crontab Man page

Resume Wikipedia de Cron

cron est un programme qui permet aux utilisateurs des systèmes Unix d’exécuter automatiquement des scripts, des commandes ou des logiciels à une date et une heure spécifiées à l’avance, ou selon un cycle défini à l’avance.

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 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)