GNU bug report logs - #6379
process substitution with a give suffix

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Peng Yu <pengyu.ut <at> gmail.com>
To: Greg Wooledge <wooledg <at> eeg.ccf.org>
Cc: bug-coreutils <at> gnu.org
Subject: Re: process substitution with a give suffix
Date: Tue, 8 Jun 2010 13:33:51 -0500

On Tue, Jun 8, 2010 at 1:02 PM, Greg Wooledge <wooledg <at> eeg.ccf.org> wrote:
> On Tue, Jun 08, 2010 at 12:53:47PM -0500, Peng Yu wrote:
>> I have a program that only accept argument with a give suffix
>>
>> ./program xxx.suffix
>>
>> If I use process substitution, which gives me /dev/fd/xx, it will not
>> work with the program. Is there a way to make sure a suffix is added
>> to the substitute process file handle in /def/fd/, so that the program
>> can work with process substitution?
>
>  mkfifo myfifo.suffix
>  something >myfifo.suffix &
>  ./program <myfifo.suffix
>  wait
>  rm myfifo.suffix

The above question was sent to bug-bash. But since it is related to
mkfilo. I redirect it to bug-coreutils.

I have more than one arguments. I tried the following code. It doesn't
seem to work for more than one arguments. Would you please let me know
what is wrong?

BTW, using fifo is going to be much faster than using a temp file as
it avoid the disk usage, right?

$ cat a.txt
In a.txt
$ cat b.txt
In b.txt
$ cat main.sh
#!/usr/bin/env bash

mkfifo a.suffix
cat a.txt >a.suffix &
mkfifo b.suffix
cat b.txt >b.suffix &
cat <a.suffix <b.suffix
wait
rm a.suffix b.suffix
$ ./main.sh
In b.txt

-- 
Regards,
Peng

Message #8 received at 6379 <at> debbugs.gnu.org (full text, mbox):

From: Eric Blake <eblake <at> redhat.com>
To: Peng Yu <pengyu.ut <at> gmail.com>
Cc: Greg Wooledge <wooledg <at> eeg.ccf.org>, 6379 <at> debbugs.gnu.org
Subject: Re: bug#6379: process substitution with a give suffix
Date: Tue, 08 Jun 2010 13:13:54 -0600

[Message part 1 (text/plain, inline)]

On 06/08/2010 12:33 PM, Peng Yu wrote:
>>  mkfifo myfifo.suffix
>>  something >myfifo.suffix &
>>  ./program <myfifo.suffix
>>  wait
>>  rm myfifo.suffix
> 
> The above question was sent to bug-bash. But since it is related to
> mkfilo. I redirect it to bug-coreutils.

Your question is about how to use various Unix tools together; rather
than directing to bug-bash or bug-coreutils, you may be better off
directing to a generic shell-programming forum.

As it is, you didn't raise any bug report about the mkfifo program, so
redirecting to coreutils didn't really buy you anything.

> 
> I have more than one arguments. I tried the following code. It doesn't
> seem to work for more than one arguments. Would you please let me know
> what is wrong?
> 
> BTW, using fifo is going to be much faster than using a temp file as
> it avoid the disk usage, right?

A fifo will have the same speed as a pipe (another name for a fifo is
named pipe; unlike 'foo | bar', where the pipe is anonymous and