08-18-2008
When you set up your cron, you set it to call the script every minute. So, every minute, cron calls your script and your script schedules an at job. When that time arrives, the system will start running all of those queued up jobs scheduled by at.
Once you have passed the time specified in the at command, cron is still going to call your script (every minute) and it is going to try to schedule the at. Since the time is now passed, those at jobs should not be scheduled, but cron and the script are going to continue to run. How do you expect them to know that the time is expired? At will know the time is already expired and refuse to schedule a new job but everything leading up to that point is still going to happen.
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LEARN ABOUT LINUX
crontab
CRONTAB(1) General Commands Manual CRONTAB(1)
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 com-
mand 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>
4th Berkeley Distribution 19 April 2010 CRONTAB(1)