Using XPath in bash with not()
The $() construct substitutes the output of the command. As such, whatever is spit out by xpath will be substituted and the shell will try to execute that as a command which is why you get the error message.
Since xpath doesn't seem to provide a different exit code based on whether a node was found, you will probably just have to compare the output to something, or test for empty:
if [ -z "$(xpath -q -e '//otherFile' "$f" 2>/dev/null)" ]; thenThis should execute the following code if xpath produced no output. To reverse the sense, use -n instead of -z (not sure which one you intended).