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CRONTAB(1) General Commands Manual CRONTAB(1)
NAME
crontab - maintain crontab files for individual users (Vixie Cron)
SYNOPSIS
crontab [ -u user ] file
crontab [ -u user ] [ -i ] { -e | -l | -r }
DESCRIPTION
crontab is the program used to install, deinstall or list the tables
used to drive the cron(8) daemon in Vixie Cron. Each user can have
their own crontab, and though these are files in
/var/spool/cron/crontabs, they are not intended to be edited directly.
If the /etc/cron.allow file exists, then you must be listed (one user
per line) therein in order to be allowed to use this command. If the
/etc/cron.allow file does not exist but the /etc/cron.deny file does
exist, then you must not be listed in the /etc/cron.deny file in order
to use this command.
If neither of these files exists, then depending on site-dependent con‐
figuration parameters, only the super user will be allowed to use this
command, or all users will be able to use this command.
If both files exist then /etc/cron.allow takes precedence. Which means
that /etc/cron.deny is not considered and your user must be listed in
/etc/cron.allow in order to be able to use the crontab.
Regardless of the existance of any of these files, the root administra‐
tive user is always allowed to setup a crontab. For standard Debian
systems, all users may use this command.
If the -u option is given, it specifies the name of the user whose
crontab is to be used (when listing) or modified (when editing). If
this option is not given, crontab examines "your" crontab, i.e., the
crontab of the person executing the command. Note that su(8) can con‐
fuse crontab and that if you are running inside of su(8) you should
always use the -u option for safety‘s sake.
The first form of this command is used to install a new crontab from
some named file or standard input if the pseudo-filename ``-‘‘ is
given.
The -l option causes the current crontab to be displayed on standard
output. See the note under DEBIAN SPECIFIC below.
The -r option causes the current crontab to be removed.
The -e option is used to edit the current crontab using the editor
specified by the VISUAL or EDITOR environment variables. After you
exit from the editor, the modified crontab will be installed automati‐
cally. If neither of the environment variables is defined, then the
default editor /usr/bin/editor is used.
The -i option modifies the -r option to prompt the user for a ‘y/Y‘
response before actually removing the crontab.
DEBIAN SPECIFIC
The "out-of-the-box" behaviour for crontab -l is to display the three
line "DO NOT EDIT THIS FILE" header that is placed at the beginning of
the crontab when it is installed. The problem is that it makes the
sequence
crontab -l | crontab -
non-idempotent -- you keep adding copies of the header. This causes
pain to scripts that use sed to edit a crontab. Therefore, the default
behaviour of the -l option has been changed to not output such header.
You may obtain the original behaviour by setting the environment vari‐
able CRONTAB_NOHEADER to ‘N‘, which will cause the crontab -l command
to emit the extraneous header.
SEE ALSO
crontab(5), cron(8)
FILES
/etc/cron.allow
/etc/cron.deny
/var/spool/cron/crontabs
There is one file for each user‘s crontab under the
/var/spool/cron/crontabs directory. Users are not allowed to edit the
files under that directory directly to ensure that only users allowed
by the system to run periodic tasks can add them, and only syntacti‐
cally correct crontabs will be written there. This is enforced by hav‐
ing the directory writable only by the crontab group and configuring
crontab command with the setgid bid set for that specific group.
STANDARDS
The crontab command conforms to IEEE Std1003.2-1992 (``POSIX‘‘). This
new command syntax differs from previous versions of Vixie Cron, as
well as from the classic SVR3 syntax.
DIAGNOSTICS
A fairly informative usage message appears if you run it with a bad
command line.
cron requires that each entry in a crontab end in a newline character.
If the last entry in a crontab is missing the newline, cron will con‐
sider the crontab (at least partially) broken and refuse to install it.
AUTHOR
Paul Vixie <paul@vix.com> is the author of cron and original creator of
this manual page. This page has also been modified for Debian by Steve
Greenland, Javier Fernandez-Sanguino and Christian Kastner.
4th Berkeley Distribution 19 April 2010 CRONTAB(1)
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 gen‐
eral form: ``run this command at this time on this date‘‘. Each user
has their own crontab, and commands in any given crontab will be exe‐
cuted as the user who owns the crontab. Uucp and News will usually
have their own crontabs, eliminating the need for explicitly running
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, com‐
ments are not allowed on the same line as environment variable set‐
tings.
