Looking at the official coreutils documentation for join: at https://www.gnu.org/software/coreutils/manual/html_node/join-invocation.html
The result of the example is incorrect. It should read: $ join file1 file2 a 1 X e 5 Y If this is the incorrect place to report a documentation bug, please let me know and point me to the correct place. The introduction of the coreutils manual does not specify where to send documentation errors.