Originální popis anglicky:
crontab - schedule periodic background work
Návod, kniha: POSIX Programmer's Manual
crontab [file]
crontab
[ -e | -l | -r
]
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
IEEE Std 1003.1-2001, the value shall be a pathname for
sh.
The values of these variables when
crontab is invoked as specified by
this volume of IEEE Std 1003.1-2001 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
/usr/lib/cron/cron.allow. If that file does not exist, the file
/usr/lib/cron/cron.deny 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.
The
crontab utility shall conform to the Base Definitions volume of
IEEE Std 1003.1-2001, 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.
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.
See the INPUT FILES section.
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>s. 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 commas. 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 asterisks, 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
/usr/lib/cron/cron.allow and
/usr/lib/cron/cron.deny 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.
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
IEEE Std 1003.1-2001, 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 .
Default.
If the
-l option is specified, the crontab entry shall be written to the
standard output.
The standard error shall be used only for diagnostic messages.
None.
None.
The following exit values shall be returned:
- 0
- Successful completion.
- >0
- An error occurred.
The user's crontab entry is not submitted, removed, edited, or listed.
The following sections are informative.
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
IEEE Std 1003.1-2001" 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.
- 1.
- Clean up core files every weekday morning at 3:15
am:
15 3 * * 1-5 find $HOME -name core 2>/dev/null | xargs rm -f
- 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:
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:
would run a command only on Mondays.
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.
None.
at
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html
.
