crontab

       A  crontab file contains instructions to the cron(8) daemon of the gen-
       eral form: ``run this command at this time on this date''.   Each  user
       has  their  own crontab, and commands in any given crontab will be exe-
       cuted as the user who owns the crontab.  Uucp  and  News  will  usually
       have  their  own  crontabs, eliminating the need for explicitly running
       su(1) as part of a cron command.

       Blank lines and leading spaces and tabs are ignored.  Lines whose first
       non-space  character  is a hash-sign (#) are comments, and are ignored.
       Note that comments are not allowed on the same line as  cron  commands,
       since  they  will  be taken to be part of the command.  Similarly, com-
       ments are not allowed on the same line  as  environment  variable  set-
       tings.

       An  active line in a crontab will be either an environment setting or a
       cron command.  The crontab file is parsed from top to  bottom,  so  any
       environment  settings  will affect only the cron commands below them in
       the file.  An environment setting is of the form,

           name = value

       where the spaces around the equal-sign (=) are optional, and any subse-
       quent non-leading spaces in value will be part of the value assigned to
       name.  The value string may be placed in quotes (single or double,  but
       matching)  to  preserve  leading or trailing blanks. To define an empty
       variable, quotes must be used. The value string is not parsed for envi-
       ronmental substitutions or replacement of variables, thus lines like

           PATH = $HOME/bin:$PATH

       will not work as you might expect. And neither will this work

           A=1
           B=2
           C=$A $B

       There will not be any subsitution for the defined variables in the last
       value.

       An alternative for setting up the commands path is using the fact  that
       many shells will treat the tilde(~) as substitution of $HOME, so if you
       use bash for your tasks you can use this:

            SHELL=/bin/bash
            PATH=~/bin:/usr/bin/:/bin

       Several environment variables are set up automatically by  the  cron(8)
       daemon.  SHELL is set to /bin/sh, and LOGNAME and HOME are set from the
       /etc/passwd  line  of   the   crontab's   owner.   PATH   is   set   to
       "/usr/bin:/bin".   HOME,  SHELL, and PATH may be overridden by settings
       in the crontab; LOGNAME is the user that the job is running  from,  and
       may not be changed.

       (Another  note:  the  LOGNAME  variable is sometimes called USER on BSD
       rity/pam_env.conf.    It   also   reads   locale    information    from
       /etc/default/locale.   However,  the  PAM  settings do NOT override the
       settings described above nor any settings in the crontab  file  itself.
       Note  in particular that if you want a PATH other than "/usr/bin:/bin",
       you will need to set it in the crontab file.

       By default, cron will send mail using the mail  "Content-Type:"  header
       of  "text/plain"  with  the  "charset="  parameter set to the charmap /
       codeset of the locale in which crond(8) is started up - ie. either  the
       default system locale, if no LC_* environment variables are set, or the
       locale specified by the LC_* environment variables  (  see  locale(7)).
       You can use different character encodings for mailed cron job output by
       setting the CONTENT_TYPE  and  CONTENT_TRANSFER_ENCODING  variables  in
       crontabs, to the correct values of the mail headers of those names.

       The  format of a cron command is very much the V7 standard, with a num-
       ber of upward-compatible extensions.  Each line has five time and  date
       fields,  followed by a command, followed by a newline character ('\n').
       The system crontab (/etc/crontab) uses the same format, except that the
       username  for  the  command is specified after the time and date fields
       and before the command. The fields may be separated by spaces or  tabs.
       The maximum permitted length for the command field is 998 characters.

       Commands  are  executed  by cron(8) when the minute, hour, and month of
       year fields match the current time, and when at least one  of  the  two
       day  fields  (day of month, or day of week) match the current time (see
       ``Note'' below).  cron(8) examines cron entries once every minute.  The
       time and date fields are:

              field          allowed values
              -----          --------------
              minute         0-59
              hour           0-23
              day of month   1-31
              month          1-12 (or names, see below)
              day of week    0-7 (0 or 7 is Sun, or use names)

       A field may be an asterisk (*), which always stands for ``first-last''.

       Ranges of numbers are allowed.  Ranges are two numbers separated with a
       hyphen.  The specified range is inclusive.  For example,  8-11  for  an
       ``hours'' entry specifies execution at hours 8, 9, 10 and 11.

       Lists are allowed.  A list is a set of numbers (or ranges) separated by
       commas.  Examples: ``1,2,5,9'', ``0-4,8-12''.

       Step values can be used in conjunction with ranges.  Following a  range
       with  ``/<number>''  specifies  skips of the number's value through the
       range.  For example, ``0-23/2'' can be used in the hours field to spec-
       ify command execution every other hour (the alternative in the V7 stan-
       dard is ``0,2,4,6,8,10,12,14,16,18,20,22'').  Steps are also  permitted
       after  an asterisk, so if you want to say ``every two hours'', just use
       ``*/2''.


