03.5. Create, monitor and kill processes
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.
- &
- bg
- fg
- jobs
- kill
- nohup
- ps
- top
- free
- uptime
- killall
foreground and background jobs
One of the great points of linux on its beginning days, was the ability to run many programs at the same time. This is done with sending programs to the background.
Normally if you run a program on the terminal, it blocks your terminal but sending a command to the background will prevent this:
xeyes &
But what if we started it normally? We can break / cancel it with Ctrl+c or suspend it using Ctrl+z.
$ 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 &
`
-l switch of
jobswill also show the process ID
nohup
The nohup command lets you run your commands even after you logged out and 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 a file:
nohup script.sh > mynohup.out 2>&1 &
kill
You can control processes by signals. Actually pressing Ctrl+c and Ctrl+z is also sending signals. Another way for this is using the kill command:
$ 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 &
If is also possible to use PIDs in from of the kill or send other signals:
| signal number | signal name | meaning |
|---|---|---|
| 1 | SIGHUP | Informing the process that its controlling terminal (like an ssh connection) is terminated |
| 15 | SIGTERM | normal termination request |
| 9 | SIGKILL | forcefully kills the proccess |
So you can do a kill -9 8733 to force process ID 8733 to close.
Now you can understand what
nohupmeans: go not answer to the SIGHUP.
killall
Will send the given signal (or 15) to all the 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
Monitoring Processes
ps
The ps command shows running processes on your computer.
$ 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
But using ps aux (= -aux) or ps -ef is also common & shows ALL processes on this system:
$ ps -aux | wc -l
293
Every process has a ProcessID (PID) and a PPID (Parent Process ID).
finding processes
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:
$ ps -C gedit -o user,pid,tty,time,comm
USER PID TT TIME COMMAND
jadi 6213 ? 00:04:49 gedit
It is also possible to use the --sort switch to sort 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
top
Processes are changing and sometimes you need to check them live. top command will help you:
$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:
$ free -m
total used free shared buffers cached
Mem: 7871 5231 2640 332 169 2195
-/+ buffers/cache: 2866 5005
Swap: 7627 0 7627
The system should not use swap in long term
uptime
The uptime command shows the time, how long the system is up, how may 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, 2.412 Responses to
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