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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

$ ps -e | grep 'd$' | sort -k4 -u
   1814 ?        00:00:00 atd
   1081 ?        00:00:17 chronyd
   1806 ?        00:01:18 crond
   6442 ?        00:00:00 gvfsd
    209 ?        00:00:00 kaluad
     98 ?        00:00:00 kauditd
    107 ?        00:00:00 kblockd
    104 ?        00:00:56 khugepaged
     99 ?        00:00:04 khungtaskd
    106 ?        00:00:00 kintegrityd
    208 ?        00:00:00 kmpath_rdacd
    103 ?        00:00:00 ksmd
   1100 ?        00:01:03 ksmtuned
      2 ?        00:00:04 kthreadd
    197 ?        00:00:00 kthrotld
1457968 ?        00:00:00 kworker/0:4H-kblockd
 424143 ?        00:00:00 kworker/11:0H-kblockd
1445549 ?        00:00:00 kworker/2:7H-kblockd
1412151 ?        00:00:00 kworker/3:1H-kblockd
 880692 ?        00:00:00 kworker/4:0H-kblockd
2870995 ?        00:00:00 kworker/5:0H-kblockd
2036758 ?        00:00:00 kworker/6:0H-kblockd
2604732 ?        00:00:26 kworker/7:0H-kblockd
1999113 ?        00:00:12 kworker/8:8H-kblockd
1420807 ?        00:00:08 kworker/9:3H-kblockd
1308614 ?        00:00:00 kworker/u24:0-nfsiod
1439735 ?        00:00:00 kworker/u24:2-xprtiod
1378172 ?        00:00:00 kworker/u24:3-nfsiod
1458029 ?        00:00:00 kworker/u24:4-nfsiod
   1851 ?        00:00:00 lockd
   1035 ?        00:00:04 lsmd
    112 ?        00:00:00 md
   1836 ?        00:00:00 nfsiod
   1668 ?        00:08:06 pmcd
   1069 ?        00:11:14 polkitd
     14 ?        00:11:46 rcu_sched
   1394 ?        00:00:00 rhsmcertd
   1028 ?        00:00:15 rpcbind
   1041 ?        00:00:00 rpciod
   1396 ?        00:00:01 rpc.statd
   1390 ?        00:07:50 rsyslogd
   1383 ?        00:00:00 sshd
      1 ?        00:56:04 systemd
   1783 ?        00:12:50 systemd-logind
    689 ?        00:03:31 systemd-udevd
   1384 ?        01:24:05 tuned
   1781 ?        00:01:39 /usr/sbin/httpd
    115 ?        00:00:00 watchdogd
   1042 ?        00:00:00 xprtiod
   1133 ?        00:00:13 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  2024 /usr/sbin/crond

Is crond SUID? How does it execute your crontab as you?

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)