crontab - maintains crontab files for individual users
crontab [-u user] <_file_ | -**> crontab [-u** user] <-l** | -r | -e> [-i**] [-s] crontab -n [ hostname ] crontab -c crontab -V
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 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 in an environment variable. If it is not set by the user, the /tmp directory is used.
-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.
-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.
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.
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. This new command syntax differs from previous versions of Vixie Cron, as well as from the classic SVR3 syntax.
An informative usage message appears if you run a crontab with a faulty command defined in it.
Paul Vixie Colin Dean