CRONTAB(5)

CRONTAB(5) | File Formats Manual | CRONTAB(5)

# 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 plus a command, in the form:

minute hour day-of-month month day-of-week user command

While lines in a user `crontab` have five fixed fields plus a command, in the form:

minute hour day-of-month month day-of-week 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 | * or 1–12 or a name (see below)
day-of-week | * or 0–7 or a name (0 or 7 is Sunday)
user | a valid username
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 field may begin with a question mark (‘?’), which indicates a single value randomly selected when the crontab file is read. If the field contains only a question mark, the value is randomly selected from the range of all possible values for the field. If the question mark precedes a range, the value is randomly selected from the range. For example, “? ?2-5 * * *” specifies that a task will be performed daily between 2:00am and and 5:59am at a time randomly selected when the crontab file is first read. As just one example, this feature can be used to prevent a large number of hosts from contacting a server simultaneously and overloading it by staggering the time at which a download script is executed.

Step values can be used in conjunction with ranges (but not random ranges which represent a single number). 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”.

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.

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 may be modified as follows:

`-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.

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

`CRON_TZ`
  The `CRON_TZ` variable can be set to an alternate time zone in order to affect when the job is run. Note that this only affects the scheduling of the job, not the time zone that the job perceives when it is run. If `CRON_TZ` is defined but empty (`CRON_TZ`=""), jobs are scheduled with respect to the local time zone.
`CRON_WITHIN`
  The `CRON_WITHIN` variable should indicate the number of seconds within a job's scheduled time that it should still be run. For example if a job is scheduled for 12:30pm and `CRON_WITHIN` is `120` (2 minutes), then the job will not be run if the system attempts to start it past 12:32pm. On a heavily loaded system, or on a system that has just been “woken up”, jobs will sometimes start later than originally intended, and by skipping non-critical jobs because of delays, system load can be lightened. If `CRON_WITHIN` is defined but empty (`CRON_WITHIN`=""), or set to some non-positive value (0, a negative number, or a non-numeric string), it is treated as if it was unset, that is the job will always run, even if it is going to start at a time long past its scheduled time.
`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 -- 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"

# SEE ALSO

crontab(1), cron(8)

# 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”.
- Months or days of the week can be specified by name.
- Mailing after a successful run can be suppressed with `-n`.
- Logging can be suppressed with `-q`.
- 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.

# AUTHORS

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

February 26, 2022 | NetBSD 10.1