What does the 'df' command display?

The 'df' command is a standard utility in Unix and Linux systems used to report the amount of disk space available on the file system. When you run the 'df' command, it provides a summary of the disk space usage on all mounted file systems, including the total space, used space, available space, and the mount points.

This information is essential for system administrators to monitor disk usage and ensure that file systems do not run out of available space, which could lead to system errors or application failures. 'df' gives a quick overview of storage capacity, helping in resource allocation and planning for future storage needs.

The other options pertain to different system information: memory usage, process status, and file permissions are not the focus of the 'df' command. Each of those would be reported by different commands, like 'free' for memory usage, 'ps' for process status, and 'ls -l' for file permissions.