MAN PAGES: JOIN COMMAND

join two
on a common field

For each pair
with identical fields,
write a line

The default is the first,
delimited by blanks.

unpairable lines replace
missing fields

with EMPTY

ignore differences
when comparing

obey FORMAT

use,
but suppress

check the input,
even if all lines are pairable

treat each
without trying

display this help
and exit

Note, comparisons honor the rules
If some cannot be joined,
a warning message will be given. 

*a found poem (from Linux Man Pages)