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UNIX Processes

UNIX Processes. Introduction. Processes How to list them How to terminate them Process priorities Scheduling jobs Signals. Processes. Every time you run a command under UNIX it creates a process A process is an instance of a running program

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UNIX Processes

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  1. UNIX Processes

  2. Introduction • Processes • How to list them • How to terminate them • Process priorities • Scheduling jobs • Signals

  3. Processes • Every time you run a command under UNIX it creates a process • A process is an instance of a running program • A program and a process are different things. wc is a program; each time you run the program wc, that creates a new process. • A process runs until it has finished or is killed

  4. Processes (2) • Every process has a number assigned to it called a process–ID or PID. • All the processes running on a UNIX machine are scheduled a certain amount of time on the CPU. The more that are running, the longer it will take for any one to finish

  5. Listing Processes • ps PID TTY TIME CMD 4073 pts/23 00:00:00 bash 16007 pts/23 00:00:00 ps • ps can be given different options to, e.g. list all processes on system, show the user that owns the process, etc.

  6. Child and Parent Processes • Any process can create another process • That process is then a parent process of the newly created child process • Use the pstree command to view relationships between parent and child processes (the process tree)

  7. pstree [zlizmj@unnc-cslinux ~]$ pstree -p zuczpd gconfd-2(10782) kdeinit(10733)-+-artsd(10752) |-emacs(10779)---aspell(27099) |-kdeinit(10738) |-kdeinit(10771)---bash(10783)---emacs(23700)-+-aspell(32542) | `-emacsserver(23754) |-kdeinit(10773) |-kdeinit(10774)---bash(10802) |-pam-panel-icon(10768)---pam_timestamp_c(10770) `-xload(10778)

  8. Running Job 1 $ ls $ $ ls & $ $ pwd & $ Prompt $ pwd Running Job 2 Child and Parent Processes (2) • A parent process forks to create a child process • This allows multiprocessing: Child Process Parent Process Child Process

  9. Child and Parent Processes (3) • Children run separately and simultaneously with each other and with their parents (although the parent might choose to wait for its child to finish) • Children inherit from their parents • When a process spawns to make a child process, the child is initially an exact copy of the parent, except for some differences (see next)

  10. Differences between Child and Parent Processes • They have different Process IDs (PIDs) • The have different PPIDs (parent PIDs) • Accounting information is reset for the child • For details of all the differences look up the manual page on the system call fork • All other things are initially the same, but each process has its own copy (they can modify their own copy)

  11. Being “nice” to other people! • If you are running a program on a shared machine and you know it will take some time - “nice” it • nice will run a program at a lower priority so that it doesn’t clog up the CPU • Priorities range from 19 to -20 • -20 is the highest priority, but you are not allowed to set priorities below 0 unless you are root $ nice –n 20 find . -name unix.ps –print

  12. Job Control: top • A program which shows you information about the top CPU processes • Updates this information at regular intervals • Type q to quit • Type k to kill a process (to kill means to send a signal) • Type u (return) followed by a username (return) to just see the processes belonging to that user

  13. Killing Processes • Use top • Alternatively, use kill • kill <signal number> <PID> • E.g. kill -15 25718 • “-15” is the signal number – here, it means “stop the process cleanly” (i.e. close any files it is using) • More about signals later… • “-9” means “kill the process whatever” • Useful if all else fails! • killall <signal number> <process name> will send the signal to every process with that name.

