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PROGRAM:

NAME


crontab — schedule periodic background work

SYNOPSIS


crontab [file]

crontab [−e|−l|−r]

DESCRIPTION


The crontab utility shall create, replace, or edit a user's crontab entry; a crontab entry
is a list of commands and the times at which they shall be executed. The new crontab entry
can be input by specifying file or input from standard input if no file operand is
specified, or by using an editor, if −e is specified.

Upon execution of a command from a crontab entry, the implementation shall supply a
default environment, defining at least the following environment variables:

HOME A pathname of the user's home directory.

LOGNAME The user's login name.

PATH A string representing a search path guaranteed to find all of the standard
utilities.

SHELL A pathname of the command interpreter. When crontab is invoked as specified by
this volume of POSIX.1‐2008, the value shall be a pathname for sh.

The values of these variables when crontab is invoked as specified by this volume of
POSIX.1‐2008 shall not affect the default values provided when the scheduled command is
run.

If standard output and standard error are not redirected by commands executed from the
crontab entry, any generated output or errors shall be mailed, via an implementation-
defined method, to the user.

Users shall be permitted to use crontab if their names appear in the file cron.allow which
is located in an implementation-defined directory. If that file does not exist, the file
cron.deny, which is located in an implementation-defined directory, shall be checked to
determine whether the user shall be denied access to crontab. If neither file exists,
only a process with appropriate privileges shall be allowed to submit a job. If only
cron.deny exists and is empty, global usage shall be permitted. The cron.allow and
cron.deny files shall consist of one user name per line.

OPTIONS


The crontab utility shall conform to the Base Definitions volume of POSIX.1‐2008, Section
12.2, Utility Syntax Guidelines.

The following options shall be supported:

−e Edit a copy of the invoking user's crontab entry, or create an empty entry to
edit if the crontab entry does not exist. When editing is complete, the entry
shall be installed as the user's crontab entry.

−l (The letter ell.) List the invoking user's crontab entry.

−r Remove the invoking user's crontab entry.

OPERANDS


The following operand shall be supported:

file The pathname of a file that contains specifications, in the format defined in
the INPUT FILES section, for crontab entries.

STDIN


See the INPUT FILES section.

INPUT FILES


In the POSIX locale, the user or application shall ensure that a crontab entry is a text
file consisting of lines of six fields each. The fields shall be separated by <blank>
characters. The first five fields shall be integer patterns that specify the following:

1. Minute [0,59]

2. Hour [0,23]

3. Day of the month [1,31]

4. Month of the year [1,12]

5. Day of the week ([0,6] with 0=Sunday)

Each of these patterns can be either an <asterisk> (meaning all valid values), an element,
or a list of elements separated by <comma> characters. An element shall be either a number
or two numbers separated by a <hyphen> (meaning an inclusive range). The specification of
days can be made by two fields (day of the month and day of the week). If month, day of
month, and day of week are all <asterisk> characters, every day shall be matched. If
either the month or day of month is specified as an element or list, but the day of week
is an <asterisk>, the month and day of month fields shall specify the days that match. If
both month and day of month are specified as an <asterisk>, but day of week is an element
or list, then only the specified days of the week match. Finally, if either the month or
day of month is specified as an element or list, and the day of week is also specified as
an element or list, then any day matching either the month and day of month, or the day of
week, shall be matched.

The sixth field of a line in a crontab entry is a string that shall be executed by sh at
the specified times. A <percent-sign> character in this field shall be translated to a
<newline>. Any character preceded by a <backslash> (including the '%') shall cause that
character to be treated literally. Only the first line (up to a '%' or end-of-line) of the
command field shall be executed by the command interpreter. The other lines shall be made
available to the command as standard input.

Blank lines and those whose first non-<blank> is '#' shall be ignored.

The text files cron.allow and cron.deny, which are located in an implementation-defined
directory, shall contain zero or more user names, one per line, of users who are,
respectively, authorized or denied access to the service underlying the crontab utility.

ENVIRONMENT VARIABLES


The following environment variables shall affect the execution of crontab:

EDITOR Determine the editor to be invoked when the −e option is specified. The default
editor shall be vi.

LANG Provide a default value for the internationalization variables that are unset or
null. (See the Base Definitions volume of POSIX.1‐2008, Section 8.2,
Internationalization Variables for the precedence of internationalization
variables used to determine the values of locale categories.)

LC_ALL If set to a non-empty string value, override the values of all the other
internationalization variables.

LC_CTYPE Determine the locale for the interpretation of sequences of bytes of text data
as characters (for example, single-byte as opposed to multi-byte characters in
arguments and input files).

LC_MESSAGES
Determine the locale that should be used to affect the format and contents of
diagnostic messages written to standard error.

NLSPATH Determine the location of message catalogs for the processing of LC_MESSAGES.

ASYNCHRONOUS EVENTS


Default.

STDOUT


If the −l option is specified, the crontab entry shall be written to the standard output.

STDERR


The standard error shall be used only for diagnostic messages.

OUTPUT FILES


None.

EXTENDED DESCRIPTION


None.

EXIT STATUS


The following exit values shall be returned:

0 Successful completion.

>0 An error occurred.

CONSEQUENCES OF ERRORS


The user's crontab entry is not submitted, removed, edited, or listed.

The following sections are informative.

APPLICATION USAGE


The format of the crontab entry shown here is guaranteed only for the POSIX locale. Other
cultures may be supported with substantially different interfaces, although
implementations are encouraged to provide comparable levels of functionality.

The default settings of the HOME, LOGNAME, PATH, and SHELL variables that are given to the
scheduled job are not affected by the settings of those variables when crontab is run; as
stated, they are defaults. The text about ``invoked as specified by this volume of
POSIX.1‐2008'' means that the implementation may provide extensions that allow these
variables to be affected at runtime, but that the user has to take explicit action in
order to access the extension, such as give a new option flag or modify the format of the
crontab entry.

A typical user error is to type only crontab; this causes the system to wait for the new
crontab entry on standard input. If end-of-file is typed (generally <control>‐D), the
crontab entry is replaced by an empty file. In this case, the user should type the
interrupt character, which prevents the crontab entry from being replaced.

EXAMPLES


1. Clean up core files every weekday morning at 3:15 am:

15 3 * * 1-5 find "$HOME" −name core −exec rm −f {} + 2>/dev/null

2. Mail a birthday greeting:

0 12 14 2 * mailx john%Happy Birthday!%Time for lunch.

3. As an example of specifying the two types of days:

0 0 1,15 * 1

would run a command on the first and fifteenth of each month, as well as on every
Monday. To specify days by only one field, the other field should be set to '*'; for
example:

0 0 * * 1

would run a command only on Mondays.

RATIONALE


All references to a cron daemon and to cron files have been omitted. Although historical
implementations have used this arrangement, there is no reason to limit future
implementations.

This description of crontab is designed to support only users with normal privileges. The
format of the input is based on the System V crontab; however, there is no requirement
here that the actual system database used by the cron daemon (or a similar mechanism) use
this format internally. For example, systems derived from BSD are likely to have an
additional field appended that indicates the user identity to be used when the job is
submitted.

The −e option was adopted from the SVID as a user convenience, although it does not exist
in all historical implementations.

FUTURE DIRECTIONS


None.

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