crontab

CRONTAB(5)                       File Formats                       CRONTAB(5)



NAME
       crontab - files used to schedule the execution of programs

DESCRIPTION
       A crontab file contains instructions for the cron(8) daemon in the
       following simplified manner: "run this command at this time on this
       date".  Each user can define their own crontab.  Commands defined in
       any given crontab are executed under the user who owns that particular
       crontab.  Uucp and News usually have their own crontabs, eliminating
       the need for explicitly running su(1) as part of a cron command.

       Blank lines, leading spaces, and tabs are ignored.  Lines whose first
       non-white space character is a pound-sign (#) are comments, and are not
       processed.  Note that comments are not allowed on the same line as cron
       commands, since they are considered a part of the command.  Similarly,
       comments are not allowed on the same line as environment variable
       settings.

       An active line in a crontab is either an environment setting or a cron
       command.  An environment setting is of the form:

          name = value

       where the white spaces around the equal-sign (=) are optional, and any
       subsequent non-leading white spaces in value is a 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 white spaces.

       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.  HOME and SHELL can be
       overridden by settings in the crontab; LOGNAME can not.

       (Note: the LOGNAME variable is sometimes called USER on BSD systems and
       is also automatically set).

       In addition to LOGNAME, HOME, and SHELL, cron(8) looks at the MAILTO
       variable if a mail needs to be send as a result of running any commands
       in that particular crontab.  If MAILTO is defined (and non-empty), mail
       is sent to the specified address.  If MAILTO is defined but empty
       (MAILTO=""), no mail is sent.  Otherwise, mail is sent to the owner of
       the crontab.  This option is useful if you decide to use /bin/mail
       instead of /usr/lib/sendmail as your mailer.  Note that /bin/mail does
       not provide aliasing and UUCP usually does not read its mail.  If
       MAILFROM is defined (and non-empty), it is used as the envelope sender
       address, otherwise, ``root'' is used.

       By default, cron sends a mail using the '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, i.e., either the default
       system locale, if no LC_* environment variables are set, or the locale
       specified by the LC_* environment variables (see locale(7)).  Different
       character encodings can be used for mailing cron job outputs by setting
       the CONTENT_TYPE and CONTENT_TRANSFER_ENCODING variables in a crontab
       to the correct values of the mail headers of those names.

       The CRON_TZ variable specifies the time zone specific for the cron
       table.  The user should enter a time according to the specified time
       zone into the table.  The time used for writing into a log file is
       taken from the local time zone, where the daemon is running.

       The MLS_LEVEL environment variable provides support for multiple per-
       job SELinux security contexts in the same crontab.  By default, cron
       jobs execute with the default SELinux security context of the user that
       created the crontab file.  When using multiple security levels and
       roles, this may not be sufficient, because the same user may be running
       in different roles or in different security levels.  For more
       information about roles and SELinux MLS/MCS, see selinux(8) and the
       crontab example mentioned later on in this text.  You can set the
       MLS_LEVEL variable to the SELinux security context string specifying
       the particular SELinux security context in which you want jobs to be
       run.  crond will then set the execution context of those jobs that meet
       the specifications of the particular security context.  For more
       information, see crontab(1) -s option.

       The RANDOM_DELAY variable allows delaying job startups by random amount
       of minutes with upper limit specified by the variable. The random
       scaling factor is determined during the cron daemon startup so it
       remains constant for the whole run time of the daemon.

       The format of a cron command is similar to the V7 standard, with a
       number of upward-compatible extensions.  Each line has five time-and-
       date fields followed by a username (if this is the system crontab
       file), and followed by a command.  Commands are executed by cron(8)
       when the 'minute', 'hour', and 'month of the year' fields match the
       current time, and at least one of the two 'day' fields ('day of month',
       or 'day of week') match the current time (see "Note" below).

       Note that this means that non-existent times, such as the "missing
       hours" during the daylight savings time conversion, will never match,
       causing jobs scheduled during the "missing times" not to be run.
       Similarly, times that occur more than once (again, during the daylight
       savings time conversion) will cause matching jobs to be run twice.

       cron(8) examines cron entries 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 Sunday, or use names)

       A field may contain 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. The first
       number must be less than or equal to the second one.

