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Full Discussion: Crontab command substitution problem
Top Forums Shell Programming and Scripting Crontab command substitution problem Post 302382092 by royalibrahim on Tuesday 22nd of December 2009 06:12:57 AM
Old 12-22-2009
Crontab command substitution problem
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
Last edited by royalibrahim; 12-22-2009 at 07:18 AM..
 

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LEARN ABOUT REDHAT

crond

CRON(8) 						      System Manager's Manual							   CRON(8)

NAME

       cron - daemon to execute scheduled commands (Vixie Cron)

SYNOPSIS

       cron

DESCRIPTION

       Cron should be started from /etc/rc or /etc/rc.local.  It will return immediately, so you don't need to start it with '&'.

       Cron searches /var/spool/cron for crontab files which are named after accounts in /etc/passwd; crontabs found are loaded into memory.  Cron
       also searches for /etc/crontab and the files in the /etc/cron.d/ directory, which are in a different format (see  crontab(5)).	Cron  then
       wakes up every minute, examining all stored crontabs, checking each command to see if it should be run in the current minute.  When execut-
       ing 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).

       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.  Note that the Crontab(1) command updates the modtime of the spool directory whenever it changes a crontab.

SEE ALSO

       crontab(1), crontab(5)

AUTHOR

       Paul Vixie <paul@vix.com>

4th Berkeley Distribution					 20 December 1993							   CRON(8)
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