Special variables¶

SHELL, PATH, USER, LOGNAME, HOME, LANG

Those are set up automatically by systemd, 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 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 set but empty (MAILTO="", MAILTO=), no mail will be sent. Otherwise mail is sent to the crontab's owner.
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 defined, mail is sent from this email address. Otherwise it's seen as being sent by "root".

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.

CRON_INHERIT_VARIABLES

In the top-level /etc/crontab: a white-space-separated list of variables (including control statements that get removed from the environment otherwise) to remember into other crontabs (/etc/cron.d, users' crontabs; not /etc/anacron). This allows instituting a global RANDOM_DELAY/SHELL/&c. default policy.
Elsewhere: ignored.

RANDOM_DELAY

(in minutes) environment variable translated to RandomizedDelaySec=.

DELAY

(in minutes) environment variable translated to OnBootSec=. This works like the anacrontab(5) delay and makes systemd wait the given amount of minutes after boot before starting the unit. This value can also be used to spread out the start times of @daily/@weekly/@monthly/&c. jobs on an always-on system.

START_HOURS_RANGE

(in hours) environment variable translated to the hour component of OnCalendar=. This variable is inherited from anacrontab(5), but also supported in crontab(5)s by systemd-crontab-generator(8). anacron(8) expects a time range like "start-end", but systemd-crontab-generator(8) only uses the starting hour of the range as reference. Unless you set this variable, all @daily/@weekly/@monthly/&c. jobs will run at midnight. If you do set this variable and the system was off during the hours defined in the range, persistent jobs will start at boot.

PERSISTENT

This boolean flag can override the generator's default heuristic:

yes

force all further jobs to be persistent

auto

only recognize @ keywords to be persistent (this is the default)

no

force all further jobs to not 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.

CRON_BATCH_LOADAVG_BELOW

If set and nonempty, delay starting the job until the 1-minute system load average drops below the set value. All jobs using this option join a global queue scheduling a random eligible job every at-least-30 seconds.

CRON_BATCH_THROTTLE_GROUP

If set and nonempty, all jobs with the same value form a group where no two jobs can run concurrently and no job is started within 5 minutes of another exiting.
If combined with CRON_BATCH_LOADAVG_BELOW, the job joins its CRON_BATCH_THROTTLE_GROUP queue only, but the load threshold still applies.
See EXAMPLES, CRON_BATCH_….

The format of a cron command¶

is the same as the one defined by the classic cron daemon. Each line has five time and date fields, followed by a command, followed by a new-line character. 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 between the time/date fields and the command. Fields may be separated by spaces or tabs.

Commands are executed by systemd(1) 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:

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 tilde (‘~’) character instead of the hyphen. 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 the sides 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 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 Version 7 AT&T UNIX standard form 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 at least the first three letters of the particular day or month (case doesn't matter). Ranges or lists of names are not allowed.

The rest of the line, after the fields, 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 percent (‘%’) 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 new-line. %s can be escaped as "\%", and produce a literal %.

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,

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

Please note that startup, as far as @reboot is concerned, may be before some system daemons, or other facilities, were started.

User's crontab¶

# 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 * * *       ~/bin/daily.job >> ~/tmp/out 2>&1
# run at 2:15pm on the first of every month
15 14 1 * *     ~/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"

System 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 user  command
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-crontab-generator(8) uses native units listed in systemd.cron(7) for those jobs instead; if you add those lines, your jobs may run twice.

CRON_BATCH_…¶

CRON_BATCH_THROTTLE_GROUP=first
 0 0 * * * sleep  2m # a
 0 0 * * * sleep  4m # b
 0 0 * * * sleep  4m # c
17 0 * * * sleep  2m # L

CRON_BATCH_THROTTLE_GROUP=second
 2 0 * * * sleep  3m # q
 2 0 * * * sleep 11m # w

may result in any of the following job runs (horizontal axis is time, one minute per column, other jobs unaffected):

00   05   10   15   20   25   30
aa     cccc     bbbb     LL
  qqq     wwwwwwwwwww

aa     cccc     bbbb     LL
  qqq     wwwwwwwwwww

cccc     bbbb     LL     aa
  wwwwwwwwwww     qqq

cccc     aa     bbbb     LL
  wwwwwwwwwww     qqq