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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.
• Each line that is not a comment has six fields:
  minute hour dom month weekday command
• The first five fields tell cron when to run:
  • minute (Minute of Hour) 0..59
  • hour (Hour of Day) 0..23
  • dom (Day of Month) 1..31
  • month (Month of Year) 1..12
  • weekday (Day of Week) 0..6, 0 = Sunday, 1 = Monday, …

crontab Schedules

    * * * * * * echo Every minute
    * 0 * * * * echo Every hour
    * 0 1 * * * echo Every day at 1:00ᴀᴍ
    * 0 23 * * 1-5 echo 11:00ᴘᴍ, Monday through Friday
    * 45 10,22 * * 0,6 echo 10:45ᴀᴍ and 10:45ᴘᴍ on weekends

crontab Example

    PATH=/usr/local/sbin:/usr/local/bin:/home/wilcox/bin
    MAILTO=Applin@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 checks out crontab and starts editor
• 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)