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| `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 [:BashFAQ/005: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. |
Redirect to [[glob#nullglob]]. /* all information that this article once contained was carefully fitted into the glob article. */ |
Redirect to glob#nullglob.