5 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  A program and 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

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

Listing Processes  ps PID TTY TIME CMD 4073 pts/23 00:00:00 bash 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.

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)

pstree ~]$ 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)

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

 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) Child and Parent Processes (3)

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)

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

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

Killing Processes  Use top  Alternatively, use kill  kill  E.g. kill  “-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 will send the signal to every process with that name.

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

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 &

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

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! ~]$ jobs [1] - Runningxclock -d [2] + Suspendedmore temp.txt ~]$ kill %2 ~]$ jobs [1] - Runningxclock -d [3] Terminatedmore temp.txt

Job Control (2)  jobs allows you to:  Bring a job to the foreground  fg %  Run a job in the background  bg %  Suspend a job  stop %  Terminate a job  kill %

Control Key Sequences for Processes  Some control sequences affect processes:  Ctrl-C - kill a process  Ctrl-D - exit a shell (send EOF)  Ctrl-S - suspend or pause the display of output  Ctrl-Q - resume or continue output from Ctrl-S

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)

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

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

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

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: $ crontab crontab.cron 0,30 * * * 1-5 date >> datelog

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

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

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> $

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

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  TermTerminate the process.  IgnIgnore the signal.  CoreTerminate the process and dump core.  StopStop the process.

Signal Numbers Signal Name NumberDefault Action Meaning SIGHUP1TermHangup (sent to a process when a modem or network connection is lost, terminal is closed, etc) SIGINT2TermInterrupt (generated by Ctrl-C) SIGTRAP5CoreTrace trap SIGKILL9TermKill SIGBUS10CoreBus error (invalid memory reference) SIGSEGV11CoreSegmentation violation SIGTERM15TermSoftware termination signal (default kill signal) For a complete reference see the section 7 of the manual on signal $ man 7 signal

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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