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
configuration 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
administrative 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
confuse 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
automatically. 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
variable 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
syntactically correct crontabs will be written there. This is enforced
by having 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
consider the crontab (at least partially) broken and refuse to install
it.
AUTHOR
Paul Vixie <paul@vix.com>