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  • On page 385 of the SysV UM, Vol II you mentioned, there is the builtin print command for ksh which is said to behave like echo. This command is present in something like AT&T Unix SVR4 2.1 i386 from ISC too. print "\012" yields the same result as would echo. Wonder why ksh had print and not echo built-in? Anyways really interesting. Commented Dec 31, 2013 at 1:10
  • How many such gems from a few years ago are snuck away waiting to be unearthed by an edit in 2016? +1 Commented Mar 5, 2016 at 9:16
  • +1. Thanks. Could you provide the sources (references) for the purpose of making a command builtin, for my (systematic) reading/learning? Commented Mar 16, 2016 at 9:08
  • I want to find some reference, manual or standard to systematically read or learn. I tried to look it up in the bash reference manual, and POSIX specifications. But I can't find it. I am not sure if I miss them. Commented Mar 16, 2016 at 14:49
  • @Tim: As this and other answers here explain, some commands must be built into the shell or they cannot do their work. All the others are purely optional. That's essentially what my Bourne shell source snippet says above, in fact. Since building such commands into the shell is optional, an authoritative rationale could only give one implementer's opinion. Commented Mar 16, 2016 at 15:16