Re: [Help-bash] Are all traps global to a script?

[Eric Blake]

On 12/7/18 2:00 PM, Peng Yu wrote:
> Hi,
>
> I see that traps seem to be all global. For example, if I set it in a
> function, it will have an effect outside a function.

You're demonstrating that you may lack a fundamental understanding of 
how functions work.  Functions are snippets of code that run in the 
context of the caller (if we disregard the special effects of 'local'), 
and have an impact to the environment the same as any other code run in 
that context.

> What is the best way to make effectively different traps for different
> function? Does bash have a native support for this?

The same way that you implement different traps for different portions 
of a shell script that does not use functions. You can use 'trap -p' to 
save off the previous trap definition, install your new trap, then when 
done with your code, call trap to set things back to the value you saved 
off earlier.

> $ cat ./main.sh
> #!/usr/bin/env bash
> # vim: set noexpandtab tabstop=2:
>
> trap "echo 'Hello World1!'" INT
> function f {
>      trap "echo 'Hello World!'" INT
>      echo 'in f'
> }
> f
> sleep 10
> echo 'after f'

Remember, this is EXACTLY the same as writing:

trap "echo 'Hello World1!'" INT
{
  trap "echo 'Hello World!'" INT
  echo 'in f'
}
sleep 10
echo 'after f'

(that is, instead of defining 'f' to be a snippet of shell code, then 
invoking 'f' to run that snippet of code, just run that snippet of code 
directly)

And if you can solve the problem of restoring the original trap without 
a shell function (which you can thanks to 'trap -p'), then you can solve 
it when shell functions are involved.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
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