How to check if a disk is mounted on Linux?

One can use the following command in the Linux system to check if a disk is mounted:

1. Use the command df -h to check the usage of all mounted file systems in the current system.
2. Use the “mount” command to view all currently mounted file systems and their mount points in the system.
3. Use the command `cat /etc/mtab` to view all currently mounted file systems and their mount points in the system, this command will display more detailed information.

By using the above method, you can easily view all the mounted disks in the current system.