piping stdout and stderr to different processes?

24/07/2007, 21h58

I've found name pipes (fifos) but I am confused on using them
properly. What I want to do is to take the stdout of a process and
send it to another process to be filtered. I also want to take the
stderr of the first process and send it to another process to
also be filtered. Any examples?

Thanks (an no it's not for school).

--
Linux Home Automation Neil Cherry ncherry@linuxha.com
http://www.linuxha.com/ Main site
http://linuxha.blogspot.com/ My HA Blog
Author of: Linux Smart Homes For Dummies

25/07/2007, 01h01

On Tue, 24 Jul 2007 15:58:52 -0500, Neil Cherry wrote:

> I've found name pipes (fifos) but I am confused on using them
> properly. What I want to do is to take the stdout of a process and
> send it to another process to be filtered. I also want to take the
> stderr of the first process and send it to another process to
> also be filtered. Any examples?
>
> Thanks (an no it's not for school).
>

Maybe this simple example will . I will create two named pipes.
Then I will in the background, cat these pipes to files. Next I will
create a file named "afile". To make it all happen I do a directory
listing of afile and notafile (notafile does not exist and should give an
error) and redirect the output to the named pipes. Finally I will cat out
the files with the data that the background cats read from the fifos.
First a cut and paste of my terminal, following is a breakdown of what
happens.

$ mkfifo stderrpipe
$ mkfifo stdoutpipe
$ cat stderrpipe > errorlog &
[1] 5860
$ cat stdoutpipe > outlog &
[2] 5863
$ touch afile
$ ls afile notafile > stdoutpipe 2> stderrpipe
[1]- Done cat stderrpipe > errorlog
[2]+ Done cat stdoutpipe > outlog
$ cat errorlog
ls: notafile: No such file or directory
$ cat outlog
afile

DEEPER EXPLANATIONS FOLLOW

CREATE PIPE FOR ERROR
$ mkfifo stderrpipe

CREATE PIPE FOR STDOUT
$ mkfifo stdoutpipe

CAT THE ERROR PIPE TO A FILE IN BACKGROUND
$ cat stderrpipe > errorlog &
BACKGROUND PROCESS #1
[1] 5860

CAT THE STDOUT PIPE TO A FILE IN BACKGROUND
$ cat stdoutpipe > outlog &
BACKGROUND PROCESS #2
[2] 5863

CREATE A FILE "afile"
$ touch afile

REDIRECT THE OUTPUT OF ls TO THE PIPES
$ ls afile notafile > stdoutpipe 2> stderrpipe

BOTH BACKGROUND PROCESSES FINISH WHEN THEIR
PIPES GET TO END OF FILE. THAT IS NOW, BECAUSE
THE LS COMMAND IS FINISHED GIVING THEM DATA

[1]- Done cat stderrpipe > errorlog
[2]+ Done cat stdoutpipe > outlog

OUTPUT THE FILES CREATED BY THE ABOVE cats
$ cat errorlog
ls: notafile: No such file or directory
$ cat outlog
afile

I hope that s.

stonerfish

25/07/2007, 01h01

On Tue, 24 Jul 2007 15:58:52 -0500, Neil Cherry wrote:

> I've found name pipes (fifos) but I am confused on using them
> properly. What I want to do is to take the stdout of a process and
> send it to another process to be filtered. I also want to take the
> stderr of the first process and send it to another process to
> also be filtered. Any examples?
>
> Thanks (an no it's not for school).
>

Maybe this simple example will . I will create two named pipes.
Then I will in the background, cat these pipes to files. Next I will
create a file named "afile". To make it all happen I do a directory
listing of afile and notafile (notafile does not exist and should give an
error) and redirect the output to the named pipes. Finally I will cat out
the files with the data that the background cats read from the fifos.
First a cut and paste of my terminal, following is a breakdown of what
happens.

