UNIX Processes

1. UNIX Processes

2. The UNIX Process • A process is an instance of a program in execution. • Created by another parent process as its child. • One process can be parent of multiple children. • The shell is the parent of the commands you run. • The shell’s parent is the init process.

3. PID – The Process ID When born, logged in the Process Table with unique number Used to track and kill a process Shell’s PID is stored in variable $$ To see or show value of $$use echo $$ PPID – The Process ID of the Parent Can be used to kill a child process Two Important Process Attributes

4. Other Process Attributes Inherited from Parent • Real UID and GID • Effective UID and GID • Current directory • File descriptors • umask value • Environment variables

5. Displaying Process Status with the ps Command • $ psDisplay a simple listing of your processes • PID TT STAT TIME COMMAND • 64199 p3 S 0:00.00 -ksh (ksh) • 64218 p3 R+ 0:00.00 ps • $ ps –a Display all the users’ processes • $ ps –e Display all processes including system processes • $ ps –f Display a full listing of your processes • $ ps –l Display a long detailed listing of processes • UID PID PPID CPU PRI NI VSZ RSS MWCHAN STAT TT TIME COMMAND • 83641 75606 75605 0 20 0 1996 540 pause S p3 0:00.00 -ksh (ksh) • 83641 75610 75606 0 96 0 5776 992 - R+ p3 0:00.00 ps –l

6. Daemons in UNIX • Daemons are system processes that lack terminal control. Examples: • lpsched • mail, mailx • cron • Cannot read from terminal nor write to terminal • ps –e shows all daemons running.

7. Exporting and Environment Variables • A local variable is not visible in child processes: • $x=5 ; shCreate a child process • $ echo $x • No value in child! • An exported variable is visible in all child processes: • $x=5 ; export x x is now environmental • $sh ; echo $x Create a child process • 5 • Changes made in child are not available in parent: • $x=7Value set in child • $exit Child dies; back to parent • $echo $x • 5Original value in parent

8. The UNIX Process Cycle • A parent process forks a child by replicating its own process image. • The child process execs (overwrites) this image with that of another • program. • While the child is running, parent may • wait for child to complete execution (foreground execution). • continue with its other tasks (background execution). • The process terminates and parent picks up exit status of child. • Kernel removes entry for dead child from process table.

9. When the Child Dies Before the Parent • When child dies, it leaves behind an exit status in process table. • Parent process may • pick up exit status; remove child entry in process table; • child is now completely dead OR • may not wait; child entry not removed from process table; • child in zombie state. • Zombies can’t be killed but are harmless (they take up space); • shown as <defunct> in ps output.

10. When the Parent Dies Before the Child • Child becomes an orphan. • Child adopted by init process. • PPID of child changes to 1 (init process). • Orphan processes continue to operate until killed or • ends normally. • When child dies, init picks up the exit status.

11. Signals • Used by kernel to communicate with processes (e.g., notification • of occurrence of an event, e.g. a keyboard press). • Process may • perform the default action OR • ignore the signal OR • catch the signal and invoke a signal-handling function (trap). • Some important signals • SIGINT (interrupt key) SIGQUIT (quit and core dump) • SIGSTOP (suspend key) SIGKILL (a sure kill, but not a keyboard • event)

12. Terminating Processes with the kill Command • No keyboard event generates a kill. • Kill signals cannot be trapped. You must kill using this command. • kill <pid> • Example: $ kill 154339 (kills process 154339) • Results are not reversible.

13. Running a Job in the Background • Job is run in background with &. Frees keyboard screen for other • uses. Example: sort empl.lst > junk2 & • If you logout before it’s finished, the job may be terminated. • Using nohup (no hang up) and &, the job will run even after logging • out. Example: nohupsort empl.lst > junk2 & • Use nohup with every command in a pipeline. • Example: nohupgrep book | nohup sort > sortedfile &

14. Job Control • Job control supported by most shells but not Bourne. • Job accessed by job number usually shown in brackets e.g., [1]. • Apart from background execution, job control allows • suspending a job ([Ctrl-z]). • bringing a background job to foreground: fg %1 • moving a suspended job to foreground or background: bg %find • The jobs command lists all running/ suspended jobs. • The kill command can also take a job id e.g., kill %3

15. Scheduling a Process to Run Periodically • cron • Kernel started daemon (UNIX’s chronograph) • Wakes up each minute to execute what is in crontabs file • crontab • Takes as input a user edited file • Used to insert jobs in crontabs file to be executed by cron daemon • crontab –r (removes content from a user’s crontab file) • crontab –l (lists content of user’s crontab file) • crontab<file> (adds content of user’s file to crontabs)

16. crontab File Format • minutes hours days of month months days of week command • Minutes: 00 to 59, can be expressed as a range e.g., 00-15, or 0,15,30,45 • Hours : 0 to 23 • Day in month: 0 to max # days in month • Month: 1 to 12 • Days of week: 0 to 6, where 0=Sunday • Use asterisk (*) to indicate all • Use comma (,) to indicate a set of values • Use hyphen (-) to indicate a range of values

17. crontabs File Format Examples • 00-10 17 10,20,30 * 1,3 find / -newer .last_time -print >bkup • Execute the find command either 3 days of the month or Monday and • Wednesday of each month. • 55 17 * * 4 find / -newer .last_time -print > bkup • Execute the find command every Thursday at 5:55 pm. • 0,30 * * * * find / -newer .last_time -print >bkup • Execute the find command every 30 minutes on the half hour.

18. Scheduling a Process to Run Later • at hh:mmor at <keywords> Examples: • at 15:08 at noon + 1 year • at> flush.sh >rep.lst at> cleanup.sh > clean.lst • ctl-d ctl-d • batch - put in batch queue and run the job later • runs when resource utilization levels are low. • have lower priority than user foreground jobs. • Example: batch < checkusers.sh • System may limit use of these commands.