Don't try to execute directories, or give a better error message

[andychu]

(24) I tested zsh and mksh, and they return 126 here.

OK, I tried zsh, mksh and dash, and they all try to execute the directory. yash and bash detects the empty string. This is interesting. I tried more shells: posh, ksh and busybox sh try to execute the directory, and csh, tcsh and fish detects the empty string. Anyway, I think it's better to detect the empty string rather than trying to execute the directory.

I guess it's a special case.

The following cases may also be handled specially. I think one can just skip directory names in searching executable files from PATH. Directories are not executable even if the permission contains an executable flag.

osh$ ..
  ..
  ^~
[ interactive ]:1: Can't execute '/home/murase/bin/..': Permission denied
osh$ mkdir -p foo/bar
osh$ PATH=foo:$PATH
osh$ bar
  bar
  ^~~
[ interactive ]:9: Can't execute 'foo/bar': Permission denied

[andychu]

Shells disagree on this: many appear to add a special case, some just execute the directory and fail with 126.

From issue #653