$ mkfifo stderrpipe
$ mkfifo stdoutpipe
$ cat stderrpipe > errorlog &
[1] 5860
$ cat stdoutpipe > outlog &
[2] 5863
$ touch afile
$ ls afile notafile > stdoutpipe 2> stderrpipe
[1]- Done cat stderrpipe > errorlog
[2]+ Done cat stdoutpipe > outlog
$ cat errorlog
ls: notafile: No such file or directory
$ cat outlog
afile

DEEPER EXPLANATIONS FOLLOW

CREATE PIPE FOR ERROR
$ mkfifo stderrpipe

CREATE PIPE FOR STDOUT
$ mkfifo stdoutpipe

CAT THE ERROR PIPE TO A FILE IN BACKGROUND
$ cat stderrpipe > errorlog &
BACKGROUND PROCESS #1
[1] 5860

CAT THE STDOUT PIPE TO A FILE IN BACKGROUND
$ cat stdoutpipe > outlog &
BACKGROUND PROCESS #2
[2] 5863

CREATE A FILE "afile"
$ touch afile

REDIRECT THE OUTPUT OF ls TO THE PIPES
$ ls afile notafile > stdoutpipe 2> stderrpipe

BOTH BACKGROUND PROCESSES FINISH WHEN THEIR
PIPES GET TO END OF FILE. THAT IS NOW, BECAUSE
THE LS COMMAND IS FINISHED GIVING THEM DATA

[1]- Done cat stderrpipe > errorlog
[2]+ Done cat stdoutpipe > outlog

OUTPUT THE FILES CREATED BY THE ABOVE cats
$ cat errorlog
ls: notafile: No such file or directory
$ cat outlog
afile

I hope that s.

stonerfish

25/07/2007, 10h27

2007-07-24, 15:58(-05), Neil Cherry:
> I've found name pipes (fifos) but I am confused on using them
> properly. What I want to do is to take the stdout of a process and
> send it to another process to be filtered. I also want to take the
> stderr of the first process and send it to another process to
> also be filtered. Any examples?
[...]

No need for named pipes here.

{
{
cm1 3>&- |
cmd2 2>&3 3>&-
} 2>&1 >&4 4>&- |
cmd3 3>&- 4>&-
} 3>&2 4>&1

--
Stéphane

25/07/2007, 10h27

2007-07-24, 15:58(-05), Neil Cherry:
> I've found name pipes (fifos) but I am confused on using them
> properly. What I want to do is to take the stdout of a process and
> send it to another process to be filtered. I also want to take the
> stderr of the first process and send it to another process to
> also be filtered. Any examples?
[...]

No need for named pipes here.

