Weight: 4
Candidates should be able to perform basic process management.
Objectives
- 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
- &
- bg
- fg
- jobs
- kill
- nohup
- ps
- top
- free
- uptime
- killall
- pgrep
- pkill
- watch
- screen
- tmux
Managing processes
foreground and background jobs
One of the great points of Linux even from its beginning days is the ability to run different programs and jobs at the same time. This is done by sending programs to the background.
Normally if you run a program on the terminal, it blocks your terminal while it's running but sending a command to the background will prevent this:
xeyes &
Even when a program is running normally in the foreground, you can do two things:
- break it using Ctrl+c
- suspend or pause it using Ctrl+z
A stopped job can be brought to the foreground using fg command (or the background using bg). You can also list all the jobs by the jobs command.
$ xeyes
^Z
[1]+ Stopped xeyes
$ jobs
[1]+ Stopped xeyes
$ bg
[1]+ xeyes &
$ jobs
[1]+ Running xeyes &
$ sleep 1000 &
[2] 7395
$ jobs
[1]- Running xeyes &
[2]+ Running sleep 1000 &
$ fg %2
sleep 1000
^Z
[2]+ Stopped sleep 1000
$ jobs
[1]- Running xeyes &
[2]+ Stopped sleep 1000
$ bg sle
[2]+ sleep 1000 &
$ jobs
[1]- Running xeyes &
[2]+ Running sleep 1000 &
`
jobs -lalso shows the process ID of jobs
nohup
The nohup command lets you run your commands even after you close the terminal or logout. By default it writes its output to nohup.out:
$ nohup ping 4.2.2.4
nohup: ignoring input and appending output to ‘nohup.out’
^C$ cat nohup.out
PING 4.2.2.4 (4.2.2.4) 56(84) bytes of data.
64 bytes from 4.2.2.4: icmp_seq=1 ttl=51 time=225 ms
64 bytes from 4.2.2.4: icmp_seq=3 ttl=51 time=223 ms
--- 4.2.2.4 ping statistics ---
4 packets transmitted, 2 received, 50% packet loss, time 3010ms
rtt min/avg/max/mdev = 223.584/224.767/225.950/1.183 ms
It is common to use
2>to redirect the nohup errors to another file and use a&to run it in the background:nohup script.sh > mynohup.out 2>&1 &
kill
Despite its frightening name, the kill command sends unix signals to processes. Pressing Ctrl+c and Ctrl+z is also sending signals. By default, the kill command sends the signal 15 (which is TERM and tells to process to terminate itself).
$ jobs
[3] Running xeyes &
[4] Running sleep 1000 &
[5]- Running sleep 2000 &
[6]+ Running sleep 3000 &
$ kill %4
$ jobs
[3] Running xeyes &
[4] Terminated sleep 1000
[5]- Running sleep 2000 &
[6]+ Running sleep 3000 &
$ jobs
[3] Running xeyes &
[5]- Running sleep 2000 &
[6]+ Running sleep 3000 &
It is also possible to use PIDs instead of job numbers and kill other signals. The general format is kill -SIGNAL_ID_OR_NAME process_id:
| signal number | signal name | meaning |
|---|---|---|
| 1 | HUP | Informing the process that its controlling terminal (like an ssh connection) is terminated |
| 15 | TERM | normal termination request |
| 9 | KILL | forcefully kills the proccess |
So you can do a kill -9 8733 to force process ID 8733 to close.
Remember the
nohupcommand ? :) It means " do not respond to the hup signal ".
