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

Post by Peng Yu
Hi,
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

--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org

Bob Proulx

2018-12-03 23:22:07 UTC

Post by Peng Yu
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

Peng Yu

2018-12-04 02:51:09 UTC

Post by Peng Yu
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

(exec cmd ...)
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

Dennis Williamson

2018-12-04 03:19:05 UTC

Post by Peng Yu
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

(exec cmd ...)
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
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.

Peng Yu

2018-12-04 04:46:35 UTC

On Mon, Dec 3, 2018 at 9:19 PM Dennis Williamson

Post by Peng Yu
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

(exec cmd ...)
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
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

Pierre Gaston

2018-12-04 07:29:03 UTC

Post by Peng Yu
On Mon, Dec 3, 2018 at 9:19 PM Dennis Williamson

Post by Peng Yu
I currently use this to make sure to always run the external

command.

Post by Peng Yu
Is it the best way to do so? Thanks.
"$(type -P cmd)" arg1 arg2

env cmd arg1 arg2
also works

(exec cmd ...)
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
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

Peng Yu

2018-12-04 12:59:54 UTC

Post by Pierre Gaston

Post by Peng Yu
On Mon, Dec 3, 2018 at 9:19 PM Dennis Williamson