[SOLVED] BASH: Download Script -> While Loop Calculation Toubles...

Devcon · 07-28-2010, 10:07 AM

I understand that all while loops in Bash create a subshell when processing piped input. I have a simple download script that displays downloads using DZEN. One of the tests in my script attempts to calculate a free Y-Coordinate to place the window. As multiple instances of the script run simultaneously, I have it dump taken Y-coords to a file. The while loop reads from this file to determine a free coordinate. Enough talk: the code:

Code:

# Determine Y-Coordinate
#########################

COORDS="/tmp/WGET_XY"
CNT=$(ls /tmp/*.wget.* | wc -l)

if [ $CNT -gt 0 ]; then
	while read y; do
	if [ $DZEN_Y -eq $y ]; then
	DZEN_Y=$(( $y - $DZEN_H ))
	else
	echo "$DZEN_Y" >> "$COORDS" && break
	fi
	done < "$COORDS"
else
DZEN_Y=$((738-($CNT*26)))
echo "$DZEN_Y" >> "$COORDS"
fi

DZEN_H in this snippet is just the height of the DZEN instance. I would like the while loop to set the DZEN_Y variable to the highest free Y-coordinate less than 738. I believe the problem is that the DZEN_Y variable is not changed outside the loop; hence, the encompassing if statement eventually returns false and sets DZEN_Y to 712 if multiple instances are running.

I know you guys will have a solution. Thanks.

David the H. · 07-28-2010, 02:00 PM

That's true, except that you don't have any piped input here, except in the embedded command that sets CNT. A file redirection such as your "< $COORDS" doesn't branch off a subshell.

Code:

#creates a subshell
cat file | while xyz; do
     something
done

#does not create a subshell
while xyz; do
     something
done <file

catkin · 07-28-2010, 03:42 PM

Is $DZEN_Y initialised when if [ $DZEN_Y -eq $y ]; then is executed? Also -le would be more robust there.

Devcon · 07-28-2010, 07:08 PM

Yes. $DZEN_Y is calculated prior to that test. It defaults based on screen resolution plus a user-defined padding value. The bottom of the stack (first wget instance) is at the bottom right of the screen. Here is the debug output of the 3rd instance of the script (so DZEN_Y should be 686):

Code:

if [[ $CNT -gt 0 ]]; then
	while read y; do
	if [[ $DZEN_Y -eq $y ]]; then
	DZEN_Y=$(( $y - $DZEN_H ))
	else
	echo "$DZEN_Y" >> "$COORDS"  
	break
	fi
	done < "$COORDS"
else
DZEN_Y=$((738-($CNT*26)))
echo "$DZEN_Y" >> "$COORDS"
fi
+ [[ 2 -gt 0 ]]
+ read y
+ [[ 738 -eq 738 ]]
+ DZEN_Y=712
+ read y

I'm not sure why the 738 is properly appended to $COORDS whereas the 712 is not. Thus, all subsequent instances are stacked atop one another at y=712.

Catkin: -le would not work in this case because it would return true even if the y-coord is available. I would like the loop to break if an increment of 26 is found that is less than the values pulled from $COORDS.

Thanks for you input guys.

Devcon · 07-28-2010, 07:08 PM

Heh. Bleh. After looking at it with fresh eyes, the solution presented itself.

Code:

if [[ $CNT -gt 0 ]]; then
        while read y; do
        if [[ $DZEN_Y -eq $y ]]; then
        DZEN_Y=$(( $y - $DZEN_H ))
        else
        break
        fi
        done < "$COORDS"
echo "$DZEN_Y" >> "$COORDS"     # <- This echo statement was the culprit (this is the right position)
else
echo "$DZEN_Y" >> "$COORDS"
fi

I knew it was something stupid. The 712 was not appended because the while loop was going kaplooey before the echo statement was made. I simply had to pull the echo statement out of loop.

Thanks you guys for the suggestions. /SOLVED

grail · 07-29-2010, 01:41 AM

Well not to be overly picky but if you do the same thing in the original if and the else you may as well just put it outside.