Ubuntu Manpage: crontab - tables for driving systemd-cron

NAME

       crontab - tables for driving systemd-cron

DESCRIPTION

A crontab file contains instructions to systemd-cron of the general form: ``run this
command at this time on this date''. Each user has their own crontab, and commands in any
given crontab will be executed as the user who owns the crontab.

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, comments are not allowed on the same line as environment variable
settings.

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 subsequent 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.
The value string is not parsed for environmental 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 substitution for the defined variables in the last value.

In PATH, tilde-expansion is performed on elements starting with "~/", so this works as
expected:

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

Special variables:

SHELL, PATH, USER, LOGNAME, HOME, LANG
Those are set up automatically by systemd itself, see systemd.exec(5) SHELL
defaults to /bin/sh. SHELL and PATH may be overridden by settings in the crontab.

MAILTO
When sending output from a job, systemd.cron(7) will look at MAILTO. If MAILTO is
defined mail is sent to this email address. MAILTO may also be used to direct mail
to multiple recipients by separating recipient users with a comma. If MAILTO is
defined but empty (MAILTO=""), no mail will be sent. Otherwise mail is sent to the
owner of the crontab.
By default, this mail contains systemctl status and the full log for the failed
run, copied from the journal.

MAILFROM
When sending output from a job, systemd.cron(7) will look at MAILFROM. If MAILFROM
is defined mail is sent from this email address. Otherwise is seen as being sent
from root@HOSTNAME.

CRON_MAIL_SUCCESS
Control if (when) to send mail with output from successful jobs.
'nonempty', 'non-empty': mail is only sent if the job left anything in the journal
(i.e. wrote something to the standard output or error streams); this is the
default, and matches classic cron
'always', 'yes', 'true', '1': always send mail
'never', 'no', 'false', '0': never send mail for a successful job

Mail is always sent for failed jobs.

CRON_MAIL_FORMAT
Control the format of the content of cron-job-related messages.
'normal': systemctl status + journalctl output (incl. time, process names, the
usual) for the run; this is the default
'nometadata', 'no-metadata': raw journal contents (-o cat: just standard output +
error streams); this matches classic cron

CRON_MAIL_SUCCESS and CRON_MAIL_FORMAT, if changed in /etc/crontab, are remembered
for all other crontabs (/etc/cron.d, /etc/anacron, users' crontabs) and act as an
administrator-controlled default. They can be set to 'inherit' to get that default
back.

RANDOM_DELAY
(in minutes) environment variable is translated to RandomizedDelaySec=.

DELAY (in minutes) environment variable is translated to OnBootSec=. This works like the
'delay' field of anacrontab(5) and make systemd wait # minutes after boot before
starting the unit. This value can also be used to spread out the start times of
@daily/@weekly/@monthly... jobs on a 24/24 system.

START_HOURS_RANGE
(in hours) environment variable is translated to the 'hour' component of
OnCalendar=. This variable is inherited from anacrontab(5), but also supported in
crontab(5) by systemd-crontab-generator. Anacron expect a time range in the START-
END format (eg: 6-9), systemd-crontab-generator will only use the starting hour of
the range as reference. Unless you set this variable, all the
@daily/@weekly/@monthly/@yearly jobs will run at midnight. If you set this variable
and the system was off during the ours defined in the range, the (persistent) job
will start at boot.

PERSISTENT
With this flag, you can override the generator default heuristic.
'yes': force all further jobs to be persistent
'auto': only recognize @ keywords to be persistent
'no': force all further jobs not to be persistent

TZ, CRON_TZ
The job is scheduled in this time-zone instead of in the system time-zone. Must be
a full IANA time-zone name (as found under /usr/share/zoneinfo), or empty to reset
to the default timezone, otherwise no special semantics. Always passed to the job.

BATCH This boolean flag is translated to options CPUSchedulingPolicy=idle and
IOSchedulingClass=idle when set.

The format of a cron command is the same as the one defined by the cron daemon. Each line
has five time and date fields, followed by a command, followed by a newline character
('\n'). The system crontab (/etc/crontab) and the packages crontabs (/etc/cron.d/*) use
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.

Commands are executed by systemd 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). 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.

A random value (within the legal range) may be obtained by using the `~' character in a
field. The interval of the random value may be specified explicitly, for example ``0~30''
will result in a random value between 0 and 30 inclusive. If either (or both) of the
numbers on either side of the `~' are omitted, the appropriate limit (low or high) for the
field will be used.

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 every other hour (the alternative in
the V7 standard 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''.

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 doesn't matter). Ranges or lists of names
are not allowed.

The ``sixth'' field (the rest of the line) specifies the command to be run. The entire
command portion of the line will be executed by /bin/sh or by the shell specified in the
SHELL variable of the crontab file.

If the command contains an unescaped % character, it is instead split thereon: the part
before is run by the shell, the part after is given on the standard input stream, with
each subsequent % replaced by a newline. %s can be escaped as \%, and produce a literal
%.

systemd-crontab-generator doesn't handle multi-line command split by the % character like
vixie-cron.

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 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. One can, however, achieve the desired result by adding a test to
the command (see the last example 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, 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 errors 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
       15 14 1 * *     $HOME/bin/monthly
       # run at 10 pm on weekdays, annoy Joe
       # runs 'mail -s "It's 10 pm" joe', with 'Joe,\n\nWhere are your kids?\n' on stdin
       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. Unlike a user's
       crontab, this file has the username field, as used by /etc/crontab.

       # /etc/crontab: system-wide crontab
       # Unlike any other crontab you don't have to run the `crontab'
       # command to install the new version when you edit this file
       # and files in /etc/cron.d. These files also have username fields,
       # that none of the other crontabs do.

       SHELL=/bin/sh
       PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

       # m h dom mon dow usercommand
       17 * * * *  root  cd / && run-parts --report /etc/cron.hourly
       25 6 * * *  root  test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
       47 6 * * 7  root  test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )
       52 6 1 * *  root  test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )
       #

       This is only an example, systemd-cron uses native units instead for those jobs.
       If you add those lines, your jobs will run twice.

LIMITATIONS

       The crontab syntax does not make it possible to define  all  possible  periods  one  could
       imagine off. For example, it is not straightforward to define the last weekday of a month.
       If a task needs to be run in a specific 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.

       systemd-crontab-generator doesn't support these vixie-cron features:

       *      spawning forking daemons, the 'Service' units are all set with 'Type=oneshot'

       *      vixie-cron requires that each entry in a crontab end in a newline character. If the
              last  entry  in  a crontab is missing a newline (ie, terminated by EOF), vixie-cron
              will consider the crontab (at least partially) broken.
              systemd-crontab-generator considers this crontab as valid

DIAGNOSTICS

       You can see how your crontab where translated by typing:
       systemctl cat cron-<userid>-*

       systemctl cat does support command-line completion.

AUTHOR

       Paul Vixie <paul@vix.com> is the author of cron and original creator of this manual  page.
       This  page has also been modified for Debian by Steve Greenland, Javier Fernandez-Sanguino
       and Christian Kastner.
       This page has been reworded by Alexandre Detiste for inclusion in systemd-cron.