For a long time, I'm confused on the subtle difference between the following two usages:

>/dev/null 2>&1
2>&1 >/dev/null

Could you please give me an example to present the difference?

Regards,
Zhao

On Fri, 19 May 2023 16:51:47 -0700, hongy...@gmail.com wrote:

> For a long time, I'm confused on the subtle difference between the following two usages:
>
>>/dev/null 2>&1
> 2>&1 >/dev/null
>
> Could you please give me an example to present the difference?

The redirection
>/dev/null 2>&1
first directs stdout to /dev/null,
then directs stderr to whatever stdout is currently directed to.

If, for instance, stdout starts out being directed to /tmp/file1
and stderr starts out being directed to /tmp/file2, then
>/dev/null 2>&1
will result in both stdout and stderr directing to /dev/null

On the other hand, he redirection
2>&1 >/dev/null
first directs stderr to whatever stdout is currently directed to,
then directs stdout to /dev/null.

If, for instance, stdout starts out being directed to /tmp/file1
and stderr starts out being directed to /tmp/file2, then
2>&1 >/dev/null
will result in both stderr being directed to /tmp/file1, and
stdout being directed to /dev/null

HTH
--
Lew Pitcher
"In Skills We Trust"

On Saturday, May 20, 2023 at 8:27:32 AM UTC+8, Lew Pitcher wrote:
> On Fri, 19 May 2023 16:51:47 -0700, hongy...@gmail.com wrote:
>
> > For a long time, I'm confused on the subtle difference between the following two usages:
> >
> >>/dev/null 2>&1
> > 2>&1 >/dev/null
> >
> > Could you please give me an example to present the difference?
> The redirection
> >/dev/null 2>&1
> first directs stdout to /dev/null,
> then directs stderr to whatever stdout is currently directed to.
>
> If, for instance, stdout starts out being directed to /tmp/file1
> and stderr starts out being directed to /tmp/file2, then
> >/dev/null 2>&1
> will result in both stdout and stderr directing to /dev/null
>
>
> On the other hand, he redirection
> 2>&1 >/dev/null
> first directs stderr to whatever stdout is currently directed to,
> then directs stdout to /dev/null.
>
> If, for instance, stdout starts out being directed to /tmp/file1
> and stderr starts out being directed to /tmp/file2, then
> 2>&1 >/dev/null
> will result in both stderr being directed to /tmp/file1, and
> stdout being directed to /dev/null

Thank you for your detailed explanation. If so, the following two seem equivalent:

>/dev/null 2>&1
2>/dev/null 1>&2

For simplicity, the first should be used.
> HTH
> --
> Lew Pitcher
> "In Skills We Trust"

Zhao

On Fri, 19 May 2023 17:46:26 -0700, hongy...@gmail.com wrote:

> On Saturday, May 20, 2023 at 8:27:32 AM UTC+8, Lew Pitcher wrote:
>> On Fri, 19 May 2023 16:51:47 -0700, hongy...@gmail.com wrote:
>>
>> > For a long time, I'm confused on the subtle difference between the following two usages:
>> >
>> >>/dev/null 2>&1
>> > 2>&1 >/dev/null
>> >
>> > Could you please give me an example to present the difference?
>> The redirection
>> >/dev/null 2>&1
>> first directs stdout to /dev/null,
>> then directs stderr to whatever stdout is currently directed to.
>>
>> If, for instance, stdout starts out being directed to /tmp/file1
>> and stderr starts out being directed to /tmp/file2, then
>> >/dev/null 2>&1
>> will result in both stdout and stderr directing to /dev/null
>>
>>
>> On the other hand, he redirection
>> 2>&1 >/dev/null
>> first directs stderr to whatever stdout is currently directed to,
>> then directs stdout to /dev/null.
>>
>> If, for instance, stdout starts out being directed to /tmp/file1
>> and stderr starts out being directed to /tmp/file2, then
>> 2>&1 >/dev/null
>> will result in both stderr being directed to /tmp/file1, and
>> stdout being directed to /dev/null
>
> Thank you for your detailed explanation. If so, the following two seem equivalent:
>
>>/dev/null 2>&1
> 2>/dev/null 1>&2

Yes, they both result in stdout and stderr being directed to /dev/null

> For simplicity, the first should be used.

