CT320

CT320: Network and System Administration

Fall 2018

Periodic

See this page as a slide show

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

Daemons

$ ps -e | grep 'd$' | sort -k4 -u
 1982 ?        00:13:38 /usr/sbin/httpd
 1986 ?        00:00:00 atd
 1693 ?        00:00:35 cleanupd
 1849 ?        00:00:29 colord
 1991 ?        00:00:05 crond
29427 ?        00:00:00 cupsd
  302 ?        00:00:00 kauditd
  116 ?        00:00:00 kblockd
  126 ?        00:00:12 khugepaged
  112 ?        00:00:03 khungtaskd
  114 ?        00:00:00 kintegrityd
  156 ?        00:00:00 kmpath_rdacd
  163 ?        00:00:00 kpsmoused
  125 ?        00:00:00 ksmd
    2 ?        00:00:07 kthreadd
  135 ?        00:00:00 kthrotld
 6316 ?        00:00:02 lmgrd
 1652 ?        00:09:09 lockd
  117 ?        00:00:00 md
 1657 ?        00:00:04 nfsd
 1914 ?        00:00:00 nfsiod
 1537 ?        00:00:28 nmbd
 1225 ?        00:00:45 ntpd
 1873 ?        00:01:53 polkitd
   10 ?        02:51:23 rcu_sched
 1469 ?        00:00:00 rhsmcertd
 1185 ?        00:00:00 rpc.idmapd
 1506 ?        00:00:00 rpc.mountd
 1471 ?        00:00:07 rpc.statd
 1202 ?        00:03:30 rpcbind
  648 ?        00:00:00 rpciod
 1203 ?        00:03:40 rsyslogd
 1198 ?        00:00:00 smartd
 1637 ?        00:04:44 smbd
 1692 ?        00:00:03 smbd-notifyd
 1479 ?        00:01:03 sshd
    1 ?        00:08:01 systemd
 1866 ?        00:01:06 systemd-logind
  662 ?        00:00:00 systemd-udevd
 3147 ?        00:00:06 upowerd
  649 ?        00:00:00 xprtiod
 1845 ?        00:00:07 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 70120 Mar 29  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

Modified: 2017-12-13T11:58

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