How does the Linux command "mount -a" work?

Question

After adding a new mount point in /etc/fstab, we usually execute mount -a to reflect the change (if we want to bypass reboot), and df -kh output shows the new mount point.

How does mount -a work/impact already mounted partitions, which have reference to the /etc/fstab file?  Does it umount and then mount those partitions, or just ignore them since they are already mounted?

Answer 1

It skips ones already mounted.

https://github.com/karelzak/util-linux/blob/master/sys-utils/mount.c#L185-L193

while (mnt_context_next_mount(cxt, itr, &fs, &mntrc, &ignored) == 0) {

    const char *tgt = mnt_fs_get_target(fs);

    if (ignored) {
        if (mnt_context_is_verbose(cxt))
            printf(ignored == 1 ? _("%-25s: ignored\n") :
                          _("%-25s: already mounted\n"),
                   tgt);
    }
    // ...
}

Answer 2

It will ignore already mounted partitions.

In the same time, you can do a mount -o remount /dev/xxx on a partition already mounted, this won't cause any problem even if users are using this partition

Answer 3

The better option would be mount the new file system manually and add the new file system entry in the /etc/fstab to make sure this f/s get mounted on every reboot.

As mentioned above "mount -a" will ignore the already mounted file systems and mount only the file system which was not mounted yet (new file system).

Hope this will helps.