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Re: Bugs in ERR and RETURN traps


From: Chet Ramey
Subject: Re: Bugs in ERR and RETURN traps
Date: Tue, 24 Feb 2015 15:47:30 -0500
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:31.0) Gecko/20100101 Thunderbird/31.4.0

On 2/19/15 2:54 PM, Paul Donohue wrote:

> The RETURN trap does not see the exit status of 'return', but rather the exit 
> status of the last command before 'return' was called.
> 
> Example:
> $ test_fun()
> {
> trap 'echo returned $?' RETURN
> false # exit status is 1
> return 2
> }
> $ test_fun
> returned 1
> $
> 
> I intuitively expected the above to print 2 instead of 1.
> 
> The bash man page states "Any command associated with the RETURN trap is 
> executed before execution resumes after the function or script." This is a 
> bit vague, but it seems to imply that the RETURN trap should run after the 
> `return` command is complete (since the `return` command is part of the 
> function and the RETURN trap runs "after the function"), which would imply 
> that $? should be set to the exit status of the return command in the RETURN 
> trap. So, the documentation seems to back up my intuition here...
> 
> The problem I was actually trying to solve was to write a trap that ran only 
> if the function returned an error. An ERR trap would have been run if any 
> command within the function returned an error, which was not what I wanted. 
> So, I simply wrote a RETURN trap which checked $?, but $? did not give me the 
> return status of the function, so this didn't work.

The RETURN trap is run in the function's context, so this approach will
probably not do what you want.  The idea is that the trap allows the
function writer to do cleanup while still seeing variables in the
function's scope, for example.  It's not intended to reflect the state
after the function's execution context is unwound.

As I recall -- and this was a long time ago -- it came in as part of the
bash debugger changes and these were the semantics from the start.

Chet
-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, ITS, CWRU    address@hidden    http://cnswww.cns.cwru.edu/~chet/



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