NAME

crontab - schedule periodic background work

SYNOPSIS

crontab [file]

[UP] crontab [-e|-l|-r]


DESCRIPTION

The crontab utility shall create, replace, ^[UP]  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, ^[UP]  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.1-2017, the value shall be a pathname for sh.

The values of these variables when crontab is invoked as specified by this volume of POSIX.1-2017 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.

^[XSI] 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 XBD Utility Syntax Guidelines.

The following options shall be supported:

-e

^[UP] 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-minus> (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.

^[XSI] 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 XBD 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

^[XSI] 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, ^[UP]  edited,  or listed.

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.1-2017" 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.

SEE ALSO

at

XBD Environment Variables, Utility Syntax Guidelines

CHANGE HISTORY

First released in Issue 2.

Issue 6

This utility is marked as part of the User Portability Utilities option.

The normative text is reworded to avoid use of the term "must" for application requirements.

Issue 7

The crontab utility (except for the -e option) is moved from the User Portability Utilities option to the Base. User Portability Utilities is now an option for interactive utilities.

SD5-XCU-ERN-95 is applied, removing the references to fixed locations for the files referenced by the crontab utility.

SD5-XCU-ERN-97 is applied, updating the SYNOPSIS.

The first example is changed to remove the unreliable use of find | xargs.

POSIX.1-2008, Technical Corrigendum 2, XCU/TC2-2008/0079 [584] is applied.