Which command line can be used to search help files that mention the word "copy"?

man -k performs a keyword search (equivalent to the apropos command) against the whatis database, returning all man page entries whose name or short description contains the specified word. Running man -k copy will list every command and function whose manual page synopsis mentions 'copy', making it ideal for discovering commands when you know the concept but not the exact command name.

whatis looks up the exact command name provided and returns its one-line description - it does not search across all man pages for a keyword.

man copy attempts to open the manual page for a command literally named 'copy', which does not exist as a standard Linux command.

'copy help' is not a valid Linux command syntax; 'copy' is not a standard Linux utility.

'copy -help' is similarly invalid because 'copy' is not a standard Linux command.