Mac OS X, like many other Unix-like operating systems, includes the “autofs” file system layer that make automatic on-demand mounting of remote resources possible. See the man page for automount(8) for more details.

Such automatic mounting is orthogonal to and possible with MacFUSE. Consider sshfs, a user-space SFTP file system implementation that works with MacFUSE. The following is a quick-and-dirty example of how you could set up an autofs mount for sshfs. (There are other ways to set up autofs mounts.)

Create an /etc/fstab file (or add to an existing one) with the following entry. We will create what’s called a "static map" in autofs parlance.

$ cat /etc/fstab dummy:user@host:/remotedir /Network/name sshfs volname=volname,allow_other 0 0

You will have to customize some components of this specification. user, host, and remotedir are the SFTP user name, the SFTP server host name, and the remote directory on the SFTP server, respectively. You can choose some reasonable value for name and volname. The local mount point will be /Network/name.

Next, to keep things simple, configure key-based authentication to the SFTP server so you can log in without having to type your password.

The keyword “sshfs” in the /etc/fstab entry is the type of the file system. Given a file system type foo, the automounter will expect a mount_foo file-system-specific mounting program to exist. In our case, we don’t have a separate mounting program for sshfs. However, because of the format of the entry and how the automounter passes arguments to the mounting program, it will work if you simply copy the command-line sshfs program to /sbin/mount_sshfs. Alternatively, you can create a symbolic link as follows.

$ which sshfs $ /usr/local/bin/sshfs $ sudo ln -s /usr/local/bin/sshfs /sbin/mount_sshfs

That should be it. Run the automount program to update the state of things. The -c argument tells the automount daemon to flush any cached information.

$ sudo automount -c

If everything went well, the new mount should appear in the output of the mount command. In the following example, we used SSH as the name component of the mount point.

$ mount ... map -static on /Network/SSH (autofs, automounted)

Now, if you simply access /Network/SSH, the SFTP file system should be automatically mounted.

$ ls /Network/SSH Applications Volumes work Desktop DB bin private ...

If there is an error in mounting (say, the remote host is not reachable), you will not be permitted to access the /Network/SSH directory.

$ ls /Network/SSH ls: SSH: Operation not permitted

You can specify a timeout period after which an automounted file system will be unmounted if it has not been accessed within that period. Either use the -t argument of automount or see the /etc/autofs.conf file.