RH030 Linux Computing Essentials

1. RH030 Linux Computing Essentials Workbook 9 Managing Processes

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

7. Viewing Processes • pscommand: • 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. Linux+ Guide to Linux Certification, 2e

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– efcommand: • -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– auxcommand: Linux+ Guide to Linux Certification, 2e

11. SystemV verse BSD optionspage 441 Table 10-1: Common options to the ps command Linux+ Guide to Linux Certification, 2e

12. ps –ef Command Output Linux+ Guide to Linux Certification, 2e

13. pgrep [maxwell@station maxwell]$ pgrep -l sshd 829 sshd [maxwell@station maxwell]$ pgrep -lu maxwell2112 bash 2146 mozilla-bin 2155 mozilla-bin 2156 mozilla-bin 2157 mozilla-bin Linux+ Guide to Linux Certification, 2e

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 Linux+ Guide to Linux Certification, 2e

15. 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 Linux+ Guide to Linux Certification, 2e

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 Linux+ Guide to Linux Certification, 2e

20. Process states • ps -aux Current processor state of process • Most processes are either sleeping (S) or running (R) • Zombie process state is Z Linux+ Guide to Linux Certification, 2e

21. /proc • The pscommand 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. Linux+ Guide to Linux Certification, 2e

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 +13 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 19 1426 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 • Higher value means lower priority EXAMPLE nice +13 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. 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 • 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 Linux+ Guide to Linux Certification, 2e

30. Kill Signals • kill –signal command: • To kill a process, give kill signal and PID • If no kill signal given, SIGTERM ( 15) assumed $ kill -15 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. 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. 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. Linux+ Guide to Linux Certification, 2e

35. 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 452 - 455 • 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 Linux+ Guide to Linux Certification, 2e

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 • atqcommand: • 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 Linux+ Guide to Linux Certification, 2e

40. Example crond table entry Figure 10-6: Sample user cron table entry Linux+ Guide to Linux Certification, 2e

41. 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