The umount command can be used to temporarily unmount a directory from a partition. For example, let's say the df -Th command shows that the /var directory is mounted onto the /dev/sda3 partition.

Filesystem                   Type      Size  Used Avail Use% Mounted on
/dev/sda3                    ext4      922M   90M  770M  11% /var

Likewise, the mount command will show that the /var directory is mounted onto the /dev/sda3 partition.

/dev/sda3 on /var type ext4 (rw,relatime,seclabel)

While mounted, you can view the contents of the /var directory.

~]# ll /var
drwxr-xr-x.  9 root root  4096 May 14 15:54 log
drwxr-xr-x.  9 root root  4096 May 14 15:54 logs
drwx------.  2 root root 16384 May 14 15:47 lost+found
drwxrwxrwt.  9 root root  4096 May 14 15:55 tmp

The umount command can be used to temporarily unmount the /var directory from the /dev/sda3 partition.

umount /var

When attempting the unmount, if target is busy is displayed, the -l or --lazy flag can be used.

umount --lazy /var

Once unmounted, the /dev/sda3 parition will no longer be included in the df -Th command or in the mount command. Likewise, when attempting to view the contents of the /var directory, the following should be returned.

[root@lab1 ~]# ll /var
total 0

The mount command can be used to remount the /var directory onto /dev/sda3.

mount -t ext4 /dev/sda3 /var

Or, if the /etc/fstab file is configured to mount the /var directory to the /dev/sda3 parition at boot, restarting the system should mount the /var directory onto the /dev/sda3 parition.

UUID=7a8e7883-e6ee-4482-a7ca-665bfe94eaa8 /var    ext4   defaults   1 2