1. RH030 Linux Computing Essentials
Workbook Managing Processes
RH030 Linux Computing Essentials

2. Linux+ Guide to Linux Certification, 2e
Objectives
Today is all about understanding processes on a Linux system
Viewing processes
local processes
system processes
Terminating Processes
Kill Commands
Using kill signals
Modifying Processes
Running background processes
Modify the priority of a process
Schedule commands: at
crondtab
Linux+ Guide to Linux Certification, 2e

3. Linux+ Guide to Linux Certification, 2e
Chapter 1
A process is an instance of a running executable, identified by a process id (pid).
It maintains a set of imformation relevant to the processes
Such as virtual memory addresses, every process possesses its own distinct memory context.
A process has a uid and a collection of gid as credentials.
A process has a filesystem context, including a cwd, a umask, a root directory, and a collection of open files.
A process has a scheduling context, including a niceness value.
A process has a collection of environment variables.
The ps command can be used to examine all currently running processes.
The top command can be used to monitor all running processes.
Linux+ Guide to Linux Certification, 2e

4. Linux+ Guide to Linux Certification, 2e
Linux Processes
Linux manages tasks using processes
Processes are programs that are executing on the system
A program is really just a structured set of commands stored in an executable file which runs in memory and on CPU
A Process can spawn a subprocess, thus creating a process hierarchy with parent / child relationships
Some simple commands, such as cd, are executed by the shell itself and so do not create a separate process
Each program creates a process which is assigned a unique process identification number (PID)
Linux+ Guide to Linux Certification, 2e

5. Kernel Allocation for Processes
Processes are run by the CPU
Linux+ Guide to Linux Certification, 2e

6. All processes are generated.
User process:
Task is begun by a local user
Task is run as a process owned by the user from the local terminal
This process is now associated with that specific user.
User processes are run from the same terminal as the user who executed them.
“ tty? “ or “ pty? “- from ps output identifies it as a user process
System process:
System processes have been started by the kernel
Or they are a service which has been turned on.
They are not associated with a terminal
Services are run by a Daemon process
Which commonly run a network service
“ ? “ - from the ps output identifies it as a system process
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7. Linux+ Guide to Linux Certification, 2e
Viewing Processes
ps command:
Most commonly used command to view processes.
No arguments:
List local processes only
Brief listing only
PID, TTY, CPU time, command that started process,
– f (full) option:
Full detailed listing
User identifier (UID), PID, PPID, TTY, start time, CPU utilization, command
– u option:
See the processes that another user is using.
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8. Overview of the ps Command
$ ps [-options]
Linux+ Guide to Linux Certification, 2e

9. ps –f gives full output details
See examples page 436
A full listing shows both PID and PPID
Linux+ Guide to Linux Certification, 2e

10. Viewing All Processes Running on the System
ps – ef command: -e
Lists all processes running on the system
Brief listing only
PID, TTY, CPU time, command that started process,
–f (full) option:
Full detailed listing
User identifier (UID), PID, PPID, start time, TTY, CPU utilization, command
Or you can use
ps – aux command:
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11. SystemV verse BSD options page 441
Table 10-1: Common options to the ps command
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12. Linux+ Guide to Linux Certification, 2e
ps –ef Command Output
Linux+ Guide to Linux Certification, 2e

13. Linux+ Guide to Linux Certification, 2e
pgrep
maxwell]$ pgrep -l sshd
829 sshd
maxwell]$ pgrep -lu maxwell bash
2146 mozilla-bin
2155 mozilla-bin
2156 mozilla-bin
2157 mozilla-bin
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14. Filtering ps command Output
See example page 438
Output from a ps –ef listing can be quite long
By piping the output of the ps command through grep you can search for the specific process you want to terminate and determine the correct PID
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15. Linux+ Guide to Linux Certification, 2e
Chapter 2
Key Concepts
In Linux, the first process, /sbin/init, is started by the kernel on bootup. All other processes are the result of a parent process duplicating itself, or forking.
A process begins executing a new command through a process called execing.
Often, new commands are run by a process (often a shell) first forking, and then execing. This mechanism is referred to as the fork and exec mechanism.
Processes can always be found in one of five well defined states: runnable, voluntarily sleeping, involuntarily sleeping, stopped, or zombie.
Process ancestry can be viewed with the pstree command.
When a process dies, it is the responsibility of the process's parent to collect it's return code and resource usage information.
When a parent dies before it's children, the orphaned children are inherited by the first process (usually /sbin/init).
Linux+ Guide to Linux Certification, 2e

