Re: [Help-bash] How to make sure always run external command instead of builtin function?

[Peng Yu]

On Tue, Dec 4, 2018 at 1:29 AM Pierre Gaston <address@hidden> wrote:
>
>
>
> On Tue, Dec 4, 2018 at 6:57 AM Peng Yu <address@hidden> wrote:
>>
>> On Mon, Dec 3, 2018 at 9:19 PM Dennis Williamson
>> <address@hidden> wrote:
>> >
>> >
>> >
>> > On Mon, Dec 3, 2018, 8:51 PM Peng Yu <address@hidden wrote:
>> >>
>> >> On Mon, Dec 3, 2018 at 5:22 PM Bob Proulx <address@hidden> wrote:
>> >> >
>> >> > Eric Blake wrote:
>> >> > > Peng Yu wrote:
>> >> > > > I currently use this to make sure to always run the external 
>> >> > > > command.
>> >> > > > Is it the best way to do so? Thanks.
>> >> > > >
>> >> > > > "$(type -P cmd)" arg1 arg2
>> >> > >
>> >> > > env cmd arg1 arg2
>> >> > > also works
>> >> >
>> >> > There is also another way too:
>> >> >
>> >> >   (exec cmd ...)
>> >> >
>> >> > As I learned just the other day:
>> >> >
>> >> >   http://lists.gnu.org/archive/html/bug-bash/2018-09/msg00104.html
>> >> >   http://lists.gnu.org/archive/html/bug-bash/2018-09/msg00101.html
>> >> >
>> >> > Bob
>> >> >
>> >>
>> >> This is good. (exec cmd) is faster.
>> >>
>> >> $  ./main_ls.sh
>> >> time for ((i=0;i<300;++i))
>> >> do
>> >>     (exec ls)
>> >> done > /dev/null
>> >>
>> >> real    0m2.231s
>> >> user    0m0.677s
>> >> sys    0m1.013s
>> >>
>> >> time for ((i=0;i<300;++i))
>> >> do
>> >>     "$(type -P ls)"
>> >> done > /dev/null
>> >>
>> >> real    0m2.742s
>> >> user    0m0.830s
>> >> sys    0m1.352s
>> >> $ cat main_ls.sh
>> >> #!/usr/bin/env bash
>> >> # vim: set noexpandtab tabstop=2:
>> >>
>> >> set -v
>> >> time for ((i=0;i<300;++i))
>> >> do
>> >>     (exec ls)
>> >> done > /dev/null
>> >>
>> >> time for ((i=0;i<300;++i))
>> >> do
>> >>     "$(type -P ls)"
>> >> done > /dev/null
>> >>
>> >>
>> >> --
>> >> Regards,
>> >> Peng
>> >
>> >
>> > You didn't time the env version.
>>
>> I assume that it is slower as involves an extra external command 'env'.
>>
>> --
>> Regards,
>> Peng
>>
>
> Maybe, maybe not, env is most likely not forking again, so :
>
> * (exec ls):  you have probably a fork and an exec.
> * env ls:  you have probably a fork an exec and then another exec.
>
> The expensive operation is (I assume) forking so the results may not be so 
> different..in fact:
>
> $ cat envexec
> time for ((i=0;i<300;++i))
> do
>     (exec ls)
> done > /dev/null
>
> time for ((i=0;i<300;++i))
> do
>    env ls
> done > /dev/null
>
> $ bash envexec
>
> real    0m1,140s
> user    0m0,272s
> sys     0m0,232s
>
> real    0m1,075s
> user    0m0,020s
> sys     0m0,072s

In my case, it can be slower.

time for ((i=0;i<300;++i))
do
    (exec ls)
done > /dev/null

real    0m2.104s
user    0m0.663s
sys    0m0.950s

time for ((i=0;i<300;++i))
do
    env ls
done > /dev/null

real    0m4.491s
user    0m1.074s
sys    0m2.329s




-- 
Regards,
Peng