If the /etc/cron.allow file exists, then you must be listed therein in order to be allowed to use this command.If the /etc/cron.allow file does not exist but the /etc/file does exist, then you must not be listed in the /etc/file in order to use this command.You should use absolute path names for commands like /bin/ls. Crontab is the program used to install, deinstall or list the tables used to drive the cron 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.Clock changes of more than 3 hours are considered to be corrections to the clock, and the new time is used immediately.In Debian and Redhat cron treats the files in /etc/cron.d as extensions to the /etc/crontab file (they follow the special format of that file, i.e. The intended purpose of this feature is to allow packages that require finer control of their scheduling than the /etc/cron.Blank lines and leading spaces and tabs are ignored.
SHELL is set to /bin/sh, and LOGNAME and HOME are set from the /etc/passwd line of the crontab's owner. HOME, SHELL, and PATH may be overridden by settings in the crontab; LOGNAME is the user that the job is running from, and may not be changed.
Files must conform to the same naming convention as used by run-parts: they must consist solely of upper- and lower-case letters, digits, underscores, and hyphens.
Like /etc/crontab, the files in the /etc/cron.d directory are monitored for changes.
Additionally, cron checks each minute to see if its spool directory's modtime (or the modtime on /etc/crontab) has changed, and if it has, cron will then examine the modtime on all crontabs and reload those which have changed.
Thus cron need not be restarted whenever a crontab file is modified.
Search for updating crontab:
When executing commands, any output is mailed to the owner of the crontab (or to the user named in the MAILTO environment variable in the crontab, if such exists).