Re: man bash does not list 'in' as a builtin command

[Robert Elz]

    Date:        Mon, 25 Nov 2019 13:43:41 -0800
    From:        Peter Benjamin <pete@peterbenjamin.com>
    Message-ID:  
<4dc457e0135603025cd500acdc95db53f9d30482.camel@peterbenjamin.com>

  | Description:
  |  'in' is a builtin command and is not listed in the man page as such.

Others have indicated what it is, but just for completeness, the man
page *does* include it...

RESERVED WORDS
       Reserved words are words that have a special meaning to the shell.  The
       following words are recognized as reserved when unquoted and either the
       first word of a simple command (see SHELL GRAMMAR below) or the third
       word of a case or for command:

       ! case  coproc  do done elif else esac fi for function if in select
       then until while { } time [[ ]]

If you really have some burning desire to use "in" (or any of the others
listed there) as a command name (I doubt any will ever become any kind of
builtin, that would be bizarre) in bash or any other Bourne-type shell
(they *all* treat in as a reserved work) you can change the word (as your
 ./in did) or quote it somehow, like
        'in' "in" \in i\n \i\n

kre

ps: you really should read the whole manual page, beginning to end.