Unix File Permissions

What do r w and x really mean for a file?

For a file, "read" and "write" are pretty intuitive. The x for "execute" means that the kernel may attempt to run the file. For that to work, the file must be an executable (output from a compiler) or a shell script with a "#!" first line. For a directory, things are a little more complex. With a directory, "write" permission means that you can create new files in the directory or remove old files. It sometimes surprises people that you can remove a file which you cannot read. The unix rm command will test for that and issue a warning, but you can suppress that warning with -f. And warning or no, if you want to remove an unreadable file from a writable directory, you may. And rmdir will not bother to check at all.

What do r w and x really mean for a directory?

A directory is a file too, and "read" permission means you can read it. But you really cannot do very much without x permission as well. With directories, you usually have both read and execute permission or neither. On a directory, that x is officially called "search permission". You need x to use a directory in a pathname. So if you try "cat /etc/passwd", you will need x on / and /etc. You also need x to cd into a directory. Suppose you have read but not search (x) permission on a directory. What can you do? Not much. You can use "ls" to view the file names. Even "ls -l" will not work. Read access without search permission is not very useful. Still that is better than having only write permission on a directory...that is completely useless. I have not seen any other documentation that states this explicitly, so let me repeat it: write but no execute permission on a directory grants nothing at all.Suppose you have search (x) permission but no read permission on a directory. Now you can open files in the directory if you happen to know the file's name. You can cd into the directory. And that is it. You cannot even create a new file. Adding write permission will allow you to create files. And you can then delete files if you happen to know their name.