Hi Martin,
thanks for reply.
> > # run an empty for cycle. we can see from the output that bash ran :, then
> > # false and stopped to it. which is the right behavior
> > for (( :; false; )); do :; done
>
> This is a syntax error. 'for' takes arithmetic expressions, not shell
> commands.
Sorry, my bad. I just wanted to make the command as simple as possible to
demonstrate the order of execution and I didn't notice that I got into an
invalid command. It's the same with this, valid, command:
for (( ; 1 > 2; )); do :; done
> > # now run this while cycle. it just cycle the debug trap forever
> > while false; do :; done
> The 'false' command is never executed so never has the chance to return
> an exit status of "false" (1).
To have both commands similar, let's assume the while as this:
while (( 1 > 2 )); do :; done
Thanks for explaining me why it happen, but now I don't know why for cycle
works and while does not.
The for cycle run the condition (after setting: t() { return 1; }; trap t DEBUG; shopt -s extdebug)
bash-4.3$ for (( ; 1 > 2; )); do :; done
+ (( 1 ))
++ t
++ return 1
+ (( 1 > 2 ))
++ t
++ return 1
but the while cycle does not:
bash-4.3$ while (( 1 > 2 )); do :; done
++ t
++ return 1
++ t
++ return 1
.. loop forever ..
Is there any workaround to achieve the functionality I want? That is:
Prevent execution of everything by default. I will decide what to run by
using eval from the debug trap.
Thanks,
Tomas