crontab man page on aLinux

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CRONTAB(1)							    CRONTAB(1)

       crontab - manipulate per-user crontabs (Dillon's Cron)

       crontab file [-u user] - replace crontab from file

       crontab - [-u user] - replace crontab from stdin

       crontab -l [user] - list crontab for user

       crontab -e [user] - edit crontab for user

       crontab -d [user] - delete crontab for user

       crontab -c dir - specify crontab directory

       crontab	manipulates the crontab for a particular user.	Only the supe‐
       ruser may specify a different user and/or crontab directory.  Generally
       the  -e	option	is  used  to  edit  your  crontab.   crontab  will use
       /usr/bin/vi or the editor specified by your VISUAL environment variable
       to edit the crontab.

       Unlike other crond/crontabs, this crontab does not try to do everything
       under the sun.  Frankly, a shell script is much more able to manipulate
       the  environment	 then  cron  and I see no particular reason to use the
       user's shell (from his password entry) to run cron commands  when  this
       requires	 special  casing of non-user crontabs, such as those for UUCP.
       When a crontab command is run, this crontab runs it  with  /bin/sh  and
       sets up only three environment variables: USER, HOME, and SHELL.

       crond  automatically detects changes in the time.  Reverse-indexed time
       changes less then an hour old will NOT re-run crontab commands  already
       issued  in  the	recovered period. Forward-indexed changes less then an
       hour into the future will issue missed commands exactly once.   Changes
       greater	then an hour into the past or future cause crond to resynchro‐
       nize and not issue missed commands.  No attempt will be made  to	 issue
       commands	 lost  due  to	a reboot, and commands are not reissued if the
       previously issued command is still running.  For example, if you have a
       crontab	command	 'sleep	 70'  that you wish to run once a minute, cron
       will only be able to issue the command once every two minutes.  If  you
       do  not	like this feature, you can run your commands in the background
       with an '&'.

       The crontab format is roughly similar to that used  by  vixiecron,  but
       without complex features.  Individual fields may contain a time, a time
       range, a time range with a skip factor, a symbolic range for the day of
       week and month in year, and additional subranges delimited with commas.
       Blank lines in the crontab or lines that begin  with  a	hash  (#)  are
       ignored.	 If you specify both a day in the month and a day of week, the
       result is effectively ORd... the crontab entry will be run on the spec‐
       ified day of week and on the specified day in the month.

       # at 6:10 a.m. every day
       10 6 * * * date

       # every two hours at the top of the hour
       0 */2 * * * date

       # every two hours from 11p.m. to 7a.m., and at 8a.m.
       0 23-7/2,8 * * * date

       # at 11:00 a.m. on the 4th and on every mon, tue, wed
       0 11 4 * mon-wed date

       # 4:00 a.m. on january 1st
       0 4 1 jan * date

       # once an hour, all output appended to log file
       0 4 1 jan * date >>/var/log/messages 2>&1

       The  command  portion  of the line is run with /bin/sh -c <command> and
       may therefore contain any valid bourne shell command.  A	 common	 prac‐
       tice is to run your command with exec to keep the process table unclut‐
       tered.  It is also common to redirect output to a log file.  If you  do
       not,  and  the command generates output on stdout or stderr, the result
       will be mailed to the user in question.	If you use this mechanism  for
       special	users,	such  as UUCP, you may want to create an alias for the
       user to direct the mail to someone else, such as root or postmaster.

       Internally, this cron uses a quick indexing system to reduce CPU	 over‐
       head  when  looking  for commands to execute.  Several hundred crontabs
       with several thousand entries can be handled  without  using  noticable
       CPU resources.

       Ought  to  be able to have several crontab files for any given user, as
       an organizational tool.

       Matthew Dillon (

				  1 May 1994			    CRONTAB(1)

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