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20090605 Friday June 05, 2009

Possibly the best shell programming mistake ever

A colleague, lets call him Lewis, just popped over with the most bizarrely behaving shell script I have seen.

The problem was that the script would hang while the automounter timed out an attempt to NFS mount a file system on the customer's system.

I narrowed it down to something in a shell function that looked like this:

# Make a copy even if the destination already exists.
safe_copy()
{
 	typeset src="$1"
	typeset dst="$2"

	/* Nothing to copy */
	if [ ! -f $src ] ; then
		return
	fi

        if [ ! -h $src -a ! -h $dst -a ! -d $dst ] ; then
		cp -p $src $dst || exit 1
	fi
}

safe_copy was called with a file as the $1 and a file as $2.

I laughed when saw the problem. Funny how you can read something and miss such an obvious mistake!

Thankfully the script has quietly been fixed.


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( Jun 05 2009, 05:18:39 PM BST ) Permalink
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Comments:

"Lewis" has been writing too much C I guess. :-)

Posted by Brett Monroe on June 05, 2009 at 07:45 PM BST #

I should point out that "Lewis" did not write the script. Perhaps he's been reading to much C or even D!

Posted by Chris Gerhard on June 06, 2009 at 11:37 AM BST #

As someone that does a lot of C but not much shell, any hints for the rest of us that haven't figured it out yet?

Posted by Sam Shinozaki on June 08, 2009 at 04:57 PM BST #

@Sam.

The problem is the comment.

/* does not delimit comments in the shell. Instead thanks to globbing it expands to every file in the root directory.

Posted by Chris Gerhard on June 08, 2009 at 06:08 PM BST #

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