Mount points

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A mount point is a directory or file at which a new file system, directory, or file is made accessible. To mount a file system or a directory, the mount point must be a directory; and to mount a file, the mount point must be a file.

Typically, a file system, directory, or file is mounted over an empty mount point, but that is not required. If the file or directory that serves as the mount point contains any data, that data is not accessible while it is mounted over by another file or directory. In effect, the mounted file or directory covers what was previously in that directory. The original directory or file that has been mounted over is accessible again once the mount over it is undone.

When a file system is mounted over a directory, the permissions of the root directory of the mounted file system take precedence over the permissions of the mount point. The one exception involves the .. (dot dot) parent directory entry in the mounted-over directory. In order for the operating system to access the new file system, the mount point parent directory information must be available.

For example, if the current working directory is /home/frank, the command cd .. changes the working directory to /home. If /home/frank directory is the root of a mounted file system, the operating system must find the parent directory information in the /home/frank directory in order for the cd .. command to succeed.

For any command that requires parent directory information in order to succeed, users must have search permission in the mounted-over directory. Failure of the mounted-over directory to grant search permission can have unpredictable results, especially since the mounted-over directory permissions are not visible. A common problem is failure of the pwd command. Without search permission in the mounted-over directory, the pwd command returns this message:

pwd: Permission denied

This problem can be avoided by always setting the permissions of the mounted-over directory to at least 111.