1. Linux+ Guide to Linux Certification, Third Edition
Chapter 9
Managing Linux Processes

2. Objectives
Categorize the different types of processes on a Linux system
View processes using standard Linux utilities
Explain the difference between common kill signals
Describe how binary programs and shell scripts are executed
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3. Objectives (continued)
Create and manipulate background processes
Use standard Linux utilities to modify the priority of a process
Schedule commands to execute in the future using the at daemon
Schedule commands to execute repetitively using the cron daemon
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4. Linux Processes
Program: structured set of commands stored in an executable file
Executed to create a process
Process: program running in memory and on CPU
User process: process begun by user on a terminal
Daemon process: system process
Not associated with a terminal
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5. Linux Processes (continued)
Process ID (PID): unique identifier assigned to a process
Child process: process started by another process (parent process)
Parent process: process that has started other processes (child processes)
Parent Process ID (PPID): PID of the parent process
The init daemon has a PID of 1 and a PPID of 0
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6. Linux Processes (continued)
Figure 9-1: Parent and child processes
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7. Linux Processes (continued)
Figure 9-2: Process genealogy
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8. Viewing Processes ps command: view processes
Most versatile and common process viewing utility
No arguments: lists all processes running in current shell
PID, terminal, command that started process, CPU time
–f (full) option: more complete information
User identifier (UID), PPID, start time, CPU utilization
-e option: displays the entire list of processes across all terminals including daemons
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9. Viewing Processes (continued)
Process flag: indicates particular features of the process
Process state: current processor state of process
Most processes sleeping (S) or running (R)
Zombie process: process finished, but parent has not released child process’s PID
Defunct process
Process state is Z
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10. Viewing Processes (continued)
Process priority (PRI): priority used by the kernel for the process
Higher value means lower priority
Nice value (NI): can be used to affect the process priority indirectly
Higher value means a greater chance of low priority
pstree command: displays the lineage of a process by tracing its PPIDs until the init daemon
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11. Viewing Processes (continued)
Table 9-1: Common options to the ps command
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12. Viewing Processes (continued)
top command: displays interactive screen listing processes
Organized by processor time
Processes using most processor time listed first
Rogue process: faulty process
Consumes excessive system resources
top command can be used to change PRI or kill processes
Rogue processes can be killed immediately when identified
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13. Killing Processes
kill command: sends a kill signal to a process to terminate it
64 types of kill signals
Affect processes in different ways
-l option: displays list of kill signal names and associated numbers
To kill a process, give kill signal and PID
If no kill signal given, the default kill signal, SIGTERM, is used
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14. Killing Processes (continued)
Table 9-2: Common administrative kill signals
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15. Killing Processes (continued)
Trapping: ignoring a kill signal
The SIGKILL signal cannot be trapped by any process
Use only as last resort
When parent process receives a kill signal, parent process terminates all child processes before terminating itself
To kill several related processes send signal to parent process
Kill parent process in order to kill zombie processes
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16. Killing Processes (continued)
killall command: kills multiple processes of the same name in one command
Takes kill signal number as an option
Uses process name instead of PID
If no kill signal given, the default kill signal, SIGTERM, is used
Can use top command to kill processes
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17. Process Execution Three main types of executable commands
Binary programs
e.g., ls, find, grep
Shell scripts
Shell functions
e.g., cd, exit
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18. Process Execution (continued)
Forking: act of creating new BASH shell or subshell
Carried out by fork function in BASH shell
Subshell executes program or shell script using exec function
Original shell waits for subshell to complete
When done, subshell kills itself
Control returns to original shell
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19. Process Execution (continued)
Figure 9-3: Process forking
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20. Running Processes in the Background
Foreground processes: BASH shell must wait for termination to display prompt and accept new commands
Background processes: BASH shell does not wait for termination
Append & metacharacter to command
Upon execution, user receives BASH shell prompt immediately
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21. Running Processes in the Background (continued)
jobs command: lists background job IDs for processes running in current shell
To terminate background process:
Send kill signal to PID
Send kill signal to background job ID
Prefix job ID with % character
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22. Running Processes in the Background (continued)
foreground (fg) command: move a background process to the foreground
Use Ctrl+z to pause a foreground process
Assigns the process a background job ID
background (bg) command: send an existing process to the background
Provide background job ID as argument
jobs command indicates the two most recent background processes
