Periodic Processes (cron)

• There are lots of things to automate in Unix
• Many of them are done routinely
• You (the administrator) have better things to do
• Cron (and some appropriate shell scripts) can do them for you

• A program that runs other programs at specified times
• Lots of flexibility in when to run things
  • "Every Monday at 8:00 am"
  • "Every first of the month, at noon"
  • "Every hour of every day, at 13 minutes after the hour"
• Times are specified in a cryptic format we'll get to shortly
• All commands are run with /bin/sh with no environment variables set
  • What you run shouldn't depend on any features of other shells, or environment variables, unless you make sure those things are available
• Timing weirdness
  • Most crons do not take care of commands missed while the machine was down
  • Some crons do not understand daylight savings time switches, or do not handle them perfectly
    • On the last Sunday of October, the time period from 1:00 am to 2:00 am happens twice, so commands may run twice
    • On the first Sunday in April, the time period from 2:00 am to 3:00 am doesn't exist, so commands may not get run
    • Safest bet is to avoid 1:00-3:00 am in your cron jobs

• System V
  • Flexible, with per user configuration files
  • Users can be allowed or denied permission to use cron
• BSD
  • One configuration file
  • Only root can use it
    • Though commands can be run as other users from the configuration file
  • "vixie cron" is a freeware System V-ish cron, written to replace the cron on BSD systems
    • Comes with OpenBSD , Redhat Linux, etc.
  • We won't talk about BSD-style cron here, since you're unlikely to see it any more.

• One line per command
• Comments begin with "#"
• whitespace between fields is insignificant
• Fields
  • minute
  • hour
  • day
  • month
  • weekday
  • command
• Recommendation:
  • Start your crontab file with the line

   #minute hour day month weekday command

• Line up the columns in your entries to make the fields fit under their name
• It beats trying to re-remember (or look up) the order of the fields each time you edit the file
• Time fields
  • ranges
    • minute: 0-59
    • hour: 0-23
    • day of month: 1-31
    • month: 1-12
    • day of week: 0-6
      • Sunday is 0
  • specification
    • • matches anything
    • single integer
      • matches exactly
    • comma-separated integers (e.g. 1,11,35)
      • matches any number in the list
    • hyphen-separated integers (e.g. 1-5)
      • matches any number in the range
  • One "gotcha"
    • The "day of week" field is "OR"ed with the other date fields, not "AND"ed.
      • So the specification 30 4 13 * 5 happens at 4:30 am on the 13th of every month and at 4:30 am every Friday
        • Not at 4:30 am on every Friday the 13th
        • If that's what you want, you'll need to write a shell script, run it every 13th, and have the script check to see if it's Friday
• Command field
  • Any valid sh command line
  • Most versions allow "%" as a replacement for a newline
    • First line is command, following lines provide stdin for the command (I've never used this feature, so I'm not sure what you'd use it for)
  • Caution: Your comands will run with the Bourne Shell, and with none of your environment
    • If you need an environment variable, you'd better define it
    • You can't use things like '~' on the command line, because that's a feature of just about every shell but the Bourne Shell
      • You can, however, run a csh or ksh script if there are certain features you need
• Some examples
  • 59 23 * * * /users/geoff/bin/mail-summary
    • Every day, at 23:59, run the mail-summary command (which supposedly does some sort of mail summary! ;^) )
  • 30 16 * * 1-5 echo "Time to go home" | mailx -s "Get a life!" joebob
    • Every weekday, at 4:30 pm, send email to joebob telling him to go home
  • 23 22 9,24 * * /usr/local/bin/print-paychecks
    • On the 9th and 24th of every month, at 10:23 pm, print the employee paychecks
  • The textbook has a few other examples

• Called "crontab" ("cron table")
• Write the file anywhere, put it into place with the crontab command
• The crontab command can also list the currently-installed crontab file (with crontab -l)
• Some versions of the crontab command can directly edit the crontab file, using the editor specified in the EDITOR environment variable (crontab -e)
• Root can usually specify a username to manipulate that user's crontab, rather than root's

• Two files, in version-specific directory
  • cron.allow
    • if exists, only users listed in cron.allow may use cron
  • cron.deny
    • if cron.allow does not exist, but cron.deny does, everyone except those listed may use cron
  • If neither file exists, anyone may use cron

• Cleaning directories
  • core files, old '*.o' files, old 'a.out' files
    • Example
      • find / -xdev -name core -atime +7 -exec rm -f {} ';'
      • removes all files named "core" on the root filesystem that haven't been accessed in the last 7 days
• User maintenance
  • accounting
  • updating
• log file maintenance
  • they can get big in a hurry

• arguments
  • A time specification (somewhat free-form)
    • 17:22
    • 10:00 tuesday
    • now + 2 minutes
• Enter commands, end with CNTL-D
• Commands run with your full environment, in the current directory, just like you typed them while logged in
• Premissions
  • Like with cron
    • at.allow
    • at.deny
• batch
  • essentially equivalent to at now