1. Linux+ Guide to Linux Certification, Second Edition
Chapter 10
Managing Linux Processes

2. Linux+ Guide to Linux Certification, 2e
Objectives
Categorize the different types of processes on a Linux system
View processes using standard Linux utilities
Illustrate the difference between common kill signals
Describe how binary programs and shell scripts are executed
Linux+ Guide to Linux Certification, 2e

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
Linux+ Guide to Linux Certification, 2e

4. Linux+ Guide to Linux Certification, 2e
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
Linux+ Guide to Linux Certification, 2e

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): Parent process’s PID
Linux+ Guide to Linux Certification, 2e

6. Linux Processes (continued)
Figure 10-1: Parent and child processes
Linux+ Guide to Linux Certification, 2e

7. Linux Processes (continued)
Figure 10-2: Process genealogy
Linux+ Guide to Linux Certification, 2e

8. Linux+ Guide to Linux Certification, 2e
Viewing Processes
ps command: View processes
Most 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
Linux+ Guide to Linux Certification, 2e

9. Viewing Processes (continued)
Process state: Current processor state of process
Most processes sleeping (S) or running (R)
Zombie process: Process finished, but parent has not released PID
Defunct process
Process state is Z
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10. Viewing Processes (continued)
Process priority (PRI): Determines how many processor time slices process will receive
Higher value means lower priority
Nice value (NI): Indirectly represents priority
Linux+ Guide to Linux Certification, 2e

11. Viewing Processes (continued)
Table 10-1: Common options to the ps command
Linux+ Guide to Linux Certification, 2e

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
Linux+ Guide to Linux Certification, 2e

13. Linux+ Guide to Linux Certification, 2e
Killing Processes
kill signal: Sent to a process by kill command
64 types of kill signals
Affect processes in different ways
kill command: Kills all instances of a process by command name
To kill a process, give kill signal and PID
If no kill signal given, SIGTERM assumed
Linux+ Guide to Linux Certification, 2e

14. Killing Processes (continued)
Table 10-2: Common administrative kill signals
Linux+ Guide to Linux Certification, 2e

15. Killing Processes (continued)
Trapping: Ignore a kill signal
Kill signals sent to processes having children
Parent process terminates all child processes before terminating itself
Often necessary to kill parent process in order to kill zombie processes
Linux+ Guide to Linux Certification, 2e

16. Killing Processes (continued)
killall command: Kills all instances of a process by command name
Uses process name instead of PID
Can use top command to kill processes
Linux+ Guide to Linux Certification, 2e

17. Linux+ Guide to Linux Certification, 2e
Process Execution
Three main types of executable commands
Binary programs
Shell scripts
Shell functions
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18. Process Execution (continued)
Forking: Act of creating new BASH shell child process from a parent
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
Linux+ Guide to Linux Certification, 2e

19. Process Execution (continued)
Figure 10-3: Process forking
Linux+ Guide to Linux Certification, 2e

20. Running Processes in the Background
Foreground processes: BASH shell must wait for termination
Background processes: BASH shell does not wait for termination
& metacharacter
Upon execution, user receives BASH shell prompt immediately
Linux+ Guide to Linux Certification, 2e

21. Running Processes in the Background (continued)
jobs command: Lists background processes running in current shell
foreground (fg) command: Run a background process in foreground
Foreground process may be paused with ctrl Z
background (bg) command: Run a foreground process in background
Linux+ Guide to Linux Certification, 2e

22. Running Processes in the Background (continued)
Jobs command marks two most recent background processes
Commands operate on most recent process, by default
Linux+ Guide to Linux Certification, 2e

23. Linux+ Guide to Linux Certification, 2e
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
Linux+ Guide to Linux Certification, 2e

24. Process Priorities (continued)
PRI dictates number of time slices a process gets
Cannot change PRI value directly
Set NI to indirectly affect priority
Processes start with NI of 0
nice command: Change a process’s priority as it starts
Linux+ Guide to Linux Certification, 2e

25. Process Priorities (continued)
Figure 10-4: The nice value scale
Linux+ Guide to Linux Certification, 2e

26. Process Priorities (continued)
renice command: Alter NI of a process after it is running
Only root user may change NI to a negative value
May also change NI of running processes using top utility
Linux+ Guide to Linux Certification, 2e

27. Linux+ Guide to Linux Certification, 2e
Scheduling Commands
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

28. Scheduling Commands with atd
at command: Schedule commands and tasks to run at a preset time
–l option: View a list of scheduled jobs
Regular users see only their own 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

29. Scheduling Commands with atd (continued)
atq command: Alternative method to view scheduled jobs
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 file, mailed to user
Linux+ Guide to Linux Certification, 2e

30. Scheduling Commands with atd (continued)
Table 10-3: Common at commands
Linux+ Guide to Linux Certification, 2e

31. Scheduling Commands with atd (continued)
/etc/at.allow: File listing all users able to use at command
/etc/at.deny: File listing all the users not able to use at command
On Fedora Linux systems, no /etc/at.allow file exists
Empty by default
All users allowed to use at daemon
Linux+ Guide to Linux Certification, 2e

32. Scheduling Commands with crond
Cron tables: Files specifying tasks for cron daemon to run
Specify when commands should be executed
User and system cron tables
Six fields separated by space or tab characters
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33. Scheduling Commands with crond (continued)
Figure 10-5: User cron table format
Linux+ Guide to Linux Certification, 2e

34. Scheduling Commands with crond (continued)
Figure 10-6: Sample user cron table entry
Linux+ Guide to Linux Certification, 2e

35. Scheduling Commands with crond (continued)
/var/spool/cron: Stores user cron tables
/etc/crontab: Contains default system cron table
/etc/cron.d: Contains additional system cron tables
Linux+ Guide to Linux Certification, 2e

36. Linux+ Guide to Linux Certification, 2e
User Cron Tables
/etc/cron.allow: Lists users able to use cron command
/etc/cron.deny: Lists users not able to access cron command
Linux+ Guide to Linux Certification, 2e

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 all scheduled jobs
Linux+ Guide to Linux Certification, 2e

38. Linux+ Guide to Linux Certification, 2e
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/crontable)
Linux+ Guide to Linux Certification, 2e

39. The System Cron Table (continued)
Initial section of cron table specifies execution environment
Remainder similar to user cron table entries
Sixth field specifies who to run command as
Remaining fields represent command to run
run-parts command: Execute all files in a directory
Linux+ Guide to Linux Certification, 2e