Which command can you use to display a summary of disk usage for all file systems?

The command that can be used to display a summary of disk usage for all file systems is "df." This command provides an overview of available and used disk space on all mounted file systems, showing the capacity, used space, available space, and the mount points. It is particularly useful for system administrators to monitor disk space and ensure that systems do not run out of storage.

The "du" command, while related to disk usage, is primarily used to estimate the space consumed by files and directories in a specific location, rather than providing an overview across all file systems. The other options, "diskusage" and "summary," do not exist as standard commands in Linux for this purpose. Therefore, the "df" command is the appropriate tool to summarize disk usage across all file systems currently mounted on the system.