/etc/fstab has only two lines: root partition and debugfs, while
/etc/mtab has much more, in addition to these two, like (sysfs, proc, udev, devpts, tmpfs, cgroup, ...).
Where do the additional mount points come from?
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Those mounts are often performed by the initramfs/initrd scripts or other early-boot system initialization scripts, or on distributions that are fully using
.mount systemd unit files executed by either the real
systemd or by the mini-systemd environment within the initramfs.
For example, Debian 9 has the following
.mount units by default:
/lib/systemd/system/dev-hugepages.mount /lib/systemd/system/dev-mqueue.mount /lib/systemd/system/proc-fs-nfsd.mount /lib/systemd/system/proc-sys-fs-binfmt_misc.mount /lib/systemd/system/run-rpc_pipefs.mount /lib/systemd/system/sys-fs-fuse-connections.mount /lib/systemd/system/sys-kernel-config.mount /lib/systemd/system/sys-kernel-debug.mount
There is no requirement that partitions must be listed in
/etc/fstab. After all, you can just run
mount with everything explicitly specified in command-line arguments and mount another filesystem, without using anything from
However, what you are seeing are things that have been automatically mounted for you, already.
These days on Debian, most of these file systems (they are not partitions as there is no disk backing these file systems) are created/mounted by
Many of them are what the
systemd people refer to as "API filesystems" because they effectively are part of the API that is provided by the kernel to applications. A list of such filesystems is hardwired into the
systemd code. Observe that it is not in synch with the
Mounting API filesystems is one of the usual tasks of the system management softwares that run as process #1, even outwith Linux, from BSD (sic!)
initare not documented, but like
systemdcan be found in its code.
runit-initon Debian and by van Smoorenburg
rcare not documented. They can be found in a common
mountkernfs.sh, that both systems use. (Note that this is one of the van Smoorenburg
systemdexplicitly masks, because it is entirely spuerseded by what
system-managerin the nosh toolset, both for Linux and for the BSDs, are listed in its manual page (q.v.) accessible with