{
{
cm1 3>&- |
cmd2 2>&3 3>&-
} 2>&1 >&4 4>&- |
cmd3 3>&- 4>&-
} 3>&2 4>&1

--
Stéphane

27/07/2007, 02h41

On Wed, 25 Jul 2007 00:01:40 GMT, jellybean stonerfish wrote:
> On Tue, 24 Jul 2007 15:58:52 -0500, Neil Cherry wrote:
>
>> I've found name pipes (fifos) but I am confused on using them
>> properly. What I want to do is to take the stdout of a process and
>> send it to another process to be filtered. I also want to take the
>> stderr of the first process and send it to another process to
>> also be filtered. Any examples?
>>
>> Thanks (an no it's not for school).
>>
>
> Maybe this simple example will . I will create two named pipes.
> Then I will in the background, cat these pipes to files. Next I will
> create a file named "afile". To make it all happen I do a directory
> listing of afile and notafile (notafile does not exist and should give an
> error) and redirect the output to the named pipes. Finally I will cat out
> the files with the data that the background cats read from the fifos.
> First a cut and paste of my terminal, following is a breakdown of what
> happens.
>
> $ mkfifo stderrpipe
> $ mkfifo stdoutpipe
> $ cat stderrpipe > errorlog &
> [1] 5860
> $ cat stdoutpipe > outlog &
> [2] 5863
> $ touch afile
> $ ls afile notafile > stdoutpipe 2> stderrpipe
> [1]- Done cat stderrpipe > errorlog
> [2]+ Done cat stdoutpipe > outlog
> $ cat errorlog
> ls: notafile: No such file or directory
> $ cat outlog
> afile
>
>
> DEEPER EXPLANATIONS FOLLOW
>
> CREATE PIPE FOR ERROR
> $ mkfifo stderrpipe
>
> CREATE PIPE FOR STDOUT
> $ mkfifo stdoutpipe
>
> CAT THE ERROR PIPE TO A FILE IN BACKGROUND
> $ cat stderrpipe > errorlog &
> BACKGROUND PROCESS #1
> [1] 5860
>
> CAT THE STDOUT PIPE TO A FILE IN BACKGROUND
> $ cat stdoutpipe > outlog &
> BACKGROUND PROCESS #2
> [2] 5863
>
> CREATE A FILE "afile"
> $ touch afile
>
> REDIRECT THE OUTPUT OF ls TO THE PIPES
> $ ls afile notafile > stdoutpipe 2> stderrpipe
>
> BOTH BACKGROUND PROCESSES FINISH WHEN THEIR
> PIPES GET TO END OF FILE. THAT IS NOW, BECAUSE
> THE LS COMMAND IS FINISHED GIVING THEM DATA
>
> [1]- Done cat stderrpipe > errorlog
> [2]+ Done cat stdoutpipe > outlog
>
> OUTPUT THE FILES CREATED BY THE ABOVE cats
> $ cat errorlog
> ls: notafile: No such file or directory
> $ cat outlog
> afile
>
> I hope that s.
>
> stonerfish

Thanks! That s big time.

--
Linux Home Automation Neil Cherry ncherry@linuxha.com
http://www.linuxha.com/ Main site
http://linuxha.blogspot.com/ My HA Blog
Author of: Linux Smart Homes For Dummies

27/07/2007, 02h44

On Wed, 25 Jul 2007 09:27:33 GMT, Stephane CHAZELAS wrote:
> 2007-07-24, 15:58(-05), Neil Cherry:
>> I've found name pipes (fifos) but I am confused on using them
>> properly. What I want to do is to take the stdout of a process and
>> send it to another process to be filtered. I also want to take the
>> stderr of the first process and send it to another process to
>> also be filtered. Any examples?
> [...]
>
> No need for named pipes here.
>
> {
> {
> cm1 3>&- |
> cmd2 2>&3 3>&-
> } 2>&1 >&4 4>&- |
> cmd3 3>&- 4>&-
> } 3>&2 4>&1

Thanks.

OK now I see why I didn't get it to work. I didn't try that far.
But I have to say I'm not quite sure what I'm reading just yet.
I'll have to hit the man pages as I'm not used to the >&-
syntax.

--
Linux Home Automation Neil Cherry ncherry@linuxha.com
http://www.linuxha.com/ Main site
http://linuxha.blogspot.com/ My HA Blog
Author of: Linux Smart Homes For Dummies

27/07/2007, 09h12

On 27 Jul., 03:44, Neil Cherry <n...@.uucp> wrote:
> On Wed, 25 Jul 2007 09:27:33 GMT, Stephane CHAZELAS wrote:
> > 2007-07-24, 15:58(-05), Neil Cherry:
> >> I've found name pipes (fifos) but I am confused on using them
> >> properly. What I want to do is to take the stdout of a process and
> >> send it to another process to be filtered. I also want to take the
> >> stderr of the first process and send it to another process to
> >> also be filtered. Any examples?
> > [...]
>
> > No need for named pipes here.
>
> > {
> > {
> > cm1 3>&- |
> > cmd2 2>&3 3>&-
> > } 2>&1 >&4 4>&- |
> > cmd3 3>&- 4>&-
> > } 3>&2 4>&1
>
> Thanks.
>
> OK now I see why I didn't get it to work. I didn't try that far.
> But I have to say I'm not quite sure what I'm reading just yet.
> I'll have to hit the man pages as I'm not used to the >&-
> syntax.

N>&- close the file descriptor with number N.

