Shell Programming and Scripting

Hi all,

I am using Ubuntu 8.04 Linux. I have a cron job entry to run a shell script at a scheduled time. This script has a X application to be launched. As cron does not pick already exported environment variables and it runs in a separate environment, I need to export the DISPLAY variable to run my GUI process. So I am setting the DISPLAY environment variable through command substitution at the top of my script as follows:

Code:

export DISPLAY="`who am i | cut -d"(" -f2 | cut -d")" -f1`"

It seems, this does not work with cron (i.e) cron is not executing command substitution inside shell script or at the crontab's "command field" entry (i.e):

crontab -l output:

Code:

SHELL=/bin/bash

01 17 * * * export DISPLAY="`who am i | cut -d"(" -f2 | cut -d")" -f1`" && gui_process.sh &> error.log

I have no other option other than hardcoding the (literal) value instead of command substitution inside my script "gui_process.sh" as follows:

Code:

#!/bin/bash

export DISPLAY="xx.xx.xx.xx:0.0"
xterm +hold -e "GUI"

My questions are:

a) Why cron does not support "command substitution"?

b) Also, I have exported DISPLAY in my .bashrc file (since my login shell is bash) and I have specified this shell at the top of the crontab as: SHELL=/bin/bash. Still why cron is not reading it from ~/.bashrc while executing my script??

c) Since cron is not associated to any terminal (/dev/tty) I guess no need for trapping HUP signal?

Thank you

First off - ALL comands in a cron entry or in a cron script should have explicit paths to command files. The PATH is not the same as you have at the command line.

who am i can be implemented all kinds of ways, as an alias or some other feature of the who command. It is usually this:

Code:

/usr/bin/who am i

where "am and "i" are arguments.

You need to experiment with each simple element of your command in crontab entry - enter the command let it run, remove it from crontab, then check the log or email.

example

Code:

* * * * * /usr/bin/who am i >> /home/royalibrahim/logfile

wait a minute, remove the crontab entry and then read the logfile. Proceed iteratively.



Is there some reason not to use the shell variable $USER?

Th command "who am i" will fail when run from cron. It needs a terminal.
X Windows also needs a terminal.
What are you trying to achieve?

You can see the commands "xterm +hold -e", that I am launching xterm terminal to paint the X application.

What if I am running only on my local machine (i.e) launching the xterm terminal in my local system - not on the remote systems?

So, command substitution is not possible in whatever be the case, right? what if scripts that are need to be run by cron, have the need of command substitution? How to achieve this? Is this a deficiency in cron - is that incapable to do that?

What is the full list of environment variables that can be set for crontab other than HOME,LOGNAME,PATH,SHELL,MAILTO variables?