Yes. The common idiom is >/dev/null 2>&1

>> HTH
>> --
>> Lew Pitcher
>> "In Skills We Trust"
>
> Zhao

--
Lew Pitcher
"In Skills We Trust"

"hongy...@gmail.com" <hongyi.zhao@gmail.com>:
> For a long time, I'm confused on the subtle difference between
> the following two usages:
>
> >/dev/null 2>&1
> 2>&1 >/dev/null
>
> Could you please give me an example to present the difference?
>

Do you have access to news articles back in 2021?  Then you might
take a look at the article

Subject: Redirecting file descriptors (was: How to redirect
to non-standard file descriptors?)
From: Helmut Waitzmann <nn.throttle@xoxy.net>
Newsgroups: comp.unix.shell
Date: Tue, 05 Oct 2021 21:55:27 +0200
Message-ID: <835yubi6vk.fsf@helmutwaitzmann.news.arcor.de>
References: <irtteiFpfsqU1@mid.individual.net>

It is a followup to an article posted by Robert Latest, who asked
a similar question as you do.

If you (or any other person) don't have access to it any more, I
could either send it to you by e‐mail or repost it here.

Also
<http://al.howardknight.net/?STYPE=msgid&A=0&MSGI=%3C835yubi6vk.fsf%40helmutwaitzmann.news.arcor.de%3E>
might help you.

On Saturday, May 20, 2023 at 9:48:14 AM UTC+8, Helmut Waitzmann wrote:
> "hongy...@gmail.com" <hongy...@gmail.com>:
> > For a long time, I'm confused on the subtle difference between
> > the following two usages:
> >
> > >/dev/null 2>&1
> > 2>&1 >/dev/null
> >
> > Could you please give me an example to present the difference?
> >
> Do you have access to news articles back in 2021? Then you might
> take a look at the article
>
> Subject: Redirecting file descriptors (was: How to redirect
> to non-standard file descriptors?)
> From: Helmut Waitzmann <nn.th...@xoxy.net>
> Newsgroups: comp.unix.shell
> Date: Tue, 05 Oct 2021 21:55:27 +0200
> Message-ID: <835yubi...@helmutwaitzmann.news.arcor.de>
> References: <irttei...@mid.individual.net>
>
> It is a followup to an article posted by Robert Latest, who asked
> a similar question as you do.
>
> If you (or any other person) don't have access to it any more, I
> could either send it to you by e‐mail or repost it here.
>
> Also
> <http://al.howardknight.net/?STYPE=msgid&A=0&MSGI=%3C835yubi6vk.fsf%40helmutwaitzmann.news.arcor.de%3E>
> might help you.

I can open the above URL, but the message shown there includes some non-human readable stuff, for example:

the shell first will do the same and then tell the kernel by means=20
of the system service =E2=80=9Cdup2=E2=80=9D to copy the contents of the entry #1 to=20
the entry #2.=C2=A0 Now two entries in the process' file descriptor=20
table=C2=A0=E2=80=93 #1 and #2=C2=A0=E2=80=93 refer to the opened file =E2=80=9Coutput.txt=E2=80=9D.

Best,
Zhao

On Fri, 19 May 2023 19:58:49 -0700 (PDT)
"hongy...@gmail.com" <hongyi.zhao@gmail.com> wrote:
> > Also
> > <http://al.howardknight.net/?STYPE=msgid&A=0&MSGI=%3C835yubi6vk.fsf%40helmutwaitzmann.news.arcor.de%3E>
> > might help you.

More simply http://al.howardknight.net/?ID=165151671300 .I have that
whole thread saved , very instructive.

> I can open the above URL, but the message shown there includes some
> non-human readable stuff, for example:
>
> the shell first will do the same and then tell the kernel by means=20
> of the system service =E2=80=9Cdup2=E2=80=9D to copy the contents of the en=
> try #1 to=20
> the entry #2.=C2=A0 Now two entries in the process' file descriptor=20
> table=C2=A0=E2=80=93 #1 and #2=C2=A0=E2=80=93 refer to the opened file =E2=
> =80=9Coutput.txt=E2=80=9D.

quoted-printable strikes again. al.howardknight.net doesn't do any
decoding of the articles. =E2=80=9C means the 3 octets with hexadecimal
values E2 , 80 , 9C which correspond to a certain quote character (not
the familiar ASCII quotes) in UTF-8 encoding.

In article <e09ff1ac-02e0-49dc-a590-d4917e1f02f6n@googlegroups.com>,
hongy...@gmail.com <hongyi.zhao@gmail.com> wrote:
>For a long time, I'm confused on the subtle difference between the following two usages:
>
>>/dev/null 2>&1
>2>&1 >/dev/null
>
>Could you please give me an example to present the difference?
>

There are also documents on the Internet that go into detail about
these sort of things. For example:

<https://mywiki.wooledge.org/BashGuide/InputAndOutput#line-327>

(scrolling up a little bit on the page also gives useful info)

--
::::::::::::: Greg Andrews ::::: gerg@panix.com :::::::::::::
I have a map of the United States that's actual size.
-- Steven Wright