Janis

>
> --
> Linux Home Automation Neil Cherry nche...@linuxha.comhttp://www.linuxha.com/ Main sitehttp://linuxha.blogspot.com/ My HA Blog
> Author of: Linux Smart Homes For Dummies

27/07/2007, 10h08

2007-07-26, 20:44(-05), Neil Cherry:
[...]
>> {
>> {
>> cm1 3>&- |
>> cmd2 2>&3 3>&-
>> } 2>&1 >&4 4>&- |
>> cmd3 3>&- 4>&-
>> } 3>&2 4>&1
>
> Thanks.
>
> OK now I see why I didn't get it to work. I didn't try that far.
> But I have to say I'm not quite sure what I'm reading just yet.
> I'll have to hit the man pages as I'm not used to the >&-
> syntax.

3>&- is for closing fd 3. It's not necessary, but it's for tidy
up. None of the commands will ever try (not should they) to
access the fd 3 and 4, so it's best to close them before
executing those commands so that they can use those fds for
something else.

{
{
cm1 |
cmd2 2>&3
} 2>&1 >&4 |
cmd3
} 3>&2 4>&1

is functionnaly equivalent.

if cmd2 doesn't output anything on its stdout nor stderr, it can
even be simplified to:
{ cm1 | cmd2; } 2>&1 | cmd3

Or if you want to be sure:

{ cm1 | cmd2 > /dev/null 2>&1; } 2>&1 | cmd3

--
Stéphane

27/07/2007, 21h49

On Fri, 27 Jul 2007 09:08:23 GMT, Stephane CHAZELAS wrote:
> 2007-07-26, 20:44(-05), Neil Cherry:
> [...]
>>> {
>>> {
>>> cm1 3>&- |
>>> cmd2 2>&3 3>&-
>>> } 2>&1 >&4 4>&- |
>>> cmd3 3>&- 4>&-
>>> } 3>&2 4>&1
>>
>> Thanks.
>>
>> OK now I see why I didn't get it to work. I didn't try that far.
>> But I have to say I'm not quite sure what I'm reading just yet.
>> I'll have to hit the man pages as I'm not used to the >&-
>> syntax.
>
> 3>&- is for closing fd 3. It's not necessary, but it's for tidy
> up. None of the commands will ever try (not should they) to
> access the fd 3 and 4, so it's best to close them before
> executing those commands so that they can use those fds for
> something else.
>
> {
> {
> cm1 |
> cmd2 2>&3
> } 2>&1 >&4 |
> cmd3
> } 3>&2 4>&1
>
> is functionnaly equivalent.
>
> if cmd2 doesn't output anything on its stdout nor stderr, it can
> even be simplified to:
> { cm1 | cmd2; } 2>&1 | cmd3
>
> Or if you want to be sure:
>
> { cm1 | cmd2 > /dev/null 2>&1; } 2>&1 | cmd3
>

Thanks that s. :-)

--
Linux Home Automation Neil Cherry ncherry@linuxha.com
http://www.linuxha.com/ Main site
http://linuxha.blogspot.com/ My HA Blog
Author of: Linux Smart Homes For Dummies