killall
This command Will send the given signal (or by default 15) to all processes with the given name:
$ jobs
[3] Running xeyes &
[5]- Running sleep 2000 &
[6]+ Running sleep 3000 &
$ ps -ef | grep sleep
jadi 7864 7651 0 21:07 pts/1 00:00:00 sleep 2000
jadi 7865 7651 0 21:07 pts/1 00:00:00 sleep 3000
jadi 7977 7651 0 21:14 pts/1 00:00:00 grep sleep
$ killall sleep
[5]- Terminated sleep 2000
[6]+ Terminated sleep 3000
$ jobs
[3]+ Running xeyes &
$ ps -ef | grep sleep
jadi 7980 7651 0 21:14 pts/1 00:00:00 grep sleep
pkill
Will send the given signal (or 15) to all the processes with a specific pattern in their name:
$ jobs
[3] Running xeyes &
[5]- Running sleep 2000 &
[6]+ Running sleep 3000 &
$ ps -ef | grep sleep
jadi 7864 7651 0 21:07 pts/1 00:00:00 sleep 2000
jadi 7865 7651 0 21:07 pts/1 00:00:00 sleep 3000
jadi 7977 7651 0 21:14 pts/1 00:00:00 grep sleep
$ pkill sle
[5]- Terminated sleep 2000
[6]+ Terminated sleep 3000
$ jobs
[3]+ Running xeyes &
$ ps -ef | grep sleep
jadi 7980 7651 0 21:14 pts/1 00:00:00 grep sleep
Monitoring Processes
ps
The ps command shows running processes on your computer. Each process has a process ID shown as PID and a Parent Process ID shown as PPID.
$ sleep 1000 &
[1] 7678
$ sleep 1001 &
[2] 7679
$ xeyes &
[3] 7680
$ ps
PID TTY TIME CMD
7651 pts/1 00:00:00 bash
7678 pts/1 00:00:00 sleep
7679 pts/1 00:00:00 sleep
7680 pts/1 00:00:00 xeyes
7681 pts/1 00:00:00 ps
Two common switch combination is ps aux ( or -aux) and ps ef which shows ALL processes on a system:
$ ps -aux | wc -l
293
It is also possible to use the --sort switch to sort the output based on different fields (+ for ascending & - for descending).
$ ps -af --sort +comm,-sid
UID PID PPID C STIME TTY TIME CMD
root 5486 5478 0 19:59 pts/12 00:00:00 -su
root 4444 1169 0 19:56 tty4 00:00:00 -bash
jadi 6638 5412 0 20:10 pts/0 00:00:04 node /usr/local/bin/sslocal
jadi 7778 7651 0 20:58 pts/1 00:00:00 ps -af --sort +comm,-sid
jadi 7678 7651 0 20:48 pts/1 00:00:00 sleep 1000
jadi 7679 7651 0 20:48 pts/1 00:00:00 sleep 1001
jadi 7775 7651 0 20:58 pts/1 00:00:00 sleep 1000
jadi 7776 7651 0 20:58 pts/1 00:00:00 sleep 1000
jadi 7777 7651 0 20:58 pts/1 00:00:00 sleep 1000
root 5478 5477 0 19:59 pts/12 00:00:00 su -
root 5477 5008 0 19:59 pts/12 00:00:00 sudo su -
jadi 7680 7651 0 20:48 pts/1 00:00:01 xeyes
pgrep
You've seen that ps -ef shows processes from all users. We can grep on that and see who is running gedit and what is its process ID:
$ ps -ef | grep gedit
jadi 6213 4604 9 20:06 ? 00:04:43 gedit
jadi 7725 7651 0 20:55 pts/1 00:00:00 grep gedit
but there is also a more direct way to check the PID of all gedit processes:
$ pgrep gedit
6213
top
This is the most common tool to do simple monitoring of the system. It will update the status and will give you a good glance at the status:
$top
top - 21:00:44 up 1:16, 5 users, load average: 1.51, 1.65, 1.78
Tasks: 293 total, 1 running, 292 sleeping, 0 stopped, 0 zombie
%Cpu(s): 19.0 us, 5.0 sy, 0.0 ni, 70.9 id, 5.1 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem: 8060264 total, 5359812 used, 2700452 free, 169240 buffers
KiB Swap: 7811068 total, 0 used, 7811068 free. 2250692 cached Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
6570 jadi 20 0 1437752 546064 88312 S 18.4 6.8 12:00.96 firefox
4870 jadi 20 0 1762516 299120 75664 S 12.2 3.7 7:37.05 compiz
4492 jadi 9 -11 455152 11516 8940 S 6.1 0.1 1:06.81 pulseaudio
4532 root 20 0 389028 77116 60192 S 6.1 1.0 12:16.63 Xorg
4723 jadi 20 0 358936 8288 5512 S 6.1 0.1 9:51.52 ibus-daemon
5648 jadi 20 0 1641556 203676 102840 S 6.1 2.5 3:20.88 chrome
7082 jadi 20 0 1210748 73136 42528 S 6.1 0.9 0:36.51 Telegram
7806 jadi 20 0 33796 3004 2500 R 6.1 0.0 0:00.02 top
1 root 20 0 29528 4320 2584 S 0.0 0.1 0:01.71 init
You can see the processes, system load, uptime, CPU status, memory, ... and do some stuff:
| key during top | functionality |
|---|---|
| h | help |
| q | quit |
| M | sort based on memory usage |
| c | show full commands |
| k | kill after asking pid and signal |
free
The free command will show you info about the system memory. The default is kilobytes but you can change it with -m for megabytes, -g for gigabytes or even -b for bytes. You can also use the -h for human readable.
