CST8177 - Linux II Processes Todd Kelley kelleyt@algonquincollege.com CST8207 - Todd Kelley 1 .elinks, mail .processes .nice .ps, pstree, top .job control, jobs, fg, bg .signals, kill, killall .crontab, anacron, at 2 Topics .elinks is a text-based (character mode) web browser .we will use it to enable our scripts to retrieve web pages .in assignment 7, we use it to retrieve a weather webpage .elinks -dump -no-numbering -no-references .Example elinks -dump -no-numbering -no-references \ 'http://m.weather.gc.ca/city/pages/on-118_e.html' .Could grep this to extract information (maybe with -A option) elinks CST8177 - Todd Kelley 3 .use the mail command to send outgoing and read incoming email on the CLS .Sending outgoing email (bold font shows what the user types) $ mail username@example.com Cc: Subject: First Message from CLS This is a test message. ^D $ mail command CST8177 - Todd Kelley 4 .text mode mail reader .incoming email is stored in /var/spool/mail/ .use the mail command to read it .you'll see a list of messages, each preceded by a number (the header list) .enter a number to see that message .enter h to see the header list again .when you enter q, mail will quit and messages you read will be stored in ~/mbox .mail -f to see the messages in ~/mbox reading mail CST8177 - Todd Kelley 5 .Any program we run executes as a process .Processes have the following attributes .a process id: PID .a parent process id: PPID .a nice number (related to priority) .controlling terminal .Real (RUID) and effective (EUID) user id .Real (RGID) and effective (EGID) group id .Also: .a current working directory .a umask value .an environment (values of environment variables) Processes CST8177 - Todd Kelley 6 .We have already been using the ps command to print out information about processes running on the system .ps -ef or ps aux piped to grep is common .there are many options for printing specific info in a specific way: man ps or ps -h .ps -l # long format .ps -f versus ps -fw ps command CST8177 - Todd Kelley 7 .top displays some system information, and a list of processes, ordered on a column .the most important keys are ?, h, and q (according to man page) .load average: 5min, 10min, 15min .load average is number of processes running or in uninterruptable state (disk IO, others) .no exact rule, but if load average is more than 1-1.5 times the number of CPUs, the machine is overloaded in some way and you have a problem (your mileage may vary) top command CST8177 - Todd Kelley 8 .pstree: connects parents and children in a pictorial display .free: memory usage .vmstat: processes, memory, and more Other commands CST8177 - Todd Kelley 9 .Runnable: ready to go .Sleeping: choosing not to go .Stopped: suspended indefinitely, as in ^Z .Uninterruptable Sleep: waiting on a disk I/O operation, or similar .Zombie or Defunct: process has completed, but it's still in the process table waiting for parent to take action Process states CST8177 - Todd Kelley 10 .Each process has a priority, which you can control with the nice command .-20 highest priority, 19 lowest priority .nice [-n increment] command .nice -n 10 long_command # 10 is default .only superuser can specify negative increments .For processes already running: .renice priority -p PID or renice -n increment -p PID Nice command CST8177 - Todd Kelley 11 .your shell can run several processes for you at once .we can run commands in the background .command & .we can put a running command in the background .^Z .what jobs are there? .jobs .resume a stopped job .bg %N # background, where N is a job number .fg %N # foreground Job Control CST8177 - Todd Kelley 12 .When we type ^C when a process is running in the foreground, the process receives a SIGINT signal, which by default would cause a process to terminate. .SIGINT: ^C (default), similar to SIGTERM .SIGHUP: terminal has been closed .SIGTERM: clean up if necessary, then die .SIGKILL: die right now .We can send these signals to a process with the kill command Sending signals: kill command CST8177 - Todd Kelley 13 .kill -SIGNAL PID #send SIGNAL to process PID .When system shuts down, it .sends all processes a SIGTERM .waits a few seconds (5 or 10) .sends all processes a SIGKILL .Why not just wait for the SIGTERM to finish? .Because SIGTERM can be handled, possibly ignored, it's optional .SIGKILL cannot be handled - it works unless the process is in an uninterruptible state (maybe disk I/O, NFS) Send a signal to kill a process CST8177 - Todd Kelley 14 .If kill -9 PID (kill -SIGKILL PID) as root doesn't kill the process, it is in an uninterruptible state .if uninterruptible processes don't become interruptible, there may be a system problem (bad disk, misconfigured NFS filesystem, etc) .Reboot may be the only way to get rid of them When kill -9 PID doesn't work CST8177 - Todd Kelley 15 .summary of all the POSIX signals: http://en.wikipedia.org/wiki/Unix_signal What are the other signals? CST8177 - Todd Kelley 16 .To run a command regularly and automatically, we use the cron facility .The cron daemon process every minute checks to see if commands specified in crontab files need to be run .for now, we're concerned only with our user crontab files, which are ./var/spool/cron/* .for example, /var/spool/cron/user1 is user1's crontab file Scheduling tasks (cron) CST8177 - Todd Kelley 17 .full details from man 5 crontab .recall that is how we read section 5 of the manual (section 5 of the manual is file formats) .man crontab will give info about the crontab command (in default section 1 of the manual) .create a file containing your cron instructions (see next slide), naming that file, say, myuser.crontab .run the crontab command to submit that file's contents to be your user's crontab file: crontab < myuser.crontab .alternatively, you can edit your user's live crontab file: crontab -e Configuring your cron job CST8177 - Todd Kelley 18 -All fields must contain a value of some valid kind -Field are separated by one or more spaces -Asterisk (*) indicates the entire range # .---------------- minute (0 - 59) # | .------------- hour (0 - 23) # | | .--------- day of month (1 - 31) # | | | .------ month (1 - 12) # | | | | .--- day of week (0 - 7, both 0 and 7 are Sunday) # | | | | | 0 6 1 * * /home/user/bin/mycommand 1 6 15 * * /home/user/bin/anothercommand > /dev/null 2>&1 crontab format (man 5 crontab) crontab format (man 5 crontab) .ranges with dash are allowed: first-last .* means every value first-last .lists are allowed: first,second,third .steps indicated with '/' are allowed after ranges or asterisk: .*/2 means every second one .1-7/2 means 1,3,5,7 crontab format (cont'd) CST8177 - Todd Kelley 20 .crontab -l .list the contents of your current live crontab file .crontab -e .edit the contents of your current live crontab file .crontab .read the new contents of for your crontab file from stdin .crontab -r .remove your current crontab file common crontab options CST8177 - Todd Kelley 21 .see man 5 crontab for example crontab .really, see the example: man 5 crontab .things to watch out for .input for your commands (they run without anyone to type input) .output of commands (if you don't (re)direct output, the output will be emailed - better if you handle it) .error output of commands (same as for output above) .summary: it's best if your commands in a crontab are arranged with input and output already handled, not relying on output to be emailed by cron .if you want to email, do it explicitly in your command somehow, and test that command before putting it into your crontab example crontab CST8177 - Todd Kelley 22 .at command runs a set of commands at a later time .at command takes a TIME parameter and reads the set of commands from standard input .example (run commands at 4pm 3 days from now) .at 4pm + 3 days rm -f /home/usr/foo touch /home/usr/newfoo ^D .other at-related commands: atrm, atq .for details: man at .as with cron, you must be aware of how your commands will get their input (if any) and what will happen to their output (if any) at command CST8177 - Todd Kelley 23