIME – amount of CPU in minutes and seconds that the process has been running CMD – name of the command that launched the process.
Note – Sometimes when we execute ps command, it shows TIME as 00:00:00. It is nothing but the total accumulated CPU utilization time for any process and 00:00:00 indicates no CPU time has been given by the kernel till now. In above example we found that, for bash no CPU time has been given. This is because bash is just a parent process for different processes which needs bash for their execution and bash itself is not utilizing any CPU time till now.
Usually when we use the command ps we add parameters like -a, -u and -x. While
a= show processes for all usersu= display the process’s user/ownerx= also show processes not attached to a terminal
[email protected]:~# ps -aux | head -10USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMANDroot 1 0.0 0.3 185244 3856 ? Ss Oct11 0:07 /sbin/init auto nopromptroot 2 0.0 0.0 0 0 ? S Oct11 0:00 [kthreadd]root 4 0.0 0.0 0 0 ? I< Oct11 0:00 [kworker/0:0H]root 6 0.0 0.0 0 0 ? I< Oct11 0:00 [mm_percpu_wq]root 7 0.0 0.0 0 0 ? S Oct11 0:12 [ksoftirqd/0]root 8 0.0 0.0 0 0 ? I Oct11 0:19 [rcu_sched]root 9 0.0 0.0 0 0 ? I Oct11 0:00 [rcu_bh]root 10 0.0 0.0 0 0 ? S Oct11 0:00 [migration/0]root 11 0.0 0.0 0 0 ? S Oct11 0:01 [watchdog/0]
where column are :
USER – Specifies the user who executed the program.
PID: Process ID, shows the process identification number.
CPU%: The processor % used by the process.
MEME%: The memory % used by the process.
VSZ: The virtual size in kbytes.
RSS: In contrast with the virtual size, this shows the real memory used by the process.
TTY: Identifies the terminal from which the process was executed.
STATE: Shows information on the process’ state just as it’s priority, by running “man ps” you can see codes meaning.
START: Show when the process has started.
TIME: Shows the processor’s time occupied by the program.
C0MMAND: Shows the command used to launch the process.
We can also use ps -ef instead of ps aux . There are no differences in the output because the meanings are the same. The difference between ps -ef and ps aux is due to historical divergences between POSIX and BSD systems. At the beginning, POSIX accepted the -ef while the BSD accepted only the aux form. Both list all processes of all users. In that aspect
-eandaxare completely equivalent.
Options for ps command | Description |
ps -T | View Processes associated with a terminal |
ps -x | View all processes owned by you |
ps -o column_name | view process according to user-defined format |
It is also possible to use --sort option to sort output based on different fields (+ for ascending & - for descending). ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%mem | head -10 . With-o or –format options, ps allows us to build user-defined output formats.
Process selection commands | Description |
ps -C command_name | Select the process by the command name. |
ps p process_id | View process by process ID. |
ps -u user_name/ID | Select by user name or ID |
ps -g group_name , ps -G group_id | Select by group name or ID |
ps -t pst/0 | Display Processes by TTY |
We already know about the grep command in Linux, which searches for a pattern, and then prints the matching text in the output. What if the requirement is to apply this kind of processing to fetch select information about processes currently running in the system?
the pgrep command searches for processes currently running on the system, based on a complete or partial process name, or other specified attributes.
pgrep [options] pattern
[email protected]:~# sleep 1000 &[1] 60647[email protected]:~# sleep 2000 &[2] 60648[email protected]:~# pgrep sleep6064760648
Always use ps -ef command to make sure about process_name. There is different between process_name and the running program(like bash). compare pgrep -a and pgrep -af.
pgrep options:
-d, --delimiter <string> specify output delimiter-l, --list-name list PID and process name-a, --list-full list PID and full command line-v, --inverse negates the matching-w, --lightweight list all TID-c, --count count of matching processes-f, --full use full process name to match-g, --pgroup <PGID,...> match listed process group IDs-G, --group <GID,...> match real group IDs-n, --newest select most recently started-o, --oldest select least recently started-P, --parent <PPID,...> match only child processes of the given parent-s, --session <SID,...> match session IDs-t, --terminal <tty,...> match by controlling terminal-u, --euid <ID,...> match by effective IDs-U, --uid <ID,...> match by real IDs-x, --exact match exactly with the command name-F, --pidfile <file> read PIDs from file-L, --logpidfile fail if PID file is not locked--ns <PID> match the processes that belong to the samenamespace as <pid>--nslist <ns,...> list which namespaces will be considered forthe --ns option.Available namespaces: ipc, mnt, net, pid, user, uts-h, --help display this help and exit-V, --version output version information and exit
Real Time process monitoring ?