$ free -m
total used free shared buffers cached
Mem: 7871 5231 2640 332 169 2195
-/+ buffers/cache: 2866 5005
Swap: 7627 0 7627
A general hint: If your system is using Swap, you have memory issues.
uptime
The uptime command shows the time, systems uptime (how long the system has been running), how many users are logged in, and the load average of 1, 5 & 15 minutes:
$ uptime
21:18:52 up 1:34, 5 users, load average: 2.38, 2.64, 2.41
Although it's one of the most important KPIs of the system status, some of the experienced Linux admins do not know what the load average means. The load average shows how many processes are in the to be run queue. If this number is higher than the number of your CPU cores, you are in a bad situation. If it's close to the number of your cores constantly, it's kind of dangerous, and if it's less than 1/10th of your core numbers, your system is kind of idle. Do you remember how to check the number of your cores? Its in
/proc/cpuinfoornproc.
watch
Sometimes you have a command which shows you an output but you want to keep running it and observing the output. In these cases, the watch is your friend. It lets you run and check the output of a command in specific time intervals (default is 2 seconds).
$ watch free -h
If you have a pipe in your command, you have to quote the watched command in double-quote " or single-quote ':
$ watch "ls -ltrh | wc -l"
These are some of the switches:
-nTo specify the interval in seconds-bBeep if the command has a non-zero exit-dShows the difference between runs
Terminal Multiplexers
screen
If you are used to GUI based system, it's easy to run different terminals side to side and use them to run different programs. But if you are on a server, you need other tools to multiplex your terminal. One such command is screen.
Run it with screen and press enter to exit the welcome window into a prompt. You can use it as a normal terminal and detach from it (and let it run in the background) using the Ctrl + A and then D keys. Check the list of your screens with screen -ls and re-attach to any of them with screen -r screen-id.
Below you can see a few common switches, they all should be issued after the Ctrl + A combination.
| Key | Usage |
|---|---|
| \ | Kill all processes windows and terminate the screen |
| | | Split current window into two vertical focuses |
| Shift+S | Split current window into two horizontal focuses |
| C | Create a window in the current focus |
| Tab | Go to the next focus |
| D | Detach from window |
| K | Kill current window |
| N | Move to Next window |
| P | Move to the Previous window |
A great point about screen (and tmux) is the fact it remains running even after you logout of the system and its possible to relogin and re-attach to the same screen (or tmux)
tmux
Is a screen on steroids! It is not installed by default in most distributions and you have to install it first. The default command prefix is Ctrl+B and after running the tmux new you can issue these:
| Key | Usage |
|---|---|
| % | Split current window vertically |
| " | Split current window horizontally |
| D | Detach from the current window |
| & | Kill current window |
You can list the tmux sessions using tmux ls and re-attach to one using tmux att to connect to the last one or tmux att -t session_name to attach to a specific one.
I highly recommend being fluent in tmux. It's super useful even when you are working locally on your machine. watch the below video for the more in-depth session:
| ← 103.4 Use streams, pipes, and redirects | 103.6 Modify process execution priorities → |