25/07/2007, 01h01	#3
jellybean stonerfish Messages: n/a Hébergeur:	Re: piping stdout and stderr to different processes? On Tue, 24 Jul 2007 15:58:52 -0500, Neil Cherry wrote: > I've found name pipes (fifos) but I am confused on using them > properly. What I want to do is to take the stdout of a process and > send it to another process to be filtered. I also want to take the > stderr of the first process and send it to another process to > also be filtered. Any examples? > > Thanks (an no it's not for school). > Maybe this simple example will . I will create two named pipes. Then I will in the background, cat these pipes to files. Next I will create a file named "afile". To make it all happen I do a directory listing of afile and notafile (notafile does not exist and should give an error) and redirect the output to the named pipes. Finally I will cat out the files with the data that the background cats read from the fifos. First a cut and paste of my terminal, following is a breakdown of what happens. $ mkfifo stderrpipe $ mkfifo stdoutpipe $ cat stderrpipe > errorlog & [1] 5860 $ cat stdoutpipe > outlog & [2] 5863 $ touch afile $ ls afile notafile > stdoutpipe 2> stderrpipe [1]- Done cat stderrpipe > errorlog [2]+ Done cat stdoutpipe > outlog $ cat errorlog ls: notafile: No such file or directory $ cat outlog afile DEEPER EXPLANATIONS FOLLOW CREATE PIPE FOR ERROR $ mkfifo stderrpipe CREATE PIPE FOR STDOUT $ mkfifo stdoutpipe CAT THE ERROR PIPE TO A FILE IN BACKGROUND $ cat stderrpipe > errorlog & BACKGROUND PROCESS #1 [1] 5860 CAT THE STDOUT PIPE TO A FILE IN BACKGROUND $ cat stdoutpipe > outlog & BACKGROUND PROCESS #2 [2] 5863 CREATE A FILE "afile" $ touch afile REDIRECT THE OUTPUT OF ls TO THE PIPES $ ls afile notafile > stdoutpipe 2> stderrpipe BOTH BACKGROUND PROCESSES FINISH WHEN THEIR PIPES GET TO END OF FILE. THAT IS NOW, BECAUSE THE LS COMMAND IS FINISHED GIVING THEM DATA [1]- Done cat stderrpipe > errorlog [2]+ Done cat stdoutpipe > outlog OUTPUT THE FILES CREATED BY THE ABOVE cats $ cat errorlog ls: notafile: No such file or directory $ cat outlog afile I hope that s. stonerfish

25/07/2007, 10h27	#4
Stephane CHAZELAS Messages: n/a Hébergeur:	Re: piping stdout and stderr to different processes? 2007-07-24, 15:58(-05), Neil Cherry: > I've found name pipes (fifos) but I am confused on using them > properly. What I want to do is to take the stdout of a process and > send it to another process to be filtered. I also want to take the > stderr of the first process and send it to another process to > also be filtered. Any examples? [...] No need for named pipes here. { { cm1 3>&- \| cmd2 2>&3 3>&- } 2>&1 >&4 4>&- \| cmd3 3>&- 4>&- } 3>&2 4>&1 -- Stéphane

25/07/2007, 10h27	#5
Stephane CHAZELAS Messages: n/a Hébergeur:	Re: piping stdout and stderr to different processes? 2007-07-24, 15:58(-05), Neil Cherry: > I've found name pipes (fifos) but I am confused on using them > properly. What I want to do is to take the stdout of a process and > send it to another process to be filtered. I also want to take the > stderr of the first process and send it to another process to > also be filtered. Any examples? [...] No need for named pipes here. { { cm1 3>&- \| cmd2 2>&3 3>&- } 2>&1 >&4 4>&- \| cmd3 3>&- 4>&- } 3>&2 4>&1 -- Stéphane

