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From: Bart Schaefer <schaefer@brasslantern.com>
Message-Id: <150409114205.ZM25276@torch.brasslantern.com>
Date: Thu, 9 Apr 2015 11:42:05 -0700
In-Reply-To: <mg69er$i52$1@ger.gmane.org>
Comments: In reply to Thorsten Kampe <thorsten@thorstenkampe.de>
        "Catch `setopt nounset`" (Apr  9,  6:30pm)
References: <mg69er$i52$1@ger.gmane.org>
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To: zsh-users@zsh.org
Subject: Re: Catch `setopt nounset`
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On Apr 9,  6:30pm, Thorsten Kampe wrote:
} 
} I'd to catch the call of a parameter that does not exist with `setopt 
} nounset`.

    setopt nounset
    {
      print "$DOESNOTEXIST"
    } always {
      if (( TRY_BLOCK_ERROR ))
      then print "error"
      else print "no error"
      fi
    }

} If I replace the `catch` statement with `if result=$(testfunc)` it 
} prints "error". What am I missing here? Why does the latter work but 
} not the former?

"parameter not set" is a fatal error, so whatever the current shell is
doing is stopped when that happens.  $(testfunc) is run in a subshell,
so the current shell proceeds even though the subshell died.

You could get the same result with

  if ( testfunc )
  then
    printf "no error\n"
  else
    printf "error\n"
  fi

but of course that also suppresses all other side-effects of "testfunc",
which the "always" construct will not.