Why do the filesystems differ in “df” and “fstab?”

Question

I am looking at /etc/fstab and the output from df. There are far more filesystems under df. Why is this?

Answer 1

The reason is because fstab is not the only source of mounted partition info. As the man page for fstab explains; bold emphasis is mine:

The file fstab contains descriptive information about the various file systems. fstab is only read by programs, and not written; it is the duty of the system administrator to properly create and maintain this file.

So fstab is basically where user/administrator defined—or at least user/administrator controllable—mounts/partitions are managed; the system only reads the fstab and does not write to it.

If you want to see what other filesystems and mount points were mounted when the system booted, check the contents of mtab like this:

cat /etc/mtab

Or even /proc/mounts like this:

cat /proc/mounts

Those two files are dynamically created by the system on boot and whenever a new mount command is issued. Those files keep track of things that are mounted already and should not be touched by system users/administrators. More details on the mount man page.

So knowing that, when you run df—or df -h for human readable output—what you are seeing is a report of all mounted file systems disk space usage; not just for items from fstab but from any/all mounted file systems on the machine.