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
11021 ?        00:00:04 /usr/sbin/httpd
 1986 ?        00:00:00 atd
 1688 ?        00:00:38 cleanupd
 1866 ?        00:01:31 colord
 1982 ?        00:00:13 crond
28974 ?        00:00:00 cupsd
 3299 ?        00:00:00 gsd-clipboard
 3304 ?        00:00:00 gsd-keyboard
 3329 ?        00:00:00 gsd-smartcard
 3332 ?        00:00:00 gsd-sound
  157 ?        00:00:00 kaluad
  287 ?        00:00:00 kauditd
  117 ?        00:00:00 kblockd
  129 ?        00:00:24 khugepaged
  113 ?        00:00:10 khungtaskd
  115 ?        00:00:00 kintegrityd
  156 ?        00:00:00 kmpath_rdacd
  159 ?        00:00:00 kpsmoused
  128 ?        00:00:00 ksmd
    2 ?        00:00:15 kthreadd
  138 ?        00:00:00 kthrotld
 1465 ?        00:00:30 lmgrd
 1639 ?        00:38:53 lockd
  119 ?        00:00:00 md
 1647 ?        00:00:01 nfsd
 1908 ?        00:00:00 nfsiod
 1546 ?        00:01:36 nmbd
 1193 ?        00:02:31 ntpd
 1857 ?        00:06:00 polkitd
   10 ?        07:18:44 rcu_sched
 1516 ?        00:00:00 rhsmcertd
 1174 ?        00:00:01 rpc.idmapd
 1495 ?        00:00:00 rpc.mountd
 1459 ?        00:00:45 rpc.statd
 1179 ?        00:07:14 rpcbind
  639 ?        00:00:00 rpciod
 1460 ?        00:05:43 rsyslogd
 1199 ?        00:00:01 smartd
 1606 ?        00:04:32 smbd
 1687 ?        00:00:09 smbd-notifyd
 1591 ?        00:00:35 sshd
    1 ?        00:15:44 systemd
 1850 ?        00:02:23 systemd-logind
  671 ?        00:00:00 systemd-udevd
 3159 ?        00:00:16 upowerd
  640 ?        00:00:00 xprtiod
 1644 ?        00:00:27 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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