Final answer:
The df command is used to check filesystem usage on a system, showing space available and used on all mounted filesystems, while du shows space usage of files, and mount is for mounting filesystems.
Step-by-step explanation:
The command used to check filesystem usage on a system is the df (disk free) command. This command provides a summary of the available and used disk space on your file system. On the other hand, the du (disk usage) command is used to estimate file space usage, and the mount command is used for mounting filesystems.
When you use the df command, it shows information such as total space, used space, available space, and the mount point of each filesystem. To get a human-readable output, the df command can be used with the '-h' option (e.g., df -h), which will display the sizes in powers of 1024 (e.g., K, M, G).