Why can’t I specify an environment variable and echo it in the same command line?

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What you see is the expected behaviour. The trouble is that the parent shell evaluates $SOMEVAR on the command line before it invokes the command with the modified environment. You need to get the evaluation of $SOMEVAR deferred until after the environment is set.

Your immediate options include:

  1. SOMEVAR=BBB eval echo zzz '$SOMEVAR' zzz.
  2. SOMEVAR=BBB sh -c 'echo zzz $SOMEVAR zzz'.

Both these use single quotes to prevent the parent shell from evaluating $SOMEVAR; it is only evaluated after it is set in the environment (temporarily, for the duration of the single command).

Another option is to use the sub-shell notation (as also suggested by Marcus Kuhn in his answer):

(SOMEVAR=BBB; echo zzz $SOMEVAR zzz)

The variable is set only in the sub-shell

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