Screencapture shell script failure...noob

Hi, I'm trying to make a shell script which runs periodically and saves a file with a number appended to it. In order to increment the appended number, I'm saving the mycount variable to a file which is then read in (text file which just stores the number). The file path is pieced together with $mypath $fileName $mycount $fileExt. When I run screen capture from the command line in terminal hand coding the path, it works fine. But even though the path seems correct in the shell script, I get the following error from screencapture:

<Error>: CGImageDestinationCreate destination parameter is nil\n
screencapture: cannot write file to intended destination, ~/Desktop/testCap2.jpg

here's the shell script. I'm sure this is something obvious, but I'm looking at this through noob glasses! 😉 Any help would be greatly appreciated!

TIA, Vince
G5 Quad, 10.5.2

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shell script below
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#!/bin/sh
myPath="~/Desktop/"
fileName="screencapture"
mycount=`cat ~/Desktop/testCount.txt`
mycount=`expr $mycount + 1`
echo $mycount > ~/Desktop/testCount.txt
fileExt=".jpg"
filePath="$myPath$fileName$mycount$fileExt"
screencapture -mx -tjpeg $filePath