So it's just not immediately obvious to many why a simple extra space is a problem at all, and why it's this specific error. It can also just generally be hard to see past what you intended to write to what you actually did and how that will be parsed.

But otherwise yeah, I also don't see this as obscure at all. Was expecting something confusing when, say, fiddling with arrays or something.

(I am the author of the linked-to entry.)

   FRED=barney; set | grep FRED

   FRED=barney printenv FRED

   sh -c 'FRED=barney set'|grep ^FRED

If you add a semicolon then FRED is no longer a single command variable (it persists)

> on NetBSD, one can do this

Linux also does the expected thing:

$ FRED=barney printenv FRED

barney

It's not missing. PP is making a point about "single-command variables."

  $ FOO=BAR env | grep FOO   # Exists as an environment variable passed to env
  FOO=BAR
  $ env | grep FOO           # But only for that command
  $ FOO=BAR                  # Creates a variable (not an environment variable)
  $ env | grep FOO           # As shown by not being passed down
  $ echo $FOO                # But still accessible in this context
  BAR
  $ export FOO=BAR           # Promotes it to an environment variable (the "=BAR" is optional since it's already defined)
  $ env | grep FOO           # And as such is now visibly passed down
  FOO=BAR

   Without using printenv or grep -F (in shell script /bin/fgrep)

   $ export FRED=value
   $ set|grep FRED
   FRED='value'
   $ FRED=
   $ set|grep FRED
   FRED=''
   $ unset FRED
   $ set|grep FRED
   $ # ie, no output from set