16. How Linux Manages Processes page 435
During the boot, the first 2 main system processes are started
sched (scheduler) (pid 0)
init (initialization) (pid 1) which manages all other processes
Figure 10-2: Process genealogy
Linux+ Guide to Linux Certification, 2e

17. One process can start another process
All Processes require a unique ID within the system.
PID = Unique identifier assigned to a process
Child process: (PID):
Process started by another process
Parent Process ID (PPID):
Process that has started another process
Figure 10-1: Parent and child processes
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18. There are different types of processes
Daemon
System processes that exist for a specific system purpose
httpd daemon - exists for the sole purpose of handling internet services, running inactive in the background until needed
Parent
A process which has spawned another process
Following boot-up, a process called init is invoked
Every process, except init, has a parent process
Child
process spawned by another process
Except for init all other processes are a child processes of a parent process
Linux+ Guide to Linux Certification, 2e

19. Types of (troubled) Processes
Orphan
Created when a child process is running and the parent process is killed.
The system recognises the orphaned child process.
And passes the orphan process to init which then becomes the it’s new parent process and terminates it.
Zombie (or Defunct)
Created when a child process loses it’s connection to an existing parent process.
It keeps running but becomes “lost” in the system
And so cannot be terminated by it’s parent process.
It just keeps running forever.
Keeps using system resource by using a slot in the process table;
It cannot be stopped in a conventional manner
The only way to kill a zombie is to reboot the system
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20. Linux+ Guide to Linux Certification, 2e
Process states
ps -aux Current processor state of process
Most processes are either sleeping (S) or running (R)
Zombie process state is Z
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21. Linux+ Guide to Linux Certification, 2e
/proc
The ps command gets it’s info from /proc
/proc is a system directory created at installation
It is always created under the / directory
It is similar to the windows device manager
It holds system information.
The process information for each processes is stored in the /proc system directory using it’s PID.
The ps commands is used to view this information
Linux+ Guide to Linux Certification, 2e

22. Viewing “Top” Processes
top command:
Displays interactive screen listing processes
Organized by processor time
Processes using most processor time listed first
Identify Rogue process: Faulty process
Consumes excessive system resources
top command can be used to change a processes priority or terminate a processes.
Linux+ Guide to Linux Certification, 2e

23. Chapter 3 - Process Scheduling: nice and renice
Key Concepts
A primary task of the Linux kernel is scheduling processes.
Every process has a niceness value that influences its scheduling.
The nice and renice commands can change a process's scheduling priority.
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24. Process priority & niceness
Every process has two values which influence its scheduling.
The first is a dynamic value which is constantly being changed by the kernel as the process's priority.
The second is a fixed value, which is only occasionally (if ever) explicitly changed by a user is the process's niceness.
nice command:
Change a process’s niceness as it starts
nice vi myfile.txt
renice command:
Alter niceness of a process after it is running
Only root user may change NI to a negative value
nice
Linux+ Guide to Linux Certification, 2e

25. Setting Process Priority (PRI)
Nice value (NI): Indirectly represents priority
Determines how many processor time slices the process will receive.
Higher value means lower priority
EXAMPLE
nice vi myfile.txt
runs the vi command on myfile.txt with an increment of +13.
Linux+ Guide to Linux Certification, 2e

26. Chapter 4 - Sending Signals
Key Concepts
Signals are a low level form of inter-process communication, which arise from a variety of sources, including the kernel, the terminal, and other processes.
Signals are distinguished by signal numbers, which have conventional symbolic names and uses. The symbolic names for signal numbers can be listed with the kill -l command.
The kill command sends signals to other processes.
Upon receiving a signal, a process may either ignore it, react in a kernel specified default manner, or implement a custom signal handler.
Conventionally, signal number 15 (SIGTERM) is used to request the termination of a process.
Signal number 9 (SIGKILL) terminates a process, and cannot be overridden.
The pkill and killall commands can be used to deliver signals to processes specified by command name, or the user who owns them.
Other utilities, such as top and the GNOME System Monitor can be used to deliver signals as well.
Linux+ Guide to Linux Certification, 2e

27. Linux+ Guide to Linux Certification, 2e
It is often necessary to kill parent process in order to kill a child process
Killing a parent process will kill all child processes spawned by it.
Such as if you wished to kill all the spawned child processes.
Use the output from the ps command.
Look at the output from ps command and use it read the PID and PPID information.
Tracing from the child process up the hierarchy to the parent processes.
Linux+ Guide to Linux Certification, 2e

28. Terminating processes
You can terminate a process using the kill, killall, top
See examples p444
kill command:
Kills all instances of a specific process
By PID
Or by command name
killall command:
Uses process name instead of PID
Kills all instances of a process by command name
Linux+ Guide to Linux Certification, 2e

