Ubuntu Manpage: crontab - maintains crontab files for individual users

NAME

       crontab - maintains crontab files for individual users

SYNOPSIS

       crontab [-u user] <file | ->
       crontab [-T] <file | ->
       crontab [-u user] <-l | -r | -e> [-i] [-s]
       crontab -n [ hostname ]
       crontab -c
       crontab -V

DESCRIPTION

Crontab is the program used to install a crontab table file, remove or list the existing
tables used to serve the cron(8) daemon. Each user can have their own crontab, and though
these are files in /var/spool/, they are not intended to be edited directly. For SELinux
in MLS mode, you can define more crontabs for each range. For more information, see
selinux(8).

In this version of Cron it is possible to use a network-mounted shared
/var/spool/cron/crontabs across a cluster of hosts and specify that only one of the hosts
should run the crontab jobs in the particular directory at any one time. You may also use
crontab from any of these hosts to edit the same shared set of crontab files, and to set
and query which host should run the crontab jobs.

Scheduling cron jobs with crontab can be allowed or disallowed for different users. For
this purpose, use the cron.allow and cron.deny files. If the cron.allow file exists, a
user must be listed in it to be allowed to use crontab. If the cron.allow file does not
exist but the cron.deny file does exist, then a user must not be listed in the cron.deny
file in order to use crontab. If neither of these files exist, then only the super user
is allowed to use crontab.

Another way to restrict the scheduling of cron jobs beyond crontab is to use PAM
authentication in /etc/security/access.conf to set up users, which are allowed or
disallowed to use crontab or modify system cron jobs in the /etc/cron.d/ directory.

The temporary directory can be set using the environment variable $TMPDIR. If it is not
set by the user, the /tmp directory is used.

When listing a crontab on a terminal the output will be colorized unless an environment
variable NO_COLOR is set.

On edition or deletion of the crontab, a backup of the last crontab will be saved to
$XDG_CACHE_HOME/crontab/crontab.bak or $XDG_CACHE_HOME/crontab/crontab.<user>.bak if -u is
used. If the XDG_CACHE_HOME environment variable is not set, $HOME/.cache will be used
instead.

OPTIONS

-u Specifies the name of the user whose crontab is to be modified. If this option is
not used, crontab examines "your" crontab, i.e., the crontab of the person
executing the command. If no crontab exists for a particular user, it is created
for them the first time the crontab -u command is used under their username.

-T Test the crontab file syntax without installing it. Once an issue is found, the
validation is interrupted, so this will not return all the existing issues at the
same execution.

-l Displays the current crontab on standard output.

-r Removes the current crontab.

-e Edits 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.

-i This option modifies the -r option to prompt the user for a 'y/Y' response before
actually removing the crontab.

-s Appends the current SELinux security context string as an MLS_LEVEL setting to the
crontab file before editing / replacement occurs - see the documentation of
MLS_LEVEL in crontab(5).

-n This option is relevant only if cron(8) was started with the -c option, to enable
clustering support. It is used to set the host in the cluster which should run the
jobs specified in the crontab files in the /var/spool/cron directory. If a
hostname is supplied, the host whose hostname returned by gethostname(2) matches
the supplied hostname, will be selected to run the selected cron jobs subsequently.
If there is no host in the cluster matching the supplied hostname, or you
explicitly specify an empty hostname, then the selected jobs will not be run at
all. If the hostname is omitted, the name of the local host returned by
gethostname(2) is used. Using this option has no effect on the /etc/crontab file
and the files in the /etc/cron.d directory, which are always run, and considered
host-specific. For more information on clustering support, see cron(8).

-c This option is only relevant if cron(8) was started with the -c option, to enable
clustering support. It is used to query which host in the cluster is currently set
to run the jobs specified in the crontab files in the directory /var/spool/cron ,
as set using the -n option.

-V Print version and exit.

CAVEATS

       The files cron.allow and cron.deny cannot be used to restrict the execution of cron  jobs;
       they  only  restrict the use of crontab.  In particular, restricting access to crontab has
       no effect on an existing crontab of a user. Its jobs will continue to  be  executed  until
       the crontab is removed.

       The files cron.allow and cron.deny must be readable by the user invoking crontab.  If this
       is not the case, then they are treated as non-existent.

FILES

       /etc/cron.allow
       /etc/cron.deny

STANDARDS

       The crontab command conforms to IEEE Std1003.2-1992 (``POSIX'') with  one  exception:  For
       replacing  the  current crontab with data from standard input the - has to be specified on
       the command line if the standard input is a TTY.  This new  command  syntax  differs  from
       previous versions of Vixie Cron, as well as from the classic SVR3 syntax.

DIAGNOSTICS

       An informative usage message appears if you run a crontab with a faulty command defined in
       it.

AUTHOR

       Paul Vixie ⟨vixie@isc.org⟩
       Colin Dean ⟨colin@colin-dean.org⟩