nullglob is a Bash shell option which affects the results of glob expansion.

Normally, when a glob which does not match any filenames is expanded, it remains unchanged. Thus, you get results like this:

•  $ rm *.bak
   rm: cannot remove `*.bak': No such file or directory

The unmatched glob *.bak is not replaced with anything; it is passed to rm directly as if it were a literal filename. rm then tries to unlink it, and fails with a reasonable error message.

However, this causes some undesired results in programming. Consider an array populated like this:

•  # Bash
   files=(*)
   echo "There are ${#files[*]} files here."

This fails if there are no files which match the glob -- it will report one file, because the array is loaded with the single element *.

The nullglob option lets us avoid this problem. If it is set, then an unmatched glob is swept away entirely -- replaced with a set of zero words -- instead of remaining in place as a single word.

•  # Bash
   shopt -s nullglob
   files=(*)
   echo "There are ${#files[*]} files here."

nullglob is not on by default because there are other cases where its behavior would be extremely disconcerting. For example, ls behaves quite unexpectedly if an unmatched glob is removed from the argument list:

•  shopt -s nullglob
   ls *.xyzqj
   # This will list all the files in the current directory
   # just as if the user had typed "ls" with no args.

nullglob by itself may not be sufficient for all cases. A glob does not, by default, match files whose names begin with a period, unless the glob itself also begins with a period. If so-called "hidden files" should also be matched by a glob, then one may wish to use the dotglob option.

•  # Bash
   shopt -s dotglob
   shopt -s nullglob
   files=(*)
   echo "There are ${#files[*]} files here."

Just remember that these options remain in effect for the current shell process until disabled.