Be creative and use combination of other commands like 'watch'. We can use 'watch' in conjunction with ps command to perform Real-time Process Monitoring :
watch -n 1 'ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%mem | head'
But there is another tool for that, top.
top command is used to show the Linux processes. It provides a dynamic real-time view of the running system. Usually, this command shows the summary information of the system and the list of processes or threads which are currently managed by the Linux Kernel.
[email protected]:~# top
Tasks: 237 total, 1 running, 174 sleeping, 0 stopped, 0 zombie%Cpu(s): 0.8 us, 0.4 sy, 0.0 ni, 98.7 id, 0.1 wa, 0.0 hi, 0.0 si, 0.0 stKiB Mem : 985080 total, 167700 free, 501856 used, 315524 buff/cacheKiB Swap: 1045500 total, 448216 free, 597284 used. 256968 avail MemPID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND59562 root 20 0 41952 3604 2920 R 12.5 0.4 0:00.02 top1 root 20 0 185244 3856 2244 S 0.0 0.4 0:07.76 systemd2 root 20 0 0 0 0 S 0.0 0.0 0:00.01 kthreadd4 root 0 -20 0 0 0 I 0.0 0.0 0:00.00 kworker/0:+6 root 0 -20 0 0 0 I 0.0 0.0 0:00.00 mm_percpu_+7 root 20 0 0 0 0 S 0.0 0.0 0:12.88 ksoftirqd/08 root 20 0 0 0 0 I 0.0 0.0 0:19.92 rcu_sched9 root 20 0 0 0 0 I 0.0 0.0 0:00.00 rcu_bh10 root rt 0 0 0 0 S 0.0 0.0 0:00.00 migration/0
Top output keep refreshing until you press ‘q‘.
Where,
PID: Shows task’s unique process id
USER: User name of owner of task.
PR: Stands for priority of the task.
NI: Represents a Nice Value of task. A Negative nice value implies higher priority, and positive Nice value means lower priority.
VIRT: Total virtual memory used by the task.
RES:It is the Resident size, the non-swapped physical memory a task has used.
SHR: Represents the amount of shared memory used by a task.
%CPU: Represents the CPU usage.
%MEM: Shows the Memory usage of task.
TIME+: CPU Time, the same as ‘TIME’, but reflecting more granularity through hundredths of a second.
COMMAND:Shows the command used to launch the process.
top command option | description |
top -n 10 | Exit Top Command After Specific repetition |
top -u user1 | Display Specific User Process |
Top -d seconds.tenths | It tells delay time between screen updates |
top -h | Shows top command syntax |
Running top command hot keys | Description |
pressing ‘d‘ | change screen refresh interval (default 3.0 sec) |
Pressing ‘z‘ | display running process in color |
Pressing ‘c‘ | display absolute path of running program |
pressing ‘k‘ | kill a process after finding PID of process(without exiting) |
Pressing 'M' | sort based on memory usage |
Shift+P | Sort by CPU Utilisation |
Press ‘h‘ | Getting top command help |
To manage processes in a linux machine we can send signals signals to the process.Many Signals are defined in the linux kernels. (try man 7 signal)
Signal Name | Signal Number | Description |
SIGHUP | 1 | Hang up detected on controlling terminal or death of controlling process |
SIGINT | 2 | Issued if the user sends an interrupt signal (Ctrl + C) |
SIGQUIT | 3 | Issued if the user sends a quit signal (Ctrl + D) |
SIGKILL | 9 | If a process gets this signal it must quit immediately and will not perform any clean-up operations |
SIGTERM | 15 | Software termination signal (sent by kill by default) |
SIGCOUNT | 18 | Continue the process stopped with STOP |
STOP | 19 | Stop process |
to send signals to processes there are some commands:
kill command in Linux (located in /bin/kill), is a built-in command which is used to terminate processes manually . kill command sends a signal to a process which terminates the process.
kill {-signal | -s signal} pid
please notice that:
A user can kill all his process.
A user can not kill another user’s process.
A user can not kill processes System is using.
A root user can kill System-level-process and the process of any user.
note: If the user doesn’t specify any signal which is to be sent along with kill command then default TERM signal is sent that terminates the process.
