Bash script exits with no error

Question

I have a bash script that I'm running from DVD. This script copies multi-volume tar files from DVD to the local machine. Part-way through the copy, the script prompts the user to insert a second DVD, at which point the remaining files are copied. The script exists on the first DVD but not on the second.

This script is simply stopping after the last file is copied, but prior to starting the tar multi-volume extract operation and subsequent processing. There are no errors or messages reported. I've tried running bash with '-x' but there's nothing suspicious - not even an exit statement. Even more unfortunate is the fact that this behavior is inconsistent. Sometimes the script will stop, but other times it will continue with no problems.

I have run strace on the script. Following the conclusion of the copy operations, I see this:

read(255, "\0\0\0\0\0\0\0\0\0\0"..., 5007) = 1302
read(255, "", 5007)       = 0
exit_group(0)             = ?

I know that bash reads the script file into memory and executes it from there, but is it possible that it's trying to re-read the script file at some point and failing (since it no longer exists)? The tar files are quite large, and it takes approximately 10-15 minutes from the time the script starts to the time the last file is copied (from the second DVD).

Answer 1

I see you have already found a workaround, so I will just try to uncover what's happening:

bash isn't reading the whole script into memory, it's doing buffered reads on it, only as much as necessary each time (presumably that's for code sharing with terminal input). Before any external commands are launched, bash seeks to the exact position in the script and continues to read from there after the command finishes. You can see this if you edit the script file while it's running:

term1$ cat > test.sh
sleep 8
echo DONE
term1$ bash test.sh

While the sleep is executing, change the script from another terminal:

term2$ cat > test.sh
echo HAHA

Observe how bash becomes confused when the sleep is complete:

test.sh: line 2: A: command not found

It remembers that the position in the input file was 8 before the sleep, so it tries to read from there and is confronted with the last A from the overwritten script.

Now to your case. Normally, having a file open from a dvd locks the drive and prohibits disk change. If you nevertheless manage to change the disk, that should definitely involve an umount which should then invalidate the script fd. That's clearly not happening according to your strace output, which is a little strange. In any case, bash won't be able to read the rest of the script.