More about UNIX file permissions

Permissions are checked for each element of a pathname

• For /usr/bin/vi, the permissions of /, /usr, /usr/bin, and /usr/bin/vi are checked in that order
• Consequently, the ability to access a file depends on the most restrictive element in the path
• Permissions on a symbolic link usually don't matter, as the effective permissions are those of the file the link points to

Other special permission flags

• the "sticky" bit (-rwxrwxrwt) on a directory means that only the owner of a file can remove it from that directory
• setuid (-rwsr-xr-x) on an executable means that the executable is run under the user id of the owner of the file
• setgid (-rwxr-sr-x) on an executable means that the executable is run under the group id of the group owner of the file

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