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CT 320: Network and System Administration

CHAPTER 9: PERIODIC PROCESSES

Original slides from Dr. James Walden at Northern Kentucky University.

Topics

1. Overview
2. Cron Daemon
3. Crontab Format
4. Crontab Command
5. Common Uses

Overview

• Why should a sysadmin repeatedly type long sequences of commands to do repetitive tasks?
• Scripting and automation provide consistency and reliability, and prevent typing errors.
• Shell, Perl, or Python scripts can incorporate lengthy tasks and encode system expertise.
• Some tasks do not require human intervention, thus they are good candidates for scripting.
• Some tasks are inherently periodic, e.g. regularly scheduled backup of files and directories.

Daemons

• A daemon is a computer program that runs as a background process.
• Daemons are programs that do not require user intervention to perform their work.
• Daemons in Linux typically have names that end in the letter ‘d’:
  • syslogd: system logging daemon
  • sshd: daemon to handle incoming ssh connections
  • nfsd: network file system daemon
  • ypbind: network information service server
  • crond: task scheduler daemon

Topics

1. Overview
2. Cron Daemon
3. Crontab Format
4. Crontab Command
5. Common Uses

cron Daemon

• The cron daemon is the standard tools for running periodic commands or scripts.
• The cron daemon reads configuration files with list of commands and their associated schedules.
• The command lines are executed by default with sh, but can also use other shells.
• The cron configuration file is called a crontab, stored in /var/spool/cron/username
• Commands run using the UID and GID of the user associated with a crontab.

Topics

1. Overview
2. Cron Daemon
3. Crontab Format
4. Crontab Command
5. Common Uses

crontab Format

Comment lines starting with ‘#’ are ignored by the daemon. Otherwise:

Label	Range	Description
Minute	0–59	Minute of Hour
Hour	0–23	Hour of Day
Day	1–31	Day of Month
Month	1–12	Month of Year (or Jan, Feb, …)
Weekday	0–6	Day of Week (0=Sunday) (or Sun, Mon, …)

crontab Schedules

    # Minute, Hour, Day of Month, Month, Weekday

      *     *  *  *   *  echo Every minute
     00     *  *  *   *  echo Every hour
     00     1  *  *   *  echo Every day at 1:00ᴀᴍ
     30   */3  *  *   *  echo Every three hours, on the half-hour
     00    23  *  *   0  echo 11:00ᴘᴍ Sundays
    */5  9-17  *  * 1-5  echo Every five minutes, during working hours
     45 10,22  *  * 0,6  echo 10:45ᴀᴍ and 10:45ᴘᴍ on weekends
     00     8 25 12   *  echo Christmas morning

crontab shortcuts

    @reboot    :  Run once after reboot.
    @yearly    :  Run once a year, i.e.,  “0 0 1 1 *”.
    @annually  :  Run once a year, i.e.,  “0 0 1 1 *”.
    @monthly   :  Run once a month, i.e., “0 0 1 * *”.
    @weekly    :  Run once a week, i.e.,  “0 0 * * 0”.
    @daily     :  Run once a day, i.e.,   “0 0 * * *”.
    @hourly    :  Run once an hour, i.e., “0 * * * *”.

What cron cannot do

cron can’t help you with:

• 3rd Monday of the month
• Last Sunday of the month
• Last day of the month
• MLK day
• Election day
• Solstices
• Easter

crontab Example

    PATH=/usr/local/bin:/home/bonehead/bin:/bin:/usr/bin
    MAILTO=Bonehead@ColoState.Edu
    0 2 1-10 * * du -h -c -d=1 /

• Changes environment variables for script
• Shell initialization (.chsrc, .bashrc) are not processed
• Can mail output to user upon completion
• At 2:00ᴀᴍ on the 1st through 10th of the month, run a disk usage command (could be a script)

Topics

1. Overview
2. Cron Daemon
3. Crontab Format
4. Crontab Command
5. Common Uses

crontab Command

• crontab filename installs filename as crontab
• crontab -e edits your crontab interactively
• crontab -l lists the current crontab file
  • crontab -l >~/.crontab-backup protects you from system reconfiguration.
• crontab -r removes the crontab file
• A file exists for each user, including root
• Root can supply a user, e.g. crontab -r smith
• /etc/cron.allow and /etc/cron.deny specify which users can submit crontab files

Topics

1. Overview
2. Cron Daemon
3. Crontab Format
4. Crontab Command
5. Common Uses

Common Uses

• Simple reminder:
  30 4 25 * * mail -s "TPS" bing%End-of-month reports due
• Filesystem Backup
• Filesystem Cleanup
• Network Distribution
• Logfile Management
• Time Synchronization (or use service instead)