News Group Posts

capturing cpio output

Subject: Re: Why does redirecting stderr prevent file list display?
References: <0wiI7.39202$S4.3462556@newsread1.prod.itd.earthlink.net>
<zbCI7.41419$S4.3703529@newsread1.prod.itd.earthlink.net>
<3bf319ff$0$79557$8eec23a@newsreader.tycho.net>
<3bf3a5e8.4899124@news.earthlink.net> 
From: spcecdt@deeptht.armory.com (John DuBois)
Date: 15 Nov 2001 23:24:09 GMT

In article <3bf3a5e8.4899124@news.earthlink.net>,
Dave Dickerson <ddinaz@hotmail.com> wrote:
>On 15 Nov 2001 01:27:27 GMT, spcecdt@deeptht.armory.com (John DuBois) wrote:
>
>>exec 3>&1; find . -print | tee /dev/fd/2 2>&3 | cpio -o -O /tmp/archive.cpio 2>/tmp/errors
>
>I understand (mostly) everthing here except the reason for
>/dev/fd/2.

You want a file list to appear on stdout (fd 1).  The file list is available in
the pipe between find and cpio, so you should be able to tee it off there. 
However, file descriptors 0 and 1 are "used up" at that point for the input &
output pipes, and you don't want to use fd 2 (stderr) since you're specifically
trying to avoid mixing the file list with error output.  The solution is to
create an extra fd that is a clone of fd 1; that's what the exec 3>&1 does -
after the exec, fd 3 refers to the same thing fd 1 did.  Now all you need to do
to accomplish your goal is tee to fd 3.  However, there are two obstacles to
this.  First, tee requires a filename.  That's easily dealt with thanks to the
'dup' driver that has been included since 5.0.0 - /dev/fd/n refers to whatever
fd n does.  The second obstacle is that any fds opened via the 'exec' mechanism
other than 0, 1, and 2 are closed when the shell execs a program like tee. 
Fortunately, we can get around this by having the shell itself dup fd 3 onto fd
2 (2>&3) before exec'ing, and then tee to /dev/fd/2 (note that we could
just as well use any other fd not in use here, e.g. tee /dev/fd/4 4>&3, since
fds opened in this way are not marked to be closed on exec the way fds > 2
opened via the 'exec' mechanism are).

        John
-- 
John DuBois  spcecdt@armory.com.  KC6QKZ/AE  http://www.armory.com./~spcecdt/
 

Comments?

Jump to Comments

Many of the products and books I review are things I purchased for my own use. Some were given to me specifically for the purpose of reviewing them. I resell or can earn commissions from the sale of some of these items. Links within these pages may be affiliate links that pay me for referring you to them. That's mostly insignificant amounts of money; whenever it is not I have made my relationship plain. I also may own stock in companies mentioned here. If you have any question, please do feel free to contact me.

Specific links that take you to pages that allow you to purchase the item I reviewed are very likely to pay me a commission. Many of the books I review were given to me by the publishers specifically for the purpose of writing a review. These gifts and referral fees do not affect my opinions; I often give bad reviews anyway.

We use Google third-party advertising companies to serve ads when you visit our website. These companies may use information (not including your name, address, email address, or telephone number) about your visits to this and other websites in order to provide advertisements about goods and services of interest to you. If you would like more information about this practice and to know your choices about not having this information used by these companies, click here.