crontab
NAME
crontab - tables for driving cron
DESCRIPTION
A
crontab
file contains instructions to the
R cron (8)
daemon of the general form: ``run this command at this time on this date''.
Each user has their own crontab, and commands in any given crontab will be
executed as the user who owns the crontab. Uucp and News will usually have
their own crontabs, eliminating the need for explicitly running
R su (1)
as part of a cron command.
Blank lines and leading spaces and tabs are ignored. Lines whose first
non-space character is a hash-sign (#) are comments, and are ignored.
Note that comments are not allowed on the same line as cron 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 will be either an environment setting or a cron
command. An environment setting is of the form,
name = value
where the spaces around the equal-sign (=) are optional, and any subsequent
non-leading spaces in
value
will be part of the value assigned to
R name .
The
value
string may be placed in quotes (single or double, but matching) to preserve
leading or trailing blanks. The
value
string is
not
parsed for environmental substitutions, thus lines like
PATH = $HOME/bin:$PATH
will not work as you might expect.
Several environment variables are set up
automatically by the
R cron (8)
daemon.
SHELL is set to /bin/sh, and LOGNAME and HOME are set from the /etc/passwd
line of the crontab's owner. PATH is set to "/usr/bin:/bin".
HOME, SHELL, and PATH may be overridden by settings in the crontab;
LOGNAME is the user that the job is running from, and may not be changed.
(Another note: the LOGNAME variable is sometimes called USER on BSD systems...
on these systems, USER will be set also.)
In addition to LOGNAME, HOME, and SHELL,
R cron (8)
will look at MAILTO if it has any reason to send mail as a result of running
commands in ``this'' crontab. 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.
On the Debian GNU/Linux system, cron supports the
pam_env
module, and loads the environment specified by
R /etc/security/pam_env.conf .
However, the PAM setting do
NOT
override the settings described above nor any settings in the
crontab
file itself. Note in particular that if you want a PATH other than
"/usr/bin:/bin", you will need to set it in the crontab file.
By default, cron will send mail using the mail "Content-Type:" header of
"text/plain" with the "charset=" parameter set to the charmap / codeset of the
locale in which
R crond(8)
is started up - ie. either the default system locale, if no LC_* environment
variables are set, or the locale specified by the LC_* environment variables
( see
R locale(7) ).
You can use different character encodings for mailed cron job output by
setting the CONTENT_TYPE and CONTENT_TRANSFER_ENCODING variables in crontabs,
to the correct values of the mail headers of those names
The format of a cron command is very much the V7 standard, with a number of
upward-compatible extensions. Each line has five time and date fields,
followed by a command, followed by a newline character ('\n').
The system crontab (/etc/crontab) uses the same format, except that
the username for the command is specified after the time and
date fields and before the command. The fields may be separated
by spaces or tabs.
Commands are executed by
R cron (8)
when the minute, hour, and month of year 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 (see ``Note'' below).
R cron (8)
examines cron entries once every minute.
The time and date fields are:
field allowed values
----- --------------
minute 0-59
hour 0-23
day of month 1-31
month 1-12 (or names, see below)
day of week 0-7 (0 or 7 is Sun, or use names)
----- --------------
minute 0-59
hour 0-23
day of month 1-31
month 1-12 (or names, see below)
day of week 0-7 (0 or 7 is Sun, or use names)
A field may be an asterisk (*), which always stands for ``first-last''.
Ranges of numbers are allowed. Ranges are two numbers separated
with a hyphen. The specified range is inclusive. For example,
8-11 for an ``hours'' entry specifies execution at hours 8, 9, 10
and 11.
Lists are allowed. A list is a set of numbers (or ranges)
separated by commas. Examples: ``1,2,5,9'', ``0-4,8-12''.
Step values can be used in conjunction with ranges. Following
a range with ``/<number>'' specifies skips of the number's value
through the range. For example, ``0-23/2'' can be used in the hours
field to specify command execution every other hour (the alternative
in the V7 standard is ``0,2,4,6,8,10,12,14,16,18,20,22''). Steps are
also permitted after an asterisk, so if you want to say ``every two
hours'', just use ``*/2''.
Names can also be used for 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 ``sixth'' field (the rest of the line) specifies 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 file.
Percent-signs (%) in the command, unless escaped with backslash
(\), will be changed into newline characters, and all data
after the first % will be sent to the command as standard
input. There is no way to split a single command line onto multiple
lines, like the shell's trailing "\".
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.
``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 once a year, "0 0 1 1 *".
@annually (same as @yearly)
@monthly Run once a month, "0 0 1 * *".
@weekly Run once a week, "0 0 * * 0".
@daily Run once a day, "0 0 * * *".
@midnight (same as @daily)
@hourly Run once an hour, "0 * * * *".
------ -------
@reboot Run once, at startup.
@yearly Run once a year, "0 0 1 1 *".
@annually (same as @yearly)
@monthly Run once a month, "0 0 1 * *".
@weekly Run once a week, "0 0 * * 0".
@daily Run once a day, "0 0 * * *".
@midnight (same as @daily)
@hourly Run once an hour, "0 * * * *".
EXAMPLE CRON FILE
# use /bin/bash to run commands, instead of the default /bin/sh SHELL=/bin/bash # 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"
EXAMPLE SYSTEM CRON FILE
This has the username field, as used by /etc/crontab.
# /etc/crontab: system-wide crontab # Unlike any other crontab you don't have to run the `crontab' # command to install the new version when you edit this file # and files in /etc/cron.d. These files also have username fields, # that none of other the crontabs do. SHELL=/bin/sh PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin # m h dom mon dow user command 42 6 * * * root run-parts --report /etc/cron.daily 47 6 * * 7 root run-parts --report /etc/cron.weekly 52 6 1 * * root run-parts --report /etc/cron.monthly # # Removed invocation of anacron, as this is now handled by a # /etc/cron.d file
SEE ALSO
cron(8), crontab(1)
EXTENSIONS
When specifying day of week, both day 0 and day 7 will be considered Sunday.
BSD and ATT seem to disagree about this.
Lists and ranges are allowed to co-exist in the same field. "1-3,7-9" would
be rejected by ATT or BSD cron -- they want to see "1-3" or "7,8,9" ONLY.
Ranges can include "steps", so "1-9/2" is the same as "1,3,5,7,9".
Months or days of the week can be specified by name.
Environment variables can be set in the crontab. In BSD or ATT, the
environment handed to child processes is basically the one from /etc/rc.
Command output is mailed to the crontab owner (BSD can't do this), can be
mailed to a person other than the crontab owner (SysV can't do this), or the
feature can be turned off and no mail will be sent at all (SysV can't do this
either).
All of the `@' commands that can appear in place of the first five fields
are extensions.
AUTHOR
Paul Vixie <paul@vix.com>
-
- JavaScript Developer with C knowledge
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