Shell Programming and Scripting

cron doesn't care whether you're connected locally or from somewhere else, because it's primary design goal since it's creation was to run programs without any user interaction. That means no TTYs, no X displays by default, so it has no way of knowing what display you're running an X server at. You'll have to tell it that yourself, by whatever means (eg. writing the required DISPLAY variable to a file & reading that in your script)

As for substitution: no, cron doesn't know anything about that, because the shell can handle it just fine. The whole command you specify is passed, without any interpolation, to a shell, which then has to handle parameter & command expansion. Example: the line

Code:

* * * * * echo "$(date +'\%Y\%m\%dT\%H:\%M:\%S')" > /tmp/file

generates this file:

Code:

> cat /tmp/file
20091223T14:10:01

and this entry in the syslog

Code:

/usr/sbin/cron[11903]: (pludi) CMD (echo "$(date +'%Y%m%dT%H:%M:%S')" > /tmp/file)

You see, an expansion is done, just not by cron but by the shell it calls.

To find out the environment variables which are set when a job starts in the default shell for cron, create a one-off cron containing just an "env" command. The output from "env" will be in the mail for the user who owns the cron.

We're still not clear what you are trying to achieve but using "cron" (which is a background task scheduler) to try to start a process which needs a foreground terminal is unlikely to be satisfactory.

what you really want to do ( for best maintenance practices ) is not clutter up cron with a host of crazy commands.

Put all those crazy commands in a script somewhere, where you'll have control to debug at will. IE:

Code:

crontab file:

* * * * * /bin/ksh /home/quirkasaurus/bin/cron_env.sh /home/quirkasaurus/bin/rule_the_world.sh

Then my cron_env.sh sets up my full required environment,
and runs my script exactly in an environment I control completely, and not at the whims of however cron kicks things off.

My cron_env.sh is essentially:

Code:

. /home/quirkasaurus/.profile
$*