27/07/2007, 02h41	#6
Neil Cherry Messages: n/a Hébergeur:	Re: piping stdout and stderr to different processes? On Wed, 25 Jul 2007 00:01:40 GMT, jellybean stonerfish wrote: > On Tue, 24 Jul 2007 15:58:52 -0500, Neil Cherry wrote: > >> I've found name pipes (fifos) but I am confused on using them >> properly. What I want to do is to take the stdout of a process and >> send it to another process to be filtered. I also want to take the >> stderr of the first process and send it to another process to >> also be filtered. Any examples? >> >> Thanks (an no it's not for school). >> > > Maybe this simple example will . I will create two named pipes. > Then I will in the background, cat these pipes to files. Next I will > create a file named "afile". To make it all happen I do a directory > listing of afile and notafile (notafile does not exist and should give an > error) and redirect the output to the named pipes. Finally I will cat out > the files with the data that the background cats read from the fifos. > First a cut and paste of my terminal, following is a breakdown of what > happens. > > $ mkfifo stderrpipe > $ mkfifo stdoutpipe > $ cat stderrpipe > errorlog & > [1] 5860 > $ cat stdoutpipe > outlog & > [2] 5863 > $ touch afile > $ ls afile notafile > stdoutpipe 2> stderrpipe > [1]- Done cat stderrpipe > errorlog > [2]+ Done cat stdoutpipe > outlog > $ cat errorlog > ls: notafile: No such file or directory > $ cat outlog > afile > > > DEEPER EXPLANATIONS FOLLOW > > CREATE PIPE FOR ERROR > $ mkfifo stderrpipe > > CREATE PIPE FOR STDOUT > $ mkfifo stdoutpipe > > CAT THE ERROR PIPE TO A FILE IN BACKGROUND > $ cat stderrpipe > errorlog & > BACKGROUND PROCESS #1 > [1] 5860 > > CAT THE STDOUT PIPE TO A FILE IN BACKGROUND > $ cat stdoutpipe > outlog & > BACKGROUND PROCESS #2 > [2] 5863 > > CREATE A FILE "afile" > $ touch afile > > REDIRECT THE OUTPUT OF ls TO THE PIPES > $ ls afile notafile > stdoutpipe 2> stderrpipe > > BOTH BACKGROUND PROCESSES FINISH WHEN THEIR > PIPES GET TO END OF FILE. THAT IS NOW, BECAUSE > THE LS COMMAND IS FINISHED GIVING THEM DATA > > [1]- Done cat stderrpipe > errorlog > [2]+ Done cat stdoutpipe > outlog > > OUTPUT THE FILES CREATED BY THE ABOVE cats > $ cat errorlog > ls: notafile: No such file or directory > $ cat outlog > afile > > I hope that s. > > stonerfish Thanks! That s big time. -- Linux Home Automation Neil Cherry ncherry@linuxha.com http://www.linuxha.com/ Main site http://linuxha.blogspot.com/ My HA Blog Author of: Linux Smart Homes For Dummies

27/07/2007, 02h44	#7
Neil Cherry Messages: n/a Hébergeur:	Re: piping stdout and stderr to different processes? On Wed, 25 Jul 2007 09:27:33 GMT, Stephane CHAZELAS wrote: > 2007-07-24, 15:58(-05), Neil Cherry: >> I've found name pipes (fifos) but I am confused on using them >> properly. What I want to do is to take the stdout of a process and >> send it to another process to be filtered. I also want to take the >> stderr of the first process and send it to another process to >> also be filtered. Any examples? > [...] > > No need for named pipes here. > > { > { > cm1 3>&- \| > cmd2 2>&3 3>&- > } 2>&1 >&4 4>&- \| > cmd3 3>&- 4>&- > } 3>&2 4>&1 Thanks. OK now I see why I didn't get it to work. I didn't try that far. But I have to say I'm not quite sure what I'm reading just yet. I'll have to hit the man pages as I'm not used to the >&- syntax. -- Linux Home Automation Neil Cherry ncherry@linuxha.com http://www.linuxha.com/ Main site http://linuxha.blogspot.com/ My HA Blog Author of: Linux Smart Homes For Dummies

24/07/2007, 21h58	#1
Neil Cherry Messages: n/a Hébergeur:	piping stdout and stderr to different processes? I've found name pipes (fifos) but I am confused on using them properly. What I want to do is to take the stdout of a process and send it to another process to be filtered. I also want to take the stderr of the first process and send it to another process to also be filtered. Any examples? Thanks (an no it's not for school). -- Linux Home Automation Neil Cherry ncherry@linuxha.com http://www.linuxha.com/ Main site http://linuxha.blogspot.com/ My HA Blog Author of: Linux Smart Homes For Dummies

