Managing system processes

1. Managing system processes Unit objectives • Categorize the various types of processes, view processes using utilities, and illustrate the differences between various kill signals • Describe how binary programs and shell scripts are executed, create and manipulate background processes • Use utilities to modify the priority of a process, schedule commands using the at daemon, and schedule repetitive commands using the cron daemon.

2. Topic A: Starting, viewing and killing processes

3. Linux processes • Program • Executable file • May be executed to create a process • Process • Instance of an executing program

4. User and daemon processes • User process • Process begun by a user that runs on a terminal • Daemon process • System process that isn’t associated with a terminal

5. Processes • Process ID (PID) • Unique identifier assigned to every process • Child process • Process that was 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 that created the current process

6. Parent and child processes

7. Processes • PID not sequential • Randomized • Can have multiple children • Only One parent • PID1 PPID 0 • Kernel/ init daemon • Can trace everything back to kernel

8. Process genealogy

9. Viewing Process • PS Command • Most versatile and common utility that can view processes • Without arguments, displays processes running in the current shell • ps –f • Full output • Includes UID and PPID as well at start time and CPU usage

10. Viewing Process • ps –e or ps -ef • Entire list across all terminals and include daemon processes • Usually pipe this command to a filter like grep

11. Viewing Process • ps –l • Gives more information, including process flah and process state

12. Process states

13. Zombie process • Process that has finished executing, but whose parent hasn’t yet released its PID • Also known as a defunct process

14. Process priority and nice value • Process priority (PRI) • Number assigned to a process, used to determine how many time slices on the processor it receives • 0 High Priority • 127 low priority • Nice value (NI) • Value that indirectly represents the priority of a process • The higher the value, the lower the priority

15. Process priority and nice value • Berkeley style options • a- list processes across terminals • x- list processes not on a terminal

16. Options of the ps command

17. top command • Most common command used to display processes aside from ps • Display organized by processor time • Processes that use the most processor time are listed at the top of the screen continued

18. Rogue process • Process that has become faulty and consumes more system resources than it should • Can be used to change the priority of processes or kill them

19. Pstree command • Show processes as a tree • Shows from a specified PID

20. Killing Processes • Kill signal • Type of signal sent to a process by the kill command • Various kill signals affect processes in various ways • kill command • Command that kills all instances of a process by command name • -l switch shows different kill commands • Default is kill -15 (SIGTERM)

21. Common administrative kill signals

22. Killing Processes • pgrep • Grep for processes • Find procceses with a particular name or user • kkill • Kills processes in argument list without outputting details

23. Trapping and killall • Trapping • Process can ignore a kill signal • Use SIGKILL to get around it • killall command • Command that kills all instances of a process by command name • Kills multiple copies of a process with same name • Can also kill using the k key in top

24. An administrator types the command ps xl and the following is a portion of the output obtained: F UID PID PPID NI VSZ RSS WCHAN TTY TIME COMMAND 4 0 1707 1706 15 2304 830 wait4 tty1 0:00 ­bash 4 0 1708 1706 15 4804 420 wait4 tty1 0:00 awk 4 0 1701 1707 15 4304 820 wait4 tty1 0:00 nmap 4 0 1836 1713 15 4405 941 wait4 tty1 0:00 vi 4 0 1921 1707 15 7205 820 wait4 tty1 0:00 xterm Which process number should be killed to stop the nmap And xterm processes? A. 15 B. 820 C. 1707 D. 1921

25. An administrator types the command ps xl and the following is a portion of the output obtained: F UID PID PPID NI VSZ RSS WCHAN TTY TIME COMMAND 4 0 1707 1706 15 2304 830 wait4 tty1 0:00 ­bash 4 0 1708 1706 15 4804 420 wait4 tty1 0:00 awk 4 0 1701 1707 15 4304 820 wait4 tty1 0:00 nmap 4 0 1836 1713 15 4405 941 wait4 tty1 0:00 vi 4 0 1921 1707 15 7205 820 wait4 tty1 0:00 xterm Which process number should be killed to stop the nmap And xterm processes? A. 15 B. 820 C. 1707 D. 1921 Answer: C

