Hi,

> Am 08.03.2017 um 19:17 schrieb Reuti <reuti@staff.uni-marburg.de>:
> 
> […]
> The strange thing seems to be, that "-j1 2" is handled like "-1 2". "-j 1" should already have an option and "2" would then become the first filename making the last filename in the command line superfluous.

My investigations revealed: on a Mac the man page of `join` explains the behavior. The options -j, -j1 and -j2 are listed with the BSD version of `join` as being there for compatibility. This leads to the assumption, that nowadays -1 and -2 should better be used.

The coreutils version of `join` doesn't explain -j1 or -j2 anywhere AFAICS (but the `join` application covers them as special cases in the source). Neither on the man page, nor the info page it's mentioned. At least this should be noted somewhere. Personally I find it confusing, that "-j 3" is like "-j3", but "-j1" and -"j2" are special and need an argument. The info page even lists "-j3" in the examples (no intervening blank). It would be good if there would be a hint of this trap on the man page, as the plain statment:

       -j FIELD
              equivalent to '-1 FIELD -2 FIELD'

does not work in all cases essentially. (One could argue, that no one will use "-j1" standing alone and expecting it to work as "-j 1" as it's the default anyway, but at least "-j2" might give some surprise.)

-- Reuti