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CT320 Periodic

⏰ 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 don’t require human intervention, so they are good candidates for scripts.
• Some tasks are inherently periodic, e.g., backups.

Daemons

$ ps -e | grep 'd$' | sort -k4 -u
   1819 ?        00:00:00 atd
   1075 ?        00:00:01 chronyd
   1823 ?        00:00:04 crond
   6559 ?        00:00:00 gvfsd
    209 ?        00:00:00 kaluad
     98 ?        00:00:00 kauditd
    108 ?        00:00:00 kblockd
    105 ?        00:00:02 khugepaged
    100 ?        00:00:00 khungtaskd
    107 ?        00:00:00 kintegrityd
    208 ?        00:00:00 kmpath_rdacd
    104 ?        00:00:00 ksmd
   1172 ?        00:00:03 ksmtuned
      2 ?        00:00:00 kthreadd
    197 ?        00:00:00 kthrotld
 139710 ?        00:00:00 kworker/0:1H-kblockd
2474416 ?        00:00:00 kworker/10:1H-kblockd
1420620 ?        00:00:00 kworker/11:3H-kblockd
 189209 ?        00:00:00 kworker/1:8H-kblockd
 108959 ?        00:00:00 kworker/2:8H-kblockd
 165549 ?        00:00:00 kworker/3:0H-kblockd
  20666 ?        00:00:00 kworker/4:0H-kblockd
2490624 ?        00:00:03 kworker/5:1H-kblockd
2880431 ?        00:00:00 kworker/6:2H-kblockd
3070522 ?        00:00:04 kworker/7:0H-kblockd
2252206 ?        00:00:11 kworker/8:1H-kblockd
    431 ?        00:00:03 kworker/9:1H-kblockd
 169312 ?        00:00:00 kworker/u24:0-rpciod
  38716 ?        00:00:01 kworker/u24:1-rpciod
 200098 ?        00:00:00 kworker/u24:2-xprtiod
 121785 ?        00:00:00 kworker/u24:3-xprtiod
 136261 ?        00:00:00 kworker/u24:4-nfsiod
   1901 ?        00:00:00 lockd
   1067 ?        00:00:00 lsmd
    111 ?        00:00:00 md
   1880 ?        00:00:00 nfsiod
   1781 ?        00:00:30 pmcd
   1079 ?        00:00:45 polkitd
     14 ?        00:00:44 rcu_sched
   1418 ?        00:00:00 rhsmcertd
   1053 ?        00:00:00 rpcbind
   1057 ?        00:00:00 rpciod
   1417 ?        00:00:00 rpc.statd
   1411 ?        00:00:25 rsyslogd
   1408 ?        00:00:00 sshd
      1 ?        00:03:10 systemd
   1811 ?        00:00:45 systemd-logind
    698 ?        00:00:12 systemd-udevd
   1406 ?        00:05:27 tuned
   1808 ?        00:00:06 /usr/sbin/httpd
    114 ?        00:00:00 watchdogd
   1058 ?        00:00:00 xprtiod
   1177 ?        00:00:00 ypbind

• 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.

Permissions

$ cat /usr/lib/systemd/system/crond.service
[Unit]
Description=Command Scheduler
After=auditd.service nss-user-lookup.target systemd-user-sessions.service time-sync.target ypbind.service

[Service]
EnvironmentFile=/etc/sysconfig/crond
ExecStart=/usr/sbin/crond -n $CRONDARGS
ExecReload=/bin/kill -HUP $MAINPID
KillMode=process
Restart=on-failure
RestartSec=30s

[Install]
WantedBy=multi-user.target

$ ls -l /usr/sbin/crond
-rwxr-xr-x 1 root root 75712 Apr  6 05:40 /usr/sbin/crond

Is crond SUID? How does it execute your crontab as you?
It’s executed as root by systemd.

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:

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
  • Ramadan

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
• The super-user 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)