Programs. Program: Set of commands stored in a file Stored on disk Starting a program creates a process static Process: Program loaded in RAM dynamic

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2 Programs Program: Set of commands stored in a file Stored on disk Starting a program creates a process static Process: Program loaded in RAM dynamic

3 Types of Processes 1. User process: Process started by a user on a terminal Always run in a terminal 2. Daemon process: System process Not associated with a terminal Displays? as terminal

4 Process Family Processes do not spontaneously start Processes must be started by processes already running New process is called child process The process that started a process is called parent process

5 Process ID PID is Process ID Unique identifier assigned to a process PPID is Parent Process ID PID of a process s parent process

6 Parent and child processes

7 Ini8aliza8on Linux kernel is always PID 0 Kernel starts init daemon init daemon is always PID 1 init daemon always has PPID 0 init starts other daemons required for the run level

8 Process Genealogy

9 pstree Use pstree command to view family tree of processes

10 /proc directory Processes are listed as files in /proc directory Processes are identified in /proc directory by PID When a process is created it appears in /proc When a process is killed it is removed from /proc During system shutdown, all processes are removed from /proc /proc is considered the only dynamic directory

11 Viewing Processes ps command: View processes PID, terminal, command that started process, CPU time ps with no options lists all processes running in current shell ps f (full) More complete information User identifier (UID), PPID, start time, CPU utilization ps e (every) shows all running processes in all terminals and all daemons (no terminal)

12 Op8ons to the ps command

13 Process State Current processor state of process R running S sleeping waiting on user action Z zombie Process finished, but has not been killed Cannot run, but still using resources

14 top command Displays interactive screen listing processes Updated every second, Organized by processor time Processes using most processor time listed first Identifies Rogue process Faulty process Consumes excessive system resources top command can be used to change PRI kill processes

15 kill command Must use PID to identify specific process kill command has 64 signals Each signal kills process with unique aspects Syntax: kill signal PID kill Default: if no signal, then kill -15 is assumed (SIGTERM)

16 5 Most Common kill signals

17 Sighup kill 1 Suppose you are administering an Apache Web Server. You have just edited the configuration file and want the changes to take effect immediately. Configuration files are only read when the daemon starts. Shutting down the daemon will cause all customers connected to lose their connection. Kill -1 apached will shut down the daemon and immediately restart it. Significantly, kill -1 ensures the restarted daemon will have the SAME PID, thus customers will not lose their connections.

18 Sigint kill -2 ctrl+c {ctrl+c} is the hot-key shortcut for sigint. One of the weakest kill signal, but usually effective Most commonly used because of hot-key Sends an interrupt to the process, triggering normal shutdown.

19 Sigquit kill -3 ctrl+\ {ctrl+\} hot=key shortcut Known as core dump Before killing the process it Dumps the memory of the process to a file called core in the present working directory Useful for debugging programs Useful for retaining forensics data

20 Sigterm kill -15 Default kill signal If you don t designate a kill signal, this will be used Most commonly used signal

21 Sigkill kill -9 Known as the absolute kill signal. One of only 2 kill signal which cannot be trapped. The other is Sigstop which doesn t kill the process but stops it for later resumption. Does not send a termination signal which would allow the process to due a normal end and release resources. Only signal which is performed directly by the kernel. allowing the for clean kill. Used to kill processes which cannot be killed by safer means May not be able to kill Zombies and processes in a interrupt wait state

22 (Cont) Sigkill kill -9 Should not be first choice for killing process Stops the process by sending its memory resourses to /dev/null Process cannot do cleanup may cause data loss; crash other processes; memory resources cannot be reclaimed and reused Prevents process from doing cleanup and might have unfortunate results Can have severe consequences on production servers where repeated use of kill -9 will slowly reduce memory resources available

23 Sigstp kill -20 ctrl+z ctrl+z is another frequent kill signal Suspends (but, does not terminate) a process Process may be resumed May be trapped, unlike sigstop which also suspends a process but cannot be trapped.

24 killall command Uses program name rather than PID Whereas kill command kills a single specific process killall command kills all processes with the same program name running in all terminals Can create problems

25 Killing Processes When a kill signal is sent to a process with children process terminates all child processes before terminating itself Usually, the only way to kill a zombie process is to kill its parent process

26 Traps Kill signals can be trapped Trap tells process to ignore term signal kill -9 is only kill signal that cannot be trapped Called Absolute Kill signal (SIGKILL). Also called the Nuclear Option. Rather than send a kill signal to the process, it moves the process s memory resources to the null device /dev/null Cannot recover memory resources for reuse until system restarted

27 kill -9 kill -9 is the only kill signal that cannot be trapped Called Absolute Kill signal (SIGKILL). Also called the Nuclear Option. Rather than send a kill signal to the process, it moves the process s memory resources to the null device /dev/ null Only kill signal where the kernel directly kills process rather sending term signal May cause data loss. Memory resources cannot be reclaimed and reused until system restart.

28 Forking Act of creating new BASH shell (child) process from a parent Subshell (child ) executes program Original shell (parent) waits for subshell to complete When done, subshell (child) returns control to original shell (parent) Original shell (parent) kills subshell (child)

29 Process forking

30 Processes in the Background Foreground processes: BASH shell must wait for termination Background processes: BASH shell does not wait for termination append & to run process in background Upon execution, user receives BASH shell prompt immediately

31 Processes in the Background jobs command: Lists background processes running in current shell fg foreground command: Run a background process in foreground Foreground process may be paused with ctrl+z bg background command: Run a foreground process in background