Applescript: Check if shell command fails?

Question

I'm writing an AppleScript to watch a folder for completed downloads, then process them. If I'm downloading a file split across partial RARs (.r01, .r02, … God I hate those), they'll enter the Completed folder randomly as they're completed. The script will trigger every time one of them is completed, but I don't want it to start to auto-unarchive until all of them are done.

So here's my solution. In the script, when I detect a rar, I first verify it for completeness:

do shell script ("unrar v " & theItem)

The shell will spit out, among other things, this:

… Testing archive thefile.r19 96% … Testing archive thefile.r20 99% ... Calculating control sums of all volumes. Cannot find volume thefile.r21 … thefile.avi - CRC failed Total errors: 1

I'm new to AppleScript. What's the most efficient way to scan through that and determine if the verification has failed?

(Alternatively, unrar v is time consuming; is there any better way to approach this problem?)

Answer 1

If you change your code to:

 set output to do shell script ("unrar v " & theItem)

The variable output will hold the output of the shell script, which you could use to compare to what you are expecting for a valid output.

if output contains "CRC Failed" then
    return
end if

For avoiding having to run unrar v you could implement a check on the file's extension (either when you set theItem in the AppleScript or perhaps in whichever method you're using to monitor the folder - if you've manually made the launchd job and not used Folder Actions)

For more info on do shell script check out Apple's Technical Note 2065.