29. Kill Signals kill –l Table 10-2: Common administrative kill signals
Displays a list of all the possible kill signals.
64 different kill signals
Affect processes in different ways
Table 10-2: Common administrative kill signals
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30. Linux+ Guide to Linux Certification, 2e
Kill Signals
kill –signal command:
To kill a process, give kill signal and PID
If no kill signal given, SIGTERM ( 15) assumed
$ kill soft kill
If process will not terminate SIGKILL ( 9 ) can be used to force it.
$ kill -9 absolute kill
Linux+ Guide to Linux Certification, 2e

31. Linux+ Guide to Linux Certification, 2e
Chapter 5. Job Control
Key Concepts
The bash shell allows commands to be run in the background as "jobs".
The bash shell allows one job to run in the foreground, and can have multiple backgrounded jobs.
The jobs command will list all backgrounded jobs.
The CTRL-Z key sequence will suspend and background the current foreground job.
The bg command resumes a backgrounded job.
The fg command brings a backgrounded job to the foreground.
Linux+ Guide to Linux Certification, 2e

32. Linux+ Guide to Linux Certification, 2e
Running Processes
There are 2 areas in which processes can be run.
Foreground processes:
BASH shell must wait for process termination to continue
Background processes:
BASH shell does not wait for termination to continue
Processes can be run in the background by appending an & to the command name
When “ & “ is placed at the end of a command it causes the command to be run in the background upon execution.
The user receives a BASH shell prompt immediately.
Linux+ Guide to Linux Certification, 2e

33. Background /Foreground processes
See examples p447
jobs command:
Lists background processes running in current shell
Each background jobs is given a specfic “ %# “ by which it is referred.
It also marks the two most recent background processes
Commands operate on most recent process, by default
+ symbol = most recent process
- symbol = next most recent process
foreground (fg) command:
Run move a background process into the foreground
fg %1
background (bg) command:
To run a job in the background place a “ & “ at the end of the command-line
vi my-document &
Or type “ ctrl Z” to move a running foreground process into the background
Linux+ Guide to Linux Certification, 2e

34. Chapter 6. Scheduling Delayed Tasks: at
Key Concepts
The at command can submit commands to run at a later time.
The batch command can submit commands to run when the machines load is low.
Commands can either be entered directly, or submitted as a script.
stdout from at jobs is mailed to the user.
atq and atrm are used to examine and remove currently scheduled jobs.
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35. Linux+ Guide to Linux Certification, 2e
Scheduling Commands
Commands can be scheduled to run at a later time
Using the at or cron daemons.
at daemon (atd):
System daemon that executes tasks at a future time
cron daemon (crond):
System daemon that executes tasks repetitively in the future
Linux+ Guide to Linux Certification, 2e

36. Scheduling Commands with atd
See examples page
at <time> command:
Used to schedule commands to run at a preset time.
Runs command once only.
Example:
at now
cp /etc/passwd ~/atpass
Table 10-3: Common at commands
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37. Viewing Scheduled “ at “ Jobs
Shell environment & scheduled commands are stored in /var/spool/at
Used to allow us to view scheduled jobs queue.
The “ at “ daemon uses current shell’s environment for execution
If stdout of scheduled command has not been redirected to file,
It is mailed to user
atq command:
Each local users has their own “ at “ queue.
Regular users can only see their own jobs.
–l option: View the queued list of scheduled jobs
–c option: View system environment at scheduling time
–d option: Delete a job
–f option: Run scheduled jobs from shell script
Linux+ Guide to Linux Certification, 2e

38. Chapter 7. Scheduling Periodic Tasks: cron
Key Concepts
The cron facility is used to schedule regularly recurring tasks.
The crontab command provides a frontend to editing crontab files.
The crontab file uses 5 fields to specify timing information.
stdout from cron jobs is mailed to the user.
Linux+ Guide to Linux Certification, 2e

39. Scheduling Commands with crond
See examples page 457 – 459
crontab command:
Used to schedule commands on a preset schedule.
Local users can view and edit their own cron tables
Six fields separated by space or tab characters which determine action
–e option: Edit cron tables in vi editor
–l option: List a user cron table
–r option: Remove all scheduled jobs
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40. Example crond table entry
Figure 10-6: Sample user cron table entry
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41. Linux+ Guide to Linux Certification, 2e
Summary ps
ps –f -e
ps –ef | grep PID
kill killall
jobs
fg bg kill %1
nice renice
at crontab
Linux+ Guide to Linux Certification, 2e