By default, commands apply to most recent process
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23. Process Priorities
Time slice: amount of time a process is given on a CPU
More time slices mean more execution time on CPU
Executes faster
Usually measured in milliseconds
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24. Process Priorities (continued)
PRI dictates number of time slices a process gets
Low PRI is likely to get more time slices than high PRI
Cannot change PRI value directly
Set NI to indirectly affect priority
Negative NI value, more time slices; positive NI value, less time slices
Processes start with NI of 0
nice command: change a process’s priority as it starts
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25. Process Priorities (continued)
Figure 9-4: The nice value scale
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26. Process Priorities (continued)
renice command: alter NI of a process after it has been started
Only root user may change NI to a negative value
-u option: change the NI for all processes owned by the specified user or group
May also change NI of running processes using top utility
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27. Scheduling Commands To schedule commands to execute in the future:
at daemon (atd): system daemon that executes tasks at a future time
cron daemon (crond): system daemon that executes tasks repetitively in the future
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28. Scheduling Commands with atd
at command: schedule commands and tasks to run at a preset time
Specify the time as an argument
–l option: view a list of at job IDs (regular users see only their own jobs)
atq command: alias to at -l
–c option: view content of a specified at job
–d option: delete the specified at job
–f option: list commands to be scheduled by at from a shell script
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29. Scheduling Commands with atd (continued)
Table 9-3: Common at commands
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30. Scheduling Commands with atd (continued)
at daemon uses current shell’s environment for execution
Shell environment and scheduled commands stored in /var/spool/at
If stdout of scheduled command has not been redirected to a file, it is mailed to user
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31. Scheduling Commands with atd (continued)
/etc/at.allow: file listing all users allowed to use the at daemon
/etc/at.deny: file listing all the users not allowed to use the at daemon
If both files exist, only /etc/at.allow file is processed
On Fedora Linux systems, only /etc/at.deny file exists by default
Initially left blank, all users allowed to use at daemon
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32. Scheduling Commands with crond
Suitable for scheduling repetitive tasks
Cron tables: configuration files specifying when commands should be executed
Six fields separated by space or tab characters
First five specify times to run the command
Sixth absolute pathname to command to be executed
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33. Scheduling Commands with crond (continued)
User cron tables: represent tasks scheduled by individual users
System cron tables: contains system tasks
/var/spool/cron: stores user cron tables
/etc/crontab file: contains system cron tables
/etc/cron.d directory: contains system cron tables
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34. Scheduling Commands with crond (continued)
Figure 9-5: User cron table format
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35. Scheduling Commands with crond (continued)
Figure 9-6: Sample user cron table entry
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36. User Cron Tables
/etc/cron.allow: lists users allowed to use the cron daemon
/etc/cron.deny: lists users not allowed to use the cron daemon
If both files exist, only /etc/cron.allow file is processed
On Fedora Linux systems, only /etc/cron.deny file exists by default
Initially left blank, all users allowed to use cron daemon
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37. User Cron Tables (continued)
crontab command: view and edit user cron tables
–e option: edit cron tables in vi editor
–l option: list a user cron table
–r option: remove cron table and all scheduled jobs
-u option: used by root user to edit, list, or remove a specified user’s cron table
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38. The System Cron Table
System maintenance, backups, and CPU-intensive tasks often scheduled for non-business hours
Often scheduled by cron daemon
Entries in system cron table (/etc/crontab)
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39. The System Cron Table (continued)
Initial section of cron table specifies execution environment
Remainder similar to user cron table entries
Cron tables located in the /etc/cron.d directory are run by the system as a specified user
Can schedule administrative task by placing a shell script in the appropriate one of the following directories: /etc/cron.hourly/, /etc/cron.daily/, /etc/cron.weekly, and etc/cron.monthly/
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40. Summary Processes are programs that are executing on the system
User processes are run in the same terminal as the user who executed them, whereas daemon processes are system processes that do not run on a terminal
Every process has a parent process associated with it and, optionally, several child processes
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41. Summary (continued)
Process information is stored in the /proc filesystem; the ps, pstree and top commands can be used to view this information
Zombie and rogue processes that exist for long periods of time use up system resources and should be killed to improve system performance
You can send kill signals to a process using the kill, killall, and top commands
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42. Summary (continued)
The BASH shell creates, or forks, a subshell to execute most commands
Processes can be run in the background by appending an & to the command name
The priority of a process can be affected indirectly by altering its nice value
Commands can be scheduled to run at a later time using the at and cron daemons
Linux+ Guide to Linux Certification, 3e