Strange crontab-script interaction (bash)

[Garrigus Carraig]

I'm running Ubuntu 12.04 and bash. I've written a pair of shell scripts that allow me to set an alarm which, after ringing, unsets itself. The first, alarmset, allows me to enter a time and modifies the alarm line in the crontab. That line launches the second script, alarmring, which launches a radio player in a browser window and then comments out the alarm line in the crontab.

alarmring is behaving strangely. If I run it myself directly, it performs both actions: it launches the browser window and edits the crontab. But if I run alarmset, when the crontab launches alarmring at the appointed time, alarmring edits the crontab, but does not launch the browser window.

Finally, when crontab runs alarmring, it ignores the set -x command, whereas when I run it directly, set -x is executed. So it's as though the crontab is skipping the first ten lines.

Any ideas on what's going on? I'll paste the two scripts and the crontab below.

alarmset:

#!/bin/bash
    
    # alarmset
    
    set -x
    
    usage()
    { echo "alarmset [ hour minute | -h ]" }
    
    editcrontab() 
    { 
        echo $'/alarmring/s/^\(.*\)\(\* \* \*\)/'$2$' '$1$' \\2/' > ~/Documents/crontab_script.txt 
        crontab -l | sed --file=/home/username/Documents/crontab_script.txt > ~/Documents/new_crontab.txt crontab ~/Documents/new_crontab.txt 
    }

    ### MAIN 
    case $# in 
        2 ) editcrontab $1 $2 ;; 
        * ) usage 
            exit ;; 
    esac
    
    set +x

alarmring:

#!/bin/bash
    
    # alarmring
    
    set -x
    
    env DISPLAY=:0
    
    # Ring the alarm : launch BBC World Service in Firefox 
    firefox --new-window http://www.bbc.co.uk/radio/player/bbc_world_service
    
    # Unset the alarm : comment out the alarm line in the crontab 
    crontab -l | sed '/alarmring/s/^/#/1' > ~/Documents/new_crontab.txt 
    crontab ~/Documents/new_crontab.txt
    
    set +x

crontab:

SHELL=/bin/bash 
    PATH=~/bin:/usr/bin:/bin 
    # 
    # m h dom mon dow command 
    53 07 * * * /home/username/bin/alarmring

[schragge]

But if I run alarmset, when the crontab launches alarmring at the appointed time, alarmring edits the crontab, but does not launch the browser window.

This

env DISPLAY=:0

just displays environment on the screen (or sends it by mail when run from crontab), but doesn't set $DISPLAY for firefox. Instead, do this

 
    # Ring the alarm : launch BBC World Service in Firefox 
 DISPLAY=:0 firefox --new-window http://www.bbc.co.uk/radio/player/bbc_world_service

It probably doesn't ignore the set -x either, cron just sends the output of all commands by mail. Run mailx in terminal to see your mailbox, or just

more /var/mail/$USER

BTW, wouldn't using at be simpler?

[Garrigus Carraig]

Well, that was fast and easy. Thank you, schragge. I'm a little unclear on the syntax: Does DISPLAY=:0 need to be on the same line as the firefox command?

[rnerwein]

hi
my crontab looks (every 2 minutes): */2 * * * * /home/richi/tmp/ff
my script ff:
# not "env" --> export
DISPLAY=:0.0
export DISPLAY
/usr/bin/firefox --new-window http://www.bbc.co.uk/radio/player/bbc_world_service

this works for me !
and another hint is: for crontab jobs - always use the full pathname !!!
good luck
ciao
ups - schragge was faster

[schragge]

Does DISPLAY=:0 need to be on the same line as the firefox command?

There are two options: either on the same line like what I did (then it applies only to the command it prefaces), or on a separate line, then you need to export DISPLAY like what rnerwein did. In the last case, it applies to all commands in the script that follow it.

[Garrigus Carraig]

Thanks, rnerwein, and thanks for the pathname tip.

schragge: I don't know about at; I'll look into it. I'm a noob & glad to have come this far!

[Garrigus Carraig]

Thanks again, schragge. One more thing: Is at Ubuntu specific?

[schragge]

No, it's part of the POSIX standard. On Debian-based distributions it can be installed with

sudo apt-get install at

[Garrigus Carraig]

Haha well I've been playing around with at, and discover indeed that it does exactly what I needed. Ah well, I got some good shell script writing practice. And I've learned that at my level, anything I want to implement has probably already been implemented.