The Crontab File

1
The Crontab File
2
Objectives

• Discuss and explain the crontab file
• Learn how to create a crontab file
• Understand and use the crontab file

3
Crontab File Example

• Create a crontab file for Dave called davecron
• vi davecron E
• echo date to console every 15 minutes between
  6pm and 6am
• 0, 15, 30, 45 18 06 /bin/echo/ date
• gt /dev/console
• Save this file and exit
• This writes to the console file, since the
  monitor, as all devices, is treated as a file by
  UNIX
• cron davecron or crontab davecron
• This copes the crontab file into /var/spool/cron

4
Crontab Files

• crontab l
• Lists the crontab files
• crontab l gt HOME/backupCron
• Backs up the crontab file
• crontab e
• Allows user to edit crontab file
• crontab r
• Deletes crontab file
• crontab backupCron
• Restores crontab file if backup available

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Crontab Files

• delete cor files at 30am on the days
  1,7,14,21,26 of each month
• 30 3 1,7,14,21,26 /bin find name core exec
  rm \

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At Command

• Allows the user to submit jobs or commands to the
  crontab to be executed at a later time
• General form
• at -f ltfilenamegt -m -t lttimegt date -l
  -r
• Some Options
• f
• Specifies the file that contains the command to
  run.
• m
• Sends mail once the command has been run.

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At Command (contd)

• Some Options (contd)
• t
• Specifies at what time for the command to be run.
  
• Format hhmm. am / pm indication may follow
  the time otherwise
  a 24-hour clock is used.
• l
• Lists the commands that have been set to run.
• r
• Cancels the command set in the past

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At Command

• Some Options (contd)
• date
• Specifies the date for it to be run.
• Format month, date, year.
• The following quick days may be
  entered
• today - Indicates the current day
• tomorrow - Indicates the day following the
  current day.

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At Command (contd)

• Examples
• at 2110
• Gives atgt prompt
• atgt find / -name passed print
• atgt D
• Exits at
• This wilt list, at 2110, all files including
  passwd from root directory

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At Command (contd)

• Example (contd)
• Will also produce a warning
• warning commands will be executed using /bin/sh
  job1 at ltdategt 2110
• At 2110, the following will be displayed on
  screen
• /etc/passwd
• /etc/passwd/passed
• .