Chapter 10 Periodic Processes

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Chapter 10Periodic Processes

• Unix System Administration

2
C-Ron, C-Ron Run

• Cron is a daemon that periodically executes
  commanded defined in the crontab.
• Some Unix systems provide security to restrict
  who can run cron jobs.
• Cron jobs are executed using the Bourne shell
  (/bin/sh)

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Put It on Crons Tab

• Format of crontab
• 6 space separated commands
• field 1 minute of hour 0-59
• field 2 hour of day 0-23
• field 3 day of month 1-31
• field 4 month of year 1-12
• field 5 day of the week 1-7 (1 Sunday)
• field 6 (to end of line) command to execute

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/var/spool/cron/crontabs/
ident "_at_()root 1.19 98/07/06 SMI"
/ SVr4.0 1.1.3.1 / The root crontab
should be used to perform accounting data
collection. The rtc command is run to adjust
the real time clock if and when daylight
savings time changes. 10 3 0,4
/etc/cron.d/logchecker 10 3 0
/usr/lib/newsyslog 15 3 0 /usr/lib/fs/nfs/nfsf
ind 1 2 -x /usr/sbin/rtc
/usr/sbin/rtc -c /dev/null 21 30 3 -x
/usr/lib/gss/gsscred_clean
/usr/lib/gss/gsscred_clean
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Managing Crontab

• Edit crontabs
• Method 1
• setenv EDITOR vi
• crontab -e
• Method 2
• crontab -l mycrontab
• vi mycrontab
• crontab mycrontab
• Removing a crontab
• crontab -r

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Locking Down Cron

• /etc/cron.d/cron.allow /etc/cron.d/cron.deny
• One username per line
• See man crontab for details on how the rules are
  enforced.

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Questions To Ask Yourself Periodically

• What types of services is cron well suited for?
• What happens if the program creates output and
  you dont redirect it?
• If /bin/sh is used to execute crontab commands,
  how do you execute stuff in other shells?