26. The following is the output from a ps command: PID TTY TIME CMD 1054 pts/1 00:00:00 bash 6894 pts/1 00:00:00 nmap 7123 pts/1 00:00:00 ps The nmap process has been taking up too much processing power. What command can be used to run nmap with less resources? A. bg nmap B. stop | back 6894 C. CTRL­Q, bk nmap:6894 D. Kill 6894 : nice ­n19 nmap <arguments>

27. The following is the output from a ps command: PID TTY TIME CMD 1054 pts/1 00:00:00 bash 6894 pts/1 00:00:00 nmap 7123 pts/1 00:00:00 ps The nmap process has been taking up too much processing power. What command can be used to run nmap with less resources? A. bg nmap B. stop | back 6894 C. CTRL­Q, bk nmap:6894 D. Kill 6894 : nice ­n19 nmap <arguments> Answer: D.

28. The system administrator notices that the system is running slow. What should be done to find the processes that are using the most resources? A. Use the command "renice" to show the processes with the highest priority. B. Use the command "ps a" to list the top five processes with the highest priority. C. Use the command "jobs" to give a constantly updated list of CPU­intensive processes. D. Use the command "top" to give a real­time updated list of processes ranked by CPU usage.

29. The system administrator notices that the system is running slow. What should be done to find the processes that are using the most resources? A. Use the command "renice" to show the processes with the highest priority. B. Use the command "ps a" to list the top five processes with the highest priority. C. Use the command "jobs" to give a constantly updated list of CPU­intensive processes. D. Use the command "top" to give a real­time updated list of processes ranked by CPU usage. Answer: D.

30. Topic B: Foreground and background processes

31. Process execution • The three main types of Linux commands that you may execute: • Binary programs • Shell functions • Shell scripts

32. Forking • Act of creating a new BASH shell child process from a parent • Carried out by the fork function in the Linux kernel

33. Process forking

34. Background and foreground processes • Foreground process • Process for which the BASH shell that executed it must wait for its termination • Background process • Process that doesn’t require the BASH shell to wait for its termination • Upon execution, the user receives the BASH shell prompt immediately

35. Background processes • jobs command • Command used to see the list of background processes running in the current shell • foreground (fg) command • Command used to run a background process in the foreground • background (bg) command • Command used to run a foreground process in the background

36. Topic C: Process priorities and scheduling

37. Process priorities • Time slice • Amount of time a process is given on a CPU in a multiprocessing operating system • The more time slices a process has, the more time it has to execute on the CPU and the faster it executes • Time slices are usually measured in milliseconds

38. The nice value scale

39. The nice value and command • Processes are started with a nice value of 0 by default • On some systems, background processes are given a nice value of 4 by default to lower the chance they receive time slices • nice command • Command used to change the priority of a process as it’s started

40. The renice command • renice command • Used to alter the nice value of a process currently running • As with the nice command, only the root user may change the nice value to a negative value using renice

41. Scheduling commands • at daemon (atd) • System daemon that executes tasks at a future time • Configured with the at command • cron daemon (crond) • System daemon that executes tasks repetitively in the future • Configured using cron tables

42. Common at commands

43. Scheduling commands with atd • /etc/at.allow • A file listing all users who can use the at command • /etc/at.deny • A file listing all the users who can’t access the at command

44. crond scheduling commands • Cron table • File specifying tasks to be run by the cron daemon • User cron tables and system cron tables • Six fields separated by space or tab characters

45. User cron table format

46. User cron table entry

47. Location of cron tables • /var/spool/cron • Stores user cron tables • /etc/cron.d • Contains additional system cron tables • /etc/crontab • Default system cron table

48. User cron tables • /etc/cron.allow • File listing all users who can use the cron command • /etc/cron.deny • File listing all users who can’t access the cron command • crontab command • Command used to view and edit user cron tables

49. The system cron table • Systems are scheduled to run commands during non-business hours • Commands may perform system maintenance, back up data, or run CPU-intensive programs • Most commands are scheduled by the cron daemon from entries in the system cron table /etc/crontable

50. Chapter summary • Categorized different types of processes, viewed processes by using utilities, and learned the difference between kill signals • Described how binary programs and shell scripts are executed, created and manipulated as background processes • Used utilities to modify the priority of a process and schedule commands