NAME

crontab — tables for driving cron

DESCRIPTION

A crontab file contains instructions to the cron(8) daemon of the general form: “at these times on these dates run this command”. There may be a system crontab and each user may have their own crontab. Commands in any given crontab will be executed either as the user who owns the crontab or, in the case of the system crontab, as the user specified on the command line.

While a crontab is a text file, it is not intended to be directly edited. Creation, modification, and removal of a crontab should be done using crontab(1).

Blank lines, leading spaces, and tabs are ignored. Lines whose first non-space character is a pound sign (‘#’) are comments, and are ignored. Note that comments are not allowed on the same line as cron(8) 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 is either an environment variable setting or a cron(8) command.

Environment variable settings create the environment any command in the crontab is run in. An environment variable setting is of the form:

name = value

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.

Lines in the system crontab have six fixed fields, an optional flags field, and a command, in the form:

minute hour day-of-month month day-of-week user [flags] command

While lines in a user crontab have five fixed fields, an optional flags field, and a command, in the form:

minute hour day-of-month month day-of-week [flags] command

Fields are separated by blanks or tabs. The command may be one or more fields long. The allowed values for the fields are:

field	allowed values
`minute`	*, ~, or 0–59
`hour`	*, ~, or 0–23
`day-of-month`	*, ~, or 1–31
`month`	*, ~, 1–12, or a name (see below)
`day-of-week`	*, ~, 0–7, or a name (0 or 7 is Sunday)
`user`	a valid username
[`flags`]	runtime flags, denoted with '-'
`command`	text

Lists are allowed. A list is a set of numbers (or ranges) separated by commas. For example, “1,2,5,9” or “0–4,8–12”.

Ranges of numbers are allowed. Ranges are two numbers separated with a hyphen. The specified range is inclusive. For example, 8–11 for an hour entry specifies execution at hours 8, 9, 10 and 11.

A random value for a field may be obtained using the ‘~’ character. A value is generated every time the tab is loaded. On its own, it denotes a random value appropriate for the field. It can also be used in a range to make the interval more specific. If either of the numbers in a range are omitted, the appropriate limit (low or high) for that field will be used. For example, both “0~30” and “~30” in the minute field would result in a random value between 0 and 30.

Step values can be used in conjunction with ranges. Following a range with /number specifies skips of number through the range. For example, “0–23/2” can be used in the hour field to specify command execution every other hour. Steps are also permitted after an asterisk, so to say “every two hours”, just use “*/2”. A step value after a random range will execute the command at a random offset less than the step size. For example, to avoid a thundering herd at the top and bottom of the hour, “0~59/30” (or simply “~/30”) can be used in the minute field to specify that command execution happen twice an hour at consistent intervals.

An asterisk (‘*’) is short form for a range of all allowed values.

Names can be used in 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.

Some flags relating to process operation can be provided before the command field. Flags are denoted with '-' and may be combined.

-n command: No mail is sent after a successful run. The execution output will only be mailed if the command exits with a non-zero exit code. The -n option is an attempt to cure potentially copious volumes of mail coming from cron(8).
-q command: Execution will not be logged.
-s command: Only a single instance of command will be run concurrently. Additional instances of command will not be scheduled until the earlier one completes.

The command field (the rest of the line) is the command to be run. The entire command portion of the line, up to a newline or % character, will be executed by /bin/sh or by the shell specified in the SHELL variable of the crontab. Percent signs (‘%’) 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.

Commands are executed by cron(8) when the minute, hour, and month 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.

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.

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

string	meaning
@reboot	Run once, at startup.
@yearly	Run every January 1 (0 0 1 1 *).
@annually	The same as @yearly.
@monthly	Run the first day of every month (0 0 1 * *).
@weekly	Run every Sunday (0 0 * * 0).
@daily	Run every midnight (0 0 * * *).
@midnight	The same as @daily.
@hourly	Run every hour, on the hour (0 * * * *).

ENVIRONMENT

HOME: Set from the user's /etc/passwd entry. May be overridden by settings in the crontab.
LOGNAME: Set from the user's /etc/passwd entry. May not be overridden by settings in the crontab.
MAILTO: If MAILTO is defined and non-empty, mail is sent to the user so named. If MAILTO is defined but empty (MAILTO = ""), no mail will be sent. Otherwise mail is sent to the owner of the crontab. This is useful for pseudo-users that lack an alias that would otherwise redirect the mail to a real person.
SHELL: Set to /bin/sh. May be overridden by settings in the crontab.
USER: Set from the user's /etc/passwd entry. May not be overridden by settings in the crontab.

FILES

/etc/crontab: System crontab.
/var/cron/tabs/⟨user⟩: User crontab.

EXAMPLES

# 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
#
# 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 -- job output will be sent
# to paul, but only if $HOME/bin/monthly exits with a non-zero exit code
15 14 1 * *     -n $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"

# run hourly at a random time within the first 30 minutes of the hour
0~30 * * * *   /usr/libexec/spamd-setup

STANDARDS

The crontab file format is compliant with the IEEE Std 1003.1-2008 (“POSIX.1”) specification. The behaviours described below are all extensions to that standard:

The day-of-week field may use 7 to represent Sunday.
Ranges may include “steps”.
Random intervals are supported using the ‘~’ character.
Months or days of the week can be specified by name.
Environment variables can be set in a crontab.
Command output can be mailed to a person other than the crontab owner, or the feature can be turned off and no mail will be sent at all.
All of the ‘@’ commands that can appear in place of the first five fields.
All of the [-nqs] flags.

AUTHORS

crontab was written by Paul Vixie <vixie@isc.org>.