Ubuntu Manpage: crontab - tables for driving systemd-cron

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description
example cron file
example system cron file
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limitations
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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.