[email protected]:~# sleep 1000 &[1] 61080[email protected]:~# sleep 2000 &[2] 61081[email protected]:~# jobs -l[1]- 61080 Running sleep 1000 &[2]+ 61081 Running sleep 2000 &[email protected]:~# ps -ef | grep sleeproot 61080 57301 0 02:55 pts/18 00:00:00 sleep 1000root 61081 57301 0 02:55 pts/18 00:00:00 sleep 2000root 61085 57301 0 02:56 pts/18 00:00:00 grep --color=auto sleep[email protected]:~# kill 61080[email protected]:~# ps -ef | grep sleeproot 61081 57301 0 02:55 pts/18 00:00:00 sleep 2000root 61089 57301 0 02:56 pts/18 00:00:00 grep --color=auto sleep[1]- Terminated sleep 1000[email protected]:~# ps -ef | grep sleeproot 61081 1722 0 02:55 ? 00:00:00 sleep 2000root 61096 55644 0 02:58 pts/17 00:00:00 grep --color=auto sleep[email protected]:~# kill -9 61081[email protected]:~# ps -ef | grep sleeproot 61100 55644 0 02:58 pts/17 00:00:00 grep --color=auto sleep
use kill -l to see all signals you can send using kill.
There are two commands used to kill a process:
kill – Kill a process by ID
killall,pkill – Kill a process by name
killing a proccess by name could be realy dangerous, Before sending signal, verify which process is matching the criteria using “pgrep -l”.
killall is a tool for terminating running processes on your system based on name. In contrast, kill terminates processes based on Process ID number (PID). Like kill , killall can also send specific system signals to processes.
kill {-signal | -s signal} process_name
note1:the whole process_name should be defined ( ex : sleep not sle or slee).
note2: If no signal name is specified, SIGTERM is sent.
[email protected]:~# sleep 1000 &[1] 61836[email protected]:~# sleep 2000 &[2] 61837[email protected]:~# ps -ef | grep sleep | grep -v greproot 61836 55644 0 03:46 pts/17 00:00:00 sleep 1000root 61837 55644 0 03:46 pts/17 00:00:00 sleep 2000[email protected]:~# pkill sleep[1]- Terminated sleep 1000[2]+ Terminated sleep 2000[email protected]:~# ps -ef | grep sleep | grep -v grep
killall command example | Description |
killall -l | all signals the killall command can send |
killall -q process_name | prevent killall from complaining if specified process doesn't exist |
killall -u [user-name] | kill all processes owned by a user |
killall -o 5h | kill all processes that have now been running for more than 5 hour |
killall -y 4h | kill all precesses that less than 4 hours old |
killall -w [process-name] | causes |
The PKill command allows you to kill a program simply by specifying the name.
pkill [options] pattern
note: We don't have to define whole process_name. So it could be really dangerous!
example:
[email protected]:~# pgrep firefox62256[email protected]:~# pkill firefox
pkill command example | Description |
pkill -c [process_name] | return a count of the number of processes killed |
pkill -U [real_user_ID] | kill all the processes for a particular user |
pkill -G [real_group_ID] | kill all the programs in a particular group |
free command displays the total amount of free space available along with the amount of memory used and swap memory in the system, and also the buffers used by the kernel.
free [options]
As free displays the details of the memory related to the system , its syntax doesn’t need any arguments to be passed but it has some options!
[email protected]:~# freetotal used free shared buff/cache availableMem: 985080 432716 135464 16724 416900 339484Swap: 1045500 671864 373636
free command with no options produces the columnar output as shown above where column:
total : displays the total installed memory (MemTotal and SwapTotal i.e present in /proc/meminfo).
used : displays the used memory.
free : displays the unused memory.
shared : displays the memory used by tmpfs(Shmen i.epresent in /proc/meminfo and displays zero in case not available).
buffers : displays the memory used by kernel buffers.
cached : displays the memory used by the page cache and slabs(Cached and Slab available in /proc/meminfo).
buffers/cache : displays the sum of buffers and cache.
By default the display is in kilobytes, but you can override this using -b for bytes, -k for kilobytes, -m for megabytes, or -g for gigabytes.
[email protected]:~# free -htotal used free shared buff/cache availableMem: 961M 423M 128M 16M 410M 330MSwap: 1.0G 655M 365M
-t displays an additional line containing the total of the total, used and free columns:
[email protected]:~# free -t -htotal used free shared buff/cache availableMem: 961M 424M 127M 16M 410M 329MSwap: 1.0G 655M 365MTotal: 1.9G 1.1G 492M
Other free command options:
-h, --human show human-readable output--si use powers of 1000 not 1024-l, --lohi show detailed low and high memory statistics-t, --total show total for RAM + swap-s N, --seconds N repeat printing every N seconds-c N, --count N repeat printing N times, then exit-w, --wide wide output--help display this help and exit-V, --version output version information and exit
The uptime command shows you a one-line display that includes the current time, how long the system has been running, how many users are currently logged on, and the system load averages for the past 1, 5, and 15 minutes.
