Process Management
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In this Module you will learn about how to manage running linux processes including:
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Contents |
Process Management
Processes have a unique Process ID the PID. This number can be used to modify a process' priority or to stop it.
A process is any running executable. If process_2 has been spawned by process_1, it is called a child process. The spawningprocess_1 is called the parent process.
The process family tree
The pstree command gives a good illustration of parent and child process hierarchy.
Part of the pstree output
bash(1046) ---xinit(1085)-+-X(1086)
`-xfwm(1094)-+-xfce(1100)---xterm(1111)---bash(1113)-+-pstree(1180)
| |-soffice.bin(1139)---soffice.bin(1152)-+
-soffice.bin(1153)
| | |-
soffice.bin(1154)
| | |-
soffice.bin(1155)
| | |-
soffice.bin(1156)
| | `-
soffice.bin(1157)
| `-xclock(1138)
|-xfgnome(1109)
|-xfpager(1108)
|-xfsound(1107)
`-xscreensaver(1098)
In the above figure all the process' PIDs are shown; these are clearly incremental. The most common used options are -p to display PIDs and -h to highlight users processes only.
Finding a running process
A more direct way to determine which processes are running is to use ps. Most users have a set combination of options which work for most situations. Here are three such options:
ps -ux all processes run by the user ps T processes run under the current terminal by the user ps aux all processes on the system
It is recommended you read the ps manpage and choose your own best options!
Modifying a running process
Stopping processes The kill command can be used to send signals to processes. There are 63 signals available. The default signal terminates a process and is called SIGTERM with value 15.
kill
Syntax
kill SIGNAL process_PID Every process can choose whether or not to catch a signal except for the SIGKILL which is dealt with by the kernel. Most daemons redifine the SIGHUP to mean “re-read configuration file”.
One can also stop processes without knowing the process' PID using killall.
killall
Syntax killall SIGNAL process_NAME
Figure illustrating Interprocess signaling
Process priority and nice numbers
Nice numbers (NI) alter the CPU priority and are used to balance the CPU load in a multiuser environment. Each process is started with a default nice number of 0. Nice numbers range from 19 [lowest] to -20 [highest]. Only root can decrease the nice number of a process. Since all processes start with a default nice number of zero as a consequence negative nice numbers can only be set by root!
To modify a process' priority that is already running use renice. To set a process' priority use nice.
Syntax
Nice –<NI> <process>
renice <+/-NI> -p <PID>
Notice that renice works with PIDs and handles lists of processes at a time. A useful option to renice is the -u option which affects all processes run by a user.
Set nice number 1 for processes 234 and 765:
renice +1 -p 234 765
Set nice number -5 for xclock:
nice --5 xclock
Processes and the Shell
Background and forground processes
After you have started a process from the shell you automatically leave the shell interpreter. You will notice that no commands will respond. The reason for this is that it is possible to run programs in the foreground fg or in the background bg of a shell. When a program is running in the foreground it is possible to recover the shell prompt but only by interrupting the program for while. The interruption signal is Ctrl Z.
Stopping and starting jobs
A process started from a shell is also called a job. Once the job receives the ^Z signal it is stopped and the shell prompt is recovered. To restart the program in the background simple type: bg.
Example
[mike localhost /bin]$xclock xclock running in forground, shell prompt lost [1]+ Stopped xclock xclock received ^Z signal [mike localhost /bin]$bg shell prompt recovered, issue the bg command [1]+ xclock & xclock is running in the background [mike localhost /bin]$
Notice the [1]+ symbol above. The integer is the process' job number, which it can be referred to as. The '+' sign indicates the last modified process. A '-' sign would indicate the second last modified process.
Listing jobs
The jobs utility lists all running processes started from the current shell. The job number, the job's state (running/stopped), as well as the two last modified processes, will be listed.
The job number One can conveniently stop and start a selection of jobs using the job number. This is achieved with the fg command.
Calling job 2 to the foreground and killing job 1
Avoiding HUP with nohup
Finally there is a program called nohup which acts as a parent process independently from the user’s session. When a user logs off, the system sends a HUP to all processes owned by that process group. For example, to avoid this HUP signal a script called bigbang which attempts to calculate the age of the Universe should be started like this:
nohup bigbang &
| As a system administrator, you will required to work with continuously running processes commonly reffered to as daemons. It is important to know how to troubleshoot these processes and ensure that they are available to deliver the services required to your users. What you have learned today is critical to your success as a system administrator. |
You should run X before starting these exercises.
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