       Note:  The  day of a command's execution can be specified by two fields
       -- day of month, and day of week.  If both fields are restricted (i.e.,
       aren't  *),  the command will be run when either field matches the cur-
       rent time.  For example,
       ``30 4 1,15 * 5'' would cause a command to be run at 4:30 am on the 1st
       and  15th  of  each month, plus every Friday. One can, however, achieve
       the desired result by adding a test to the command (see the last  exam-
       ple in EXAMPLE CRON FILE below).

       Instead  of  the  first  five  fields, one of eight special strings may
       appear:

              string         meaning
              ------         -------
              @reboot        Run once, at startup.
              @yearly        Run once a year, "0 0 1 1 *".
              @annually      (same as @yearly)
              @monthly       Run once a month, "0 0 1 * *".
              @weekly        Run once a week, "0 0 * * 0".
              @daily         Run once a day, "0 0 * * *".
              @midnight      (same as @daily)
              @hourly        Run once an hour, "0 * * * *".

       Please note that startup, as far as @reboot is concerned, is  the  time
       when  the cron(8) daemon startup.  In particular, it may be before some
       system daemons, or other facilities, were startup.  This is due to  the
       boot order sequence of the machine.

EXAMPLE CRON FILE
       The following lists an example of a user crontab file.

       # use /bin/bash to run commands, instead of the default /bin/sh
       SHELL=/bin/bash
       # mail any output to `paul', no matter whose crontab this is
       MAILTO=paul
       #
       # run five minutes after midnight, every day
       5 0 * * *       $HOME/bin/daily.job >> $HOME/tmp/out 2>&1
       # run at 2:15pm on the first of every month -- output mailed to paul
       15 14 1 * *     $HOME/bin/monthly
       # run at 10 pm on weekdays, annoy Joe
       0 22 * * 1-5    mail -s "It's 10pm" joe%Joe,%%Where are your kids?%
       23 0-23/2 * * * echo "run 23 minutes after midn, 2am, 4am ..., everyday"
       5 4 * * sun     echo "run at 5 after 4 every sunday"
       # Run on every second Saturday of the month
       0 4 8-14 * *    test $(date +\%u) -eq 6 && echo "2nd Saturday"

EXAMPLE SYSTEM CRON FILE
       The  following lists the content of a regular system-wide crontab file.
       Unlinke a user's crontab, this file has the username field, as used  by
       /etc/crontab.

       # /etc/crontab: system-wide crontab
       52 6 1 * *  root  test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )
       #

SEE ALSO
       cron(8), crontab(1)

EXTENSIONS
       When  specifying  day  of week, both day 0 and day 7 will be considered
       Sunday.  BSD and AT&T seem to disagree about this.

       Lists and ranges are allowed to co-exist in the same field.   "1-3,7-9"
       would  be  rejected  by  AT&T  or BSD cron -- they want to see "1-3" or
       "7,8,9" ONLY.

       Ranges can include "steps", so "1-9/2" is the same as "1,3,5,7,9".

       Months or days of the week can be specified by name.

       Environment variables can be set in the crontab.  In BSD or  AT&T,  the
       environment  handed  to  child  processes  is  basically  the  one from
       /etc/rc.

       Command output is mailed to the crontab owner (BSD can't do this),  can
       be  mailed  to  a  person  other  than the crontab owner (SysV can't do
       this), or the feature can be turned off and no mail will be sent at all
       (SysV can't do this either).

       All  of  the  `@'  commands  that can appear in place of the first five
       fields are extensions.

LIMITATIONS
       The cron daemon runs with a defined timezone.  It  currently  does  not
       support  per-user timezones. All the tasks: system's and user's will be
       run based on the configured timezone. Even if a user specifies  the  TZ
       environment  variable in his crontab this will affect only the commands
       executed in the crontab, not the execution of the crontab  tasks  them-
       selves.

       The  crontab  syntax  does  not make it possible to define all possible
       periods one could image off. For example, it is not straightforward  to
       define the last weekday of a month. If a task needs to be run in a spe-
       cific period of time that cannot be defined in the crontab syntaxs  the
       best  approach  would  be to have the program itself check the date and
       time information and continue execution only if the period matches  the
       desired one.

       If  the program itself cannot do the checks then a wrapper script would
       be required. Useful tools that could be used for date analysis are ncal
       or  calendar  For  example, to run a program the last Saturday of every
       month you could use the following wrapper code:

       0 4 * * Sat   [ "$(date +\%e)" = "`ncal | grep $(date +\%a | sed -e 's/.$//') | sed -e 's/^.*\s\([0-9]\+\)\s*$/\1/'`" ] && echo "Last Saturday" && program_to_run

DIAGNOSTICS
Man Pages Copyright Respective Owners. Site Copyright (C) 1994 - 2019 Hurricane Electric. All Rights Reserved.