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The 'df' command, short for "disk filesystem," is used in Linux and Unix-like operating systems to report the amount of disk space used and available on filesystems. When you run the 'df' command, it provides a summary of disk space usage for all mounted filesystems, showing total size, used space, available space, and usage percentage. This information is essential for system administrators and users to manage disk resources effectively, ensuring that sufficient space is available for files and applications.

Other options, such as reporting memory usage, CPU load, or network connections, are not functions of the 'df' command. Memory usage is typically checked using commands like 'free' or 'top', CPU load can be monitored with tools like 'top' or 'vmstat', and network connections are not covered by 'df', but can be viewed with commands such as 'netstat' or 'ss'.