       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
       specify command execution for every other hour (the alternative in the
       V7 standard is "0,2,4,6,8,10,12,14,16,18,20,22").  Step values are also
       permitted after an asterisk, so if specifying a job to be run every two
       hours, you can use "*/2".

       Names can also be used for the 'month' and 'day of week' fields.  Use
       the first three letters of the particular day or month (case does not
       matter).  Ranges or lists of names are not allowed.

       If the UID of the owner is 0 (root), the first character of a crontab
       entry can be "-" character. This will prevent cron from writing a
       syslog message about the command being executed.

       The "sixth" field (the rest of the line) specifies the command to be
       run.  The entire command portion of the line, up to a newline or a "%"
       character, will be executed by /bin/sh or by the shell specified in the
       SHELL variable of the cronfile.  A "%" character in the command, unless
       escaped with a backslash (\), will be changed into newline characters,
       and all data after the first % will be sent to the command as standard
       input.

       Note: The day of a command's execution can be specified in the
       following two fields — 'day of month', and 'day of week'.  If both
       fields are restricted (i.e., do not contain the "*" character), the
       command will be run when either field matches the current 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.

EXAMPLE CRON FILE
       # use /bin/sh to run commands, no matter what /etc/passwd says
       SHELL=/bin/sh
       # mail any output to `paul', no matter whose crontab this is
       MAILTO=paul
       #
       CRON_TZ=Japan
       # 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"

Jobs in /etc/cron.d/
       The jobs in cron.d and /etc/crontab are system jobs, which are used
       usually for more than one user, thus, additionally the username is
       needed.  MAILTO on the first line is optional.

EXAMPLE OF A JOB IN /etc/cron.d/job
       #login as root
       #create job with preferred editor (e.g. vim)
       MAILTO=root
       * * * * * root touch /tmp/file

SELinux with multi level security (MLS)
       In a crontab, it is important to specify a security level by crontab -s
       or specifying the required level on the first line of the crontab.
       Each level is specified in /etc/selinux/targeted/seusers.  When using
       crontab in the MLS mode, it is especially important to:
       - check/change the actual role,
       - set correct role for directory, which is used for input/output.

EXAMPLE FOR SELINUX MLS
       # login as root
       newrole -r sysadm_r
       mkdir /tmp/SystemHigh
       chcon -l SystemHigh /tmp/SystemHigh
       crontab -e
       # write in crontab file
       MLS_LEVEL=SystemHigh
       0-59 * * * * id -Z > /tmp/SystemHigh/crontest

FILES
       /etc/crontab main system crontab file.  /var/spool/cron/ a directory
       for storing crontabs defined by users.  /etc/cron.d/ a directory for
       storing system crontabs.

SEE ALSO
       cron(8), crontab(1)

EXTENSIONS
       These special time specification "nicknames" which replace the 5
       initial time and date fields, and are prefixed with the '@' character,
       are supported:

       @reboot    :    Run once after reboot.
       @yearly    :    Run once a year, ie.  "0 0 1 1 *".
       @annually  :    Run once a year, ie.  "0 0 1 1 *".
       @monthly   :    Run once a month, ie. "0 0 1 * *".
       @weekly    :    Run once a week, ie.  "0 0 * * 0".
       @daily     :    Run once a day, ie.   "0 0 * * *".
       @hourly    :    Run once an hour, ie. "0 * * * *".

CAVEATS
       crontab files have to be regular files or symlinks to regular files,
       they must not be executable or writable for anyone else but the owner.
       This requirement can be overridden by using the -p option on the crond
       command line.  If inotify support is in use, changes in the symlinked
       crontabs are not automatically noticed by the cron daemon.  The cron
       daemon must receive a SIGHUP signal to reload the crontabs.  This is a
       limitation of the inotify API.

       cron requires that each entry in a crontab end in a newline character.
       If the last entry in a crontab is missing a newline (i.e. terminated by
       EOF), cron will consider the crontab (at least partially) broken.  A
       warning will be written to syslog.

AUTHOR
       Paul Vixie ⟨vixie@isc.org⟩



cronie                            2012-11-22                        CRONTAB(5)