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
 6763 ?        00:00:01 /usr/sbin/httpd
12435 ?        00:00:00 atd
13778 ?        00:00:23 boltd
22345 ?        00:00:08 cleanupd
12165 ?        00:01:25 colord
12436 ?        00:00:12 crond
 1644 ?        00:00:01 cupsd
13793 ?        00:00:00 gsd-clipboard
13803 ?        00:00:00 gsd-keyboard
13834 ?        00:00:00 gsd-smartcard
13841 ?        00:00:00 gsd-sound
  161 ?        00:00:00 kaluad
  310 ?        00:00:00 kauditd
  119 ?        00:00:00 kblockd
  131 ?        00:00:21 khugepaged
  113 ?        00:00:09 khungtaskd
  115 ?        00:00:00 kintegrityd
  160 ?        00:00:00 kmpath_rdacd
  172 ?        00:00:00 kpsmoused
  130 ?        00:00:00 ksmd
    2 ?        00:00:16 kthreadd
  140 ?        00:00:00 kthrotld
11754 ?        00:00:32 lmgrd
11914 ?        00:18:36 lockd
  120 ?        00:00:00 md
11924 ?        00:00:01 nfsd
12276 ?        00:00:00 nfsiod
11825 ?        00:01:20 nmbd
11495 ?        00:02:22 ntpd
12169 ?        00:05:50 polkitd
   10 ?        06:57:26 rcu_sched
11862 ?        00:00:00 rhsmcertd
11443 ?        00:00:01 rpc.idmapd
11846 ?        00:00:00 rpc.mountd
11808 ?        00:01:22 rpc.statd
11457 ?        00:09:21 rpcbind
 5869 ?        00:00:00 rpciod
11755 ?        00:04:50 rsyslogd
11456 ?        00:00:01 smartd
  513 ?        00:00:00 smbd
22344 ?        00:00:10 smbd-notifyd
11745 ?        00:01:05 sshd
    1 ?        00:15:40 systemd
12082 ?        00:02:41 systemd-logind
 5904 ?        00:00:04 systemd-udevd
13619 ?        00:00:19 upowerd
  122 ?        00:00:00 watchdogd
19434 ?        00:01:07 winbindd
 5870 ?        00:00:00 xprtiod
11967 ?        00:00:21 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?
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:

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: 2018-09-10T18:26

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