CT320: Network and System Administration

Fall 2018

Periodic

See this page as a slide show

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

Daemons

$ ps -e | grep 'd$' | sort -k4 -u
 2003 ?        00:11:15 /usr/sbin/httpd
 2006 ?        00:00:00 atd
 1899 ?        00:00:26 cleanupd
 1885 ?        00:00:26 colord
 2009 ?        00:00:04 crond
  911 ?        00:00:01 cupsd
 3354 ?        00:00:00 gsd-clipboard
 3366 ?        00:00:00 gsd-keyboard
 3389 ?        00:00:00 gsd-smartcard
 3390 ?        00:00:00 gsd-sound
  158 ?        00:00:00 kaluad
  298 ?        00:00:00 kauditd
  117 ?        00:00:00 kblockd
  129 ?        00:00:08 khugepaged
  113 ?        00:00:03 khungtaskd
  115 ?        00:00:00 kintegrityd
  157 ?        00:00:00 kmpath_rdacd
  163 ?        00:00:00 kpsmoused
  128 ?        00:00:00 ksmd
    2 ?        00:00:04 kthreadd
  138 ?        00:00:00 kthrotld
 1520 ?        00:00:10 lmgrd
 1694 ?        00:28:59 lockd
  119 ?        00:00:00 md
 1699 ?        00:00:00 nfsd
 1944 ?        00:00:00 nfsiod
 1485 ?        00:00:27 nmbd
 1240 ?        00:00:42 ntpd
 1883 ?        00:01:28 polkitd
   10 ?        02:18:05 rcu_sched
 1495 ?        00:00:00 rhsmcertd
 1195 ?        00:00:00 rpc.idmapd
 1522 ?        00:00:00 rpc.mountd
 1497 ?        00:00:54 rpc.statd
 1208 ?        00:05:25 rpcbind
  636 ?        00:00:00 rpciod
 1493 ?        00:03:18 rsyslogd
 1209 ?        00:00:00 smartd
 1349 ?        00:00:02 smbd
 1898 ?        00:00:03 smbd-notifyd
 1484 ?        00:00:44 sshd
    1 ?        00:05:42 systemd
 1680 ?        00:00:54 systemd-logind
  670 ?        00:00:00 systemd-udevd
 3187 ?        00:00:06 upowerd
  637 ?        00:00:00 xprtiod
 1641 ?        00:00:06 ypbind

Topics

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

cron Daemon

Permissions

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

[Service]
EnvironmentFile=/etc/sysconfig/crond
ExecStart=/usr/sbin/crond -n $CRONDARGS
ExecReload=/bin/kill -HUP $MAINPID
KillMode=process

[Install]
WantedBy=multi-user.target

$ ls -l /usr/sbin/crond
-rwxr-xr-x 1 root root 70208 Oct 16  2017 /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:

LabelRangeDescription
Minute0–59Minute of Hour
Hour0–23Hour of Day
Day1–31Day of Month
Month1–12Month of Year (or “Jan”, “Feb”, …)
Weekday0–6Day 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

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 /

Topics

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

crontab Command

Topics

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

Common Uses

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Modified: 2018-04-24T17:02

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