uptime [-options]
[email protected]:~# uptime03:37:00 up 3 days, 19:07, 1 user, load average: 0.00, 0.00, 0.00
Lets try uptime -h to see all of uptime availbale options:
Usage:uptime [options]Options:-p, --pretty show uptime in pretty format-h, --help display this help and exit-s, --since system up since-V, --version output version information and exitFor more details see uptime(1).
examples:
[email protected]:~# uptime -pup 3 days, 19 hours, 16 minutes[email protected]:~# uptime -s2019-10-17 08:29:08[email protected]:~# uptime -Vuptime from procps-ng 3.3.10
.
.
.
Processes deep dive ( Beyond the scope of LPIC1)
Types of Processes
Parent and Child process : The 2nd and 3rd column of the ps –f command shows process id and parent’s process id number. For each user process there’s a parent process in the system, with most of the commands having shell as their parent.
Zombie and Orphan process : After completing its execution a child process is terminated or killed and SIGCHLD updates the parent process about the termination and thus can continue the task assigned to it. But at times when the parent process is killed before the termination of the child process, the child processes becomes orphan processes, with the parent of all processes “init” process, becomes their new ppid. A process which is killed but still shows its entry in the process status or the process table is called a zombie process, they are dead and are not used.
Daemon process : They are system-related background processes that often run with the permissions of root and services requests from other processes, they most of the time run in the background and wait for processes it can work along with for ex print daemon. When ps –ef is executed, the process with ? in the tty field are daemon processes
States of a Process in Linux
Running – here it’s either running (it is the current process in the system) or it’s ready to run (it’s waiting to be assigned to one of the CPUs). use ps -r command.
Waiting – in this state, a process is waiting for an event to occur or for a system resource. Additionally, the kernel also differentiates between two types of waiting processes; interruptible waiting processes – can be interrupted by signals and uninterruptible waiting processes – are waiting directly on hardware conditions and cannot be interrupted by any event/signal.
Stopped – in this state, a process has been stopped, usually by receiving a signal. For instance, a process that is being debugged.
Zombie – here, a process is dead, it has been halted but it’s still has an entry in the process table.
Processes state codes in ps aux or ps -ef command:
Rrunning or runnable (on run queue)Duninterruptible sleep (usually IO)Sinterruptible sleep (waiting for an event to complete)Zdefunct/zombie, terminated but not reaped by its parentTstopped, either by a job control signal or because it is being traced
Some extra modifiers:
<high-priority (not nice to other users)Nlow-priority (nice to other users)Lhas pages locked into memory (for real-time and custom IO)sis a session leaderlis multi-threaded (using CLONE_THREAD, like NPTL pthreads do)+is in the foreground process group
.
https://www.thegeekdiary.com/understanding-the-job-control-commands-in-linux-bg-fg-and-ctrlz/
https://linuxhint.com/nohup_command_linux/
https://www.tecmint.com/run-linux-command-process-in-background-detach-process/
https://www.geeksforgeeks.org/screen-command-in-linux-with-examples/
https://www.tecmint.com/screen-command-examples-to-manage-linux-terminals/
https://linoxide.com/linux-command/15-examples-screen-command-linux-terminal/
https://ryanstutorials.net/linuxtutorial/processes.php
https://www.cyberciti.biz/faq/unix-linux-disown-command-examples-usage-syntax/
https://linuxize.com/post/ps-command-in-linux/
https://www.computerhope.com/jargon/p/pid.htm
https://www.tecmint.com/linux-process-management/
https://www.geeksforgeeks.org/processes-in-linuxunix/
https://www.geeksforgeeks.org/ps-command-in-linux-with-examples/
https://linuxhint.com/ps_command_linux-2/
https://www.quora.com/What-is-the-difference-between-ps-elf-and-ps-aux-in-Linux
https://www.geeksforgeeks.org/top-command-in-linux-with-examples/
https://www.tecmint.com/12-top-command-examples-in-linux/
https://www.tutorialspoint.com/unix/unix-signals-traps.htm
https://www.geeksforgeeks.org/kill-command-in-linux-with-examples/
https://www.linux.com/tutorials/how-kill-process-command-line/
https://www.linode.com/docs/tools-reference/tools/use-killall-and-kill-to-stop-processes-on-linux/
https://www.geeksforgeeks.org/free-command-linux-examples/
https://www.geeksforgeeks.org/linux-uptime-command-with-examples/
https://linuxhint.com/load_average_linux/
.
