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)