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What is /etc/mtab in Linux?

Why is it needed and advantages of having it?

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% file /etc/mtab
/etc/mtab: symbolic link to ../proc/self/mounts
% file /proc/mounts
/proc/mounts: symbolic link to self/mounts
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/etc/mtab is a compatibility mechanism. Decades ago, Unix did not have a system call for reading the existing mount information. Instead, programs that mounted filesystems were expected to coöperatively and voluntarily maintain a table in /etc/mtab of what was mounted where.

For obvious reasons, this was not an ideal mechanism.

Linux gained the notion of a "procfs", and one of the things that it gained was a kernel-maintained version of this table, in the form of a mounts pseudo-regular file. The "system call" to read the mount information out of the kernel became an open-read-close sequence against that file, followed by parsing the result from human-readable to machine-readable form (something that has some subtle catches, as you can see from the bug reports from just over a fortnight ago).

/etc/mtab thus has popularly become a symbolic link to /proc/mounts, allowing programs that had hardwired that name to keep reading a mount table from that file, which the programs that mounted and unmounted filesystems no longer have to explicitly do anything themselves to keep up to date. (Some of them still will, though, if /etc/mtab turns out to be a writable regular file. And there are a few corner cases where the normalized information in mounts that lacks all non-kernel stuff is not quite what is needed; although they do not outweigh the general problems with /etc/mtab.)

Each process can nowadays have its own individual view of what is mounted, and there are as a consequence now individual mounts files for each process in the procfs, each process's own table being accessible to it via the self symbolic link as self/mounts, and /proc/mounts is also now a compatibility mechanism. (Interestingly, neither per-process mounts nor the format of mounts are documented in the current Linux doco, although the similar mountinfo pseudo-regular file is.)

SunOS/Solaris has a similar mechanism. The /etc/mnttab file is actually a single-file filesystem, and in addition to reading the table, via an open file descriptor to that file, with the read() system call, one can watch for mount point changes with poll() and obtain various further pieces of information with ioctl().

In HP-UX, /etc/mnttab is likewise the name of the file, but as of version 11 it was still a regular file whose contents were coöperatively maintained by the system utility programs.

AIX does not export a human-readable text table that programs have to parse, and there is no equivalent file. The BSDs, similarly, have fully-fledged system calls, getfsstat() on FreeBSD and OpenBSD, for programs to obtain the mount table from the kernel in machine-readable form without marshalling it through a human-readable intermediate form.

Further reading

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  • In complement with my comment in the question here is the mtab(5) from the old days: man.cat-v.org/unix_8th/5/mtab.
    – Weijun Zhou
    Apr 23, 2019 at 19:20
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    not only /proc/mounts, but /proc/self/mounts is itself a compatibility mechanism now; it's only showing a subset of the info available in /proc/self/mountinfo. The format of /proc/self/mounts is documented in proc(5) as identical to fstab(5)
    – user313992
    Apr 24, 2019 at 2:35
  • though admittedly, fstab(5) only tells about spaces being replaced by octal escapes, while it's spaces, tabs, newlines and backslashes
    – user313992
    Apr 24, 2019 at 3:08
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    I know pseudo files and regular files, but what is a pseudo regular file?
    – gerrit
    Apr 24, 2019 at 7:16
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    @gerrit it's a regular file which has size 0 but still contains data ;-)
    – user313992
    Apr 24, 2019 at 8:37
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According to man mount:

The programs mount and umount traditionally maintained a list of currently mounted filesystems in the file /etc/mtab. This real mtab file is still supported, but on current Linux systems it is better to make it a symlink to /proc/mounts instead, because a regular mtab file maintained in userspace cannot reliably work with namespaces, containers and other advanced Linux features.

On mounting without recording in /etc/mtab:

-n, --no-mtab

Mount without writing in /etc/mtab. This is necessary for example when /etc is on a read-only filesystem.

Many more nuances are given in the manual page.

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The answer of JdeBP might be the most actual and useful, I can add something from my (a bit outdated) Linux Learning Prep Guide which also gives a view on how it is / was used before:

The difference between /etc/mtab and /proc/mounts is that /etc/mtab is the user space administration kept by mount, and /proc/mounts is the information kept by the kernel. source

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