crontab - maintains crontab files for individual users
crontab [-u user] <file | ->
crontab [-T] <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.
When listing a crontab on a terminal the output will be colorized
unless an environment variable NO_COLOR is set.
- -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.
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.
/etc/cron.allow
/etc/cron.deny
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.
An informative usage message appears if you run a crontab with a faulty command
defined in it.