Telling shell command errors from command exit statuses.

[idallen at watmath.UUCP]

>From: stanwyck at ihuxr.UUCP (Don Stanwyck)
>
>Obviously the author of the above lines expects the echo to be executed.
>If [s]he didn't, [s]he would have used || or && instead of the ;. 

Are "command not found" and "search pattern not found" the same kind of error?

    grep bleen file && date
    nosuchcommand   && date
    cmd >nosuchfile && date
    cmd nosuchglob* && date

In the shells with which I am familiar, only the first exit status is
set by the command itself.  The Shell fakes the other three exit
statuses, because the command (on the left) is never executed.  Is it
fair to fake an exit status for the user?  What business does the Shell
have doing that sort of thing?  I want to argue that the Shell's job is
to correctly start commands and return their error status.  If the
Shell can't start a command because of a Shell error, it has no
business faking a return status and thus pretending it did.

This kind of distinction isn't a problem with commands separated by '&&',
since you abandon the subsequent command on an error, regardless of
what the error is.  But you weaken the power of '||' if you lump shell
errors in with non-zero return codes from successful programs.

    grep bleen file || echo "bleen" >>file
    grez bleen file || echo "bleen" >>file

With the weak interpretation of '||', both these lines add "bleen" to
"file", because the shell fakes a non-zero return code instead of
admitting it couldn't find the command "grez".  With a stronger
interpretation of '||', you could be assured that "bleen" would be
added to "file" *only if a proper call of grep returned non zero*.

Of course, if exit statuses had been designed so that there were
stanard distinct values for "negative status" and "command didn't get
started because of error", then the Shell could fake the latter status,
just as any command could return that same status (like GREP returing
2 for an improper invocation).
-- 
        -IAN!  (Ian! D. Allen)      University of Waterloo