An active line in a crontab will be either an environment setting or a
cron command. The crontab file is parsed from top to bottom, so any
environment settings will affect only the cron commands below them in
the file. An environment setting is of the form,
name = value
where the spaces around the equal-sign (=) are optional, and any subse‐
quent 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. To define an empty
variable, quotes must be used. The value string is not parsed for envi‐
ronmental substitutions or replacement of variables, thus lines like
PATH = $HOME/bin:$PATH
will not work as you might expect. And neither will this work
A=1
B=2
C=$A $B
There will not be any subsitution for the defined variables in the last
value.
An alternative for setting up the commands path is using the fact that
many shells will treat the tilde(~) as substitution of $HOME, so if you
use bash for your tasks you can use this:
SHELL=/bin/bash
PATH=~/bin:/usr/bin/:/bin
Several environment variables are set up automatically by the 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, 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. MAILTO may also be used to direct mail to multi‐
ple recipients by separating recipient users with a comma. 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 /etc/environment and /etc/secu‐
rity/pam_env.conf. It also reads locale information from
/etc/default/locale. However, the PAM settings 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 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 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 num‐
ber 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 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). 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)
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 spec‐
ify command execution every other hour (the alternative in the V7 stan‐
dard 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 charac‐
ters, 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 cur‐
rent 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. One can, however, achieve
the desired result by adding a test to the command (see the last exam‐
ple in EXAMPLE CRON FILE below).
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 * * * *".
Please note that startup, as far as @reboot is concerned, is the time
when the cron(8) daemon startup. In particular, it may be before some
system daemons, or other facilities, were startup. This is due to the
boot order sequence of the machine.
EXAMPLE CRON FILE
The following lists an example of a user crontab 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"
# Run on every second Saturday of the month
0 4 8-14 * * test $(date +\%u) -eq 6 && echo "2nd Saturday"
EXAMPLE SYSTEM CRON FILE
The following lists the content of a regular system-wide crontab file.
Unlinke a user‘s crontab, this file 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 the other crontabs do.
SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
# m h dom mon dow usercommand
17 * * * * root cd / && run-parts --report /etc/cron.hourly
25 6 * * * root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
47 6 * * 7 root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )
52 6 1 * * root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )
#
SEE ALSO
cron(8), crontab(1)
EXTENSIONS
When specifying day of week, both day 0 and day 7 will be considered
Sunday. BSD and AT&T 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 AT&T 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 AT&T, 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.
LIMITATIONS
The cron daemon runs with a defined timezone. It currently does not
support per-user timezones. All the tasks: system‘s and user‘s will be
run based on the configured timezone. Even if a user specifies the TZ
environment variable in his crontab this will affect only the commands
executed in the crontab, not the execution of the crontab tasks them‐
selves.
The crontab syntax does not make it possible to define all possible
periods one could image off. For example, it is not straightforward to
define the last weekday of a month. If a task needs to be run in a spe‐
cific period of time that cannot be defined in the crontab syntaxs the
best approach would be to have the program itself check the date and
time information and continue execution only if the period matches the
desired one.
If the program itself cannot do the checks then a wrapper script would
be required. Useful tools that could be used for date analysis are ncal
or calendar For example, to run a program the last Saturday of every
month you could use the following wrapper code:
0 4 * * Sat [ "$(date +\%e)" = "`ncal | grep $(date +\%a | sed -e ‘s/.$//‘) | sed -e ‘s/^.*\s\([0-9]\+\)\s*$/\1/‘`" ] && echo "Last Saturday" && program_to_run
DIAGNOSTICS
cron requires that each entry in a crontab end in a newline character.
If the last entry in a crontab is missing a newline (ie, terminated by
EOF), cron will consider the crontab (at least partially) broken. A
warning will be written to syslog.
AUTHOR
Paul Vixie <paul@vix.com> is the author of cron and original creator of
this manual page. This page has also been modified for Debian by Steve
Greenland, Javier Fernandez-Sanguino and Christian Kastner.
4th Berkeley Distribution 19 April 2010 CRONTAB(5)
标签：des os io ar for art div cti sp
原文地址：http://www.cnblogs.com/IceSword-syy/p/3956910.html