  14. Process States • Processes can have one of a number of states: • 0 - running on a processor • S - sleeping (waiting for an event to complete) • R - runnable (process is on run queue) • Z – zombie

  15. Running a Process in the Background • Some commands may take a while to complete • Some may run until the user Exits (e.g. emacs) • You may want to use your command line in the meantime: • & puts a process in the background (detaches it from the terminal) • E.g. emacs &

  16. Suspending Processes • Processes can be temporarily suspended • Use Ctrl-Z • To restart a process type one of: • fg (puts process back in the foreground) • bg (will restart process, but in the background) • Suspended processes can also be killed: • Do a process listing • Use the kill command with the process ID

  17. Job Control • The jobs command produces a numbered list of background and suspended processes • You can use these job numbers to access your jobs: • Note that the job number is not the process ID! [zlizmj@unnc-cslinux ~]$ jobs [1] - Running xclock -d [2] + Suspended more temp.txt [zlizmj@unnc-cslinux ~]$ kill %2 [zlizmj@unnc-cslinux ~]$ jobs [1] - Running xclock -d [3] Terminated more temp.txt

  18. Job Control (2) • jobs allows you to: • Bring a job to the foreground • fg %<job number> • Run a job in the background • bg %<job number> • Suspend a job • stop %<job number> • Terminate a job • kill %<job number>

  19. Control Key Sequences for Processes • Some control sequences affect processes: • Ctrl-C - kill a process • Ctrl-D - exit a shell (send EOF) • Nothing to do with process-control: • Ctrl-S - suspend or pause the display of output • Ctrl-Q - resume or continue output from Ctrl-S

  20. Daemons • UNIX daemons are processes which lie dormant until they are needed for a particular service (services in windows-speak) • Commonly, their names end with a ‘d’ • Examples are: • Printer daemons (lpd) • Web server daemons (httpd) • Scheduling daemons (atd, crond)

  21. Scheduling Processes - cron • Processes can be scheduled to run at a periodic intervals: • Use the cron daemon • With this, users can schedule processes to run periodically, or at specified times • Create a text file called crontab.cron which contains lines with a date/time and command line

  22. Scheduling Processes - cron (2) • Cron jobs are allowed or denied by system administrators using the cron.allow and cron.deny files in either /var/spool/cron or /etc/crond.d • You have to register your crontab using the command crontab crontab.cron in order for the cron daemon to activate your crontab

  23. Scheduling Processes - cron (3) • Each line in crontab.cron has five fields: • Minute - (0-59) • Hour - (0-23) • Day of the month - (1-31) • Month of the year - (1-12) • Day of the week - (0-6) (Sunday is 0) • Command line - the command to be executed

  24. Using cron • Edit your crontab.cron file to contain what you want it to do: • This cron job will record the date it was run every 30 minutes from Monday to Friday, in the file datelog • Register your crontab: 0,30 * * * 1-5 date >> datelog $ crontab crontab.cron

  25. Scheduling Processes - at • You can schedule something to happen once using at • at TIME will execute at given TIME the commands given in STDIN. • It’s often more comfortable to use at TIME < filename at TIME –f filename

  26. Scheduling Processes - at (2) $ at now + 1 min $ at> who | logged $ at> ls myDir | listing.txt $ at> <EOT> job 1171280502.a at Mon Feb 12 11:41:42 2007 $ at 3am < commands job 8 at 2007-03-21 03:00

  27. Scheduling Processes - batch • The batch command can be used to queue up jobs: • These jobs will be run as soon as the system has the resources to do so $ batch at> ls myDir > listing.txt at> <EOT> $

  28. UNIX Signals • Signals are a UNIX mechanism for controlling processes • A signal is a message to a process that requires immediate attention • Signals are generated by exceptions, e.g.: • Attempts to use illegal instructions • The user pressing an interrupt key • Window resize events • A child process calling exit or terminating abnormally

  29. Signal Numbers • Each signal has a default action associated with it • Most signals can be caught from within a program. A programmer can then: • Ignore signal • Perform the default action • Execute a program specified function • The default action can be • Term Terminate the process. • Ign Ignore the signal. • top Stop the process.

  30. Signal Numbers For a complete reference see the section 7 of the manual on signal $ man 7 signal

  31. Summary • UNIX processes • How to list them • How to prioritise them • How to schedule them • Parent and Child processes • Signals • What are they for? • Types of signal

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