Notes for Week 9

1. fcron is used to schedule jobs to be done on a periodic basis. In order to use fcron:
   • fcron must be started (this will happen automatically at boot).
   • A user creates an fcrontab file (i.e., "my.fcrontab") and registers the file with fcron:

     fcrontab my.fcrontab

     (This is like the AT command in Windows.)

     This must be done every time the my.fcrontab file is changed.

     /etc/fcron.allow and /etc/fcron.deny can be used to control which users may do this.
   • A copy is saved in /var/spool/fcron and a parameter file is generated for fcron.
   • Within one minute, fcron will be notified of the changes.
   • fcron maintains a list of jobs to be done in chronological order, and sleeps until it is time to execute the next one.
   • fcrons starts a new process, running as the user, executes the command and mails the output to the user.

     You can configure SeaMonkey's mail client to access UNIX mail as follows:

     1. Click on the mail icon, and if necessary, choose File/New/Account....
     2. Click the UNIX Mailspool (Movemail) button.
     3. Continue through the setup dialogs, filling in information as appropriate:

        Server Name should be localhost

     4. The Get Msgs tool should load whatever is in your UNIX mail to the SeaMonkey client.

   • fcrontab -l will list the current fcrontab file, and fcrontab -r will remove it.

2. Useful fcrontab entries are
   • @ frequency command

     where frequency is, for example, 4m2w3d4h5 (every 4 months, 2 weeks, 3 days, 4 hours and 5 minutes)

     Note that the command must be in /bin, /sbin, /usr/bin or /usr/sbin. Otherwise, it must be a fully qualified filename. If command is a script, this also holds for any commands it executes.
   • & minutes hours dayofmonth month dayofweek command

     where "minutes hours dayofmonth month dayofweek" determines when the command is to be executed

     57 20 * * 7 would cause the job to be run every Sunday at 8:57 PM

     * * * * 3-3/2 would cause the job to be run every other Wednesday

     (every other Thursday would be 4-4/2)

   • %frequency time command

     where frequency is one of "hourly, daily, monthly, weekly, nightly" and time is of the form "minutes hours days" (as appropriate)

     nightly * 2-3 indicates the job should be run every night some time between 2:00 and 3:00 AM

3. The characters "@", "&" or "%frequency" can be followed by comma-separated options:
   • mail(no) to turn off mailing the output
   • nice(value) to set nice value (only root can do this)
   • nolog to disable logging of start/stop information for this job
   • runas(user) to run the job as a different user (only root can do this)

   See the section 5 fcrontab man page for more options.

4. Some potentially useful fcron "one-liners":
   • @nolog 1h logger $(echo ps reports $(ps aux | wc -l), /proc reports $(ls -1 /proc | grep -e '^[1-9]' | wc -l))

     The "$(pipe)" construct is a way to create a string from the output of a pipe of commands. Here, the "wc" command ("word count") returns the number of lines output by the ps command, and the grep command is used to select all of the directories in /proc that correspond to processes.

     This will create a single log entry every hour noting the number of processes running as reported by both ps and /proc. Should both numbers be the same? Why might this be interesting?

   • %nightly,nolog * 2-3 "find /tmp/ -type f -mtime +14 -delete > /tmp/delete.status 2>&1"

     The "2>&1" tells the shell to send stderr to the same place as stdout (which in this case has been redirected to the file /tmp/delete.status).

     Every night between 2 and 3 AM, this will remove files from /tmp/ which are more than two weeks old.

   • &nolog 0 10 * * 6 "mplayer -playlist http://cpr.streamguys.net/wvxu.pls -dumpaudio"
     &nolog 0 11 * * 6 "pkill mplayer ; mv stream.dump cartalk.$(date +%y%m%d).mp3"

     Records "Car Talk" from WVXU's streaming service each Saturday from 10:00 to 11:00 AM.

     The "date" command can be used to change or interrogate the date and time (like the separate commands DATE and TIME in Windows). Here it is used to format today's date; below we use it to format yesterday's.

     If your computer is off during the seasonal time change from/to Daylight Savings Time, you will need to manually change the time and restart ntp:

     date MMDDhhmm
     hwclock --systohc
     /etc/rc.d/init.d/ntp restart

     The hwclock command stores the system time, which the date command changed, into the hardware clock. We do this so that if the system crashes the time will be correct.

   • & 0 0 1 * * "mv /var/log/sys.log /var/log/sys.log.$(date -d yesterday +%y%m) ; touch /var/log/sys.log ; kill -SIGHUP $(cat /var/run/syslogd.pid)"

     Rotates the system log file on the first of each month at midnight; names the old log file with yesterday's year and month.

5. Usually it is better to use a shell script for "command". For instance, the following script is an improvement on the last example:

   #!/bin/sh
   #/sbin/logrotate
   
   for f in $* ; do
      mv /var/log/$f.log /var/log/$f.log.$(date -d yesterday +%y%m)
      touch /var/log/$f.log
   done
   
   /etc/rc.d/init.d/sysklogd restart
   

   Now the command

   logrotate auth boot daemon kern mail sys user

   will rotate all of the system logs. Note the "for" command is similar to the FOR command in Windows.
6. EXERCISES for Week 9:
   1. Set up an fcron job to run your backup script each day at 9:00 PM.
   2. Set up an fcron job to rotate one or more log files every two weeks between 1:00 and 2:00 AM Monday morning. Note that you will have to add the day to the file name (change +%y%m to +%y%m%d).

©2014, Kenneth R. Koehler. All Rights Reserved. This document may be freely reproduced provided that this copyright notice is included.

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