27/07/2007, 09h12	#8
Janis Messages: n/a Hébergeur:	Re: piping stdout and stderr to different processes? On 27 Jul., 03:44, Neil Cherry <n...@.uucp> wrote: > On Wed, 25 Jul 2007 09:27:33 GMT, Stephane CHAZELAS wrote: > > 2007-07-24, 15:58(-05), Neil Cherry: > >> I've found name pipes (fifos) but I am confused on using them > >> properly. What I want to do is to take the stdout of a process and > >> send it to another process to be filtered. I also want to take the > >> stderr of the first process and send it to another process to > >> also be filtered. Any examples? > > [...] > > > No need for named pipes here. > > > { > > { > > cm1 3>&- \| > > cmd2 2>&3 3>&- > > } 2>&1 >&4 4>&- \| > > cmd3 3>&- 4>&- > > } 3>&2 4>&1 > > Thanks. > > OK now I see why I didn't get it to work. I didn't try that far. > But I have to say I'm not quite sure what I'm reading just yet. > I'll have to hit the man pages as I'm not used to the >&- > syntax. N>&- close the file descriptor with number N. Janis > > -- > Linux Home Automation Neil Cherry nche...@linuxha.comhttp://www.linuxha.com/ Main sitehttp://linuxha.blogspot.com/ My HA Blog > Author of: Linux Smart Homes For Dummies

27/07/2007, 10h08	#9
Stephane CHAZELAS Messages: n/a Hébergeur:	Re: piping stdout and stderr to different processes? 2007-07-26, 20:44(-05), Neil Cherry: [...] >> { >> { >> cm1 3>&- \| >> cmd2 2>&3 3>&- >> } 2>&1 >&4 4>&- \| >> cmd3 3>&- 4>&- >> } 3>&2 4>&1 > > Thanks. > > OK now I see why I didn't get it to work. I didn't try that far. > But I have to say I'm not quite sure what I'm reading just yet. > I'll have to hit the man pages as I'm not used to the >&- > syntax. 3>&- is for closing fd 3. It's not necessary, but it's for tidy up. None of the commands will ever try (not should they) to access the fd 3 and 4, so it's best to close them before executing those commands so that they can use those fds for something else. { { cm1 \| cmd2 2>&3 } 2>&1 >&4 \| cmd3 } 3>&2 4>&1 is functionnaly equivalent. if cmd2 doesn't output anything on its stdout nor stderr, it can even be simplified to: { cm1 \| cmd2; } 2>&1 \| cmd3 Or if you want to be sure: { cm1 \| cmd2 > /dev/null 2>&1; } 2>&1 \| cmd3 -- Stéphane

27/07/2007, 21h49	#10
Neil Cherry Messages: n/a Hébergeur:	Re: piping stdout and stderr to different processes? On Fri, 27 Jul 2007 09:08:23 GMT, Stephane CHAZELAS wrote: > 2007-07-26, 20:44(-05), Neil Cherry: > [...] >>> { >>> { >>> cm1 3>&- \| >>> cmd2 2>&3 3>&- >>> } 2>&1 >&4 4>&- \| >>> cmd3 3>&- 4>&- >>> } 3>&2 4>&1 >> >> Thanks. >> >> OK now I see why I didn't get it to work. I didn't try that far. >> But I have to say I'm not quite sure what I'm reading just yet. >> I'll have to hit the man pages as I'm not used to the >&- >> syntax. > > 3>&- is for closing fd 3. It's not necessary, but it's for tidy > up. None of the commands will ever try (not should they) to > access the fd 3 and 4, so it's best to close them before > executing those commands so that they can use those fds for > something else. > > { > { > cm1 \| > cmd2 2>&3 > } 2>&1 >&4 \| > cmd3 > } 3>&2 4>&1 > > is functionnaly equivalent. > > if cmd2 doesn't output anything on its stdout nor stderr, it can > even be simplified to: > { cm1 \| cmd2; } 2>&1 \| cmd3 > > Or if you want to be sure: > > { cm1 \| cmd2 > /dev/null 2>&1; } 2>&1 \| cmd3 > Thanks that s. :-) -- Linux Home Automation Neil Cherry ncherry@linuxha.com http://www.linuxha.com/ Main site http://linuxha.blogspot.com/ My HA Blog Author of: Linux Smart Homes For Dummies

Outils de la discussion
Afficher une version imprimable Envoyer un lien vers cette page par email