NAME
crontab - manipulate per-user crontabs (Dillon's Cron)
SYNOPSIS
crontab file [-u user] - replace crontab from file
crontab - [-u user] - replace crontab from stdin
crontab -l [user] - list crontab for user
crontab -e [user] - edit crontab for user
crontab -d [user] - delete crontab for user
crontab -c dir - specify crontab directory
DESCRIPTION
crontab manipulates the crontab for a particular user.
Only the superuser may specify a different user and/or
crontab directory. Generally the -e option is used to edit your crontab. crontab will use /usr/bin/vi or the
editor specified by your VISUAL environment variable to
edit the crontab.
Unlike other crond/crontabs, this crontab does not try to
do everything under the sun. Frankly, a shell script is
much more able to manipulate the environment then cron and
I see no particular reason to use the user's shell (from
his password entry) to run cron commands when this
requires special casing of non-user crontabs, such as
those for UUCP. When a crontab command is run, this
crontab runs it with /bin/sh and sets up only three envi-
ronment variables: USER, HOME, and SHELL.
crond automatically detects changes in the time. Reverse-
indexed time changes less then an hour old will NOT re-run
crontab commands already issued in the recovered period.
Forward-indexed changes less then an hour into the future
will issue missed commands exactly once. Changes greater
then an hour into the past or future cause crond to resyn-
chronize and not issue missed commands. No attempt will
be made to issue commands lost due to a reboot, and com-
mands are not reissued if the previously issued command is
still running. For example, if you have a crontab command
'sleep 70' that you wish to run once a minute, cron will
only be able to issue the command once every two minutes.
If you do not like this feature, you can run your commands
in the background with an '&'.
The crontab format is roughly similar to that used by vix-
iecron, but without complex features. Individual fields
may contain a time, a time range, a time range with a skip
factor, a symbolic range for the day of week and month in
year, and additional subranges delimited with commas.
Blank lines in the crontab or lines that begin with a hash
(#) are ignored. If you specify both a day in the month
and a day of week, the result is effectively ORd... the
crontab entry will be run on the specified day of week and
on the specified day in the month.
# MIN HOUR DAY MONTH DAYOFWEEK COMMAND
# at 6:10 a.m. every day
10 6 * * * date
# every two hours at the top of the hour
0 */2 * * * date
# every two hours from 11p.m. to 7a.m., and at 8a.m.
0 23-7/2,8 * * * date
# at 11:00 a.m. on the 4th and on every mon, tue, wed
0 11 4 * mon-wed date
# 4:00 a.m. on january 1st
0 4 1 jan * date
# once an hour, all output appended to log file
0 4 1 jan * date >>/var/log/messages 2>&1
The command portion of the line is run with /bin/sh -c
<command> and may therefore contain any valid bourne shell
command. A common practice is to run your command with
exec to keep the process table uncluttered. It is also
common to redirect output to a log file. If you do not,
and the command generates output on stdout or stderr, the
result will be mailed to the user in question. If you use
this mechanism for special users, such as UUCP, you may
want to create an alias for the user to direct the mail to
someone else, such as root or postmaster.
Internally, this cron uses a quick indexing system to
reduce CPU overhead when looking for commands to execute.
Several hundred crontabs with several thousand entries can
be handled without using noticable CPU resources.
BUGS
Ought to be able to have several crontab files for any
given user, as an organizational tool.
AUTHOR
Matthew Dillon (dillon@apollo.west.oic.com)
1 May 1994 1
Dat wat in het bold staat gaat het om. Dus wat doe je nou? Dit: `crontab -e`, dan start (waarschijnlijk) vi op. Je kan dan je crontab editten, en een line zoals in het tweede dikgedrukte gedeelte toevoegen. Als je de weg niet kent in vi kan je `:h` intikken als je in vi zit, of op de shell `VISUAL=joe` (als je joe fijn vind, of anders pico), en dan `export VISUAL`. Als je vervolgens `crontab -e` intikt, kan je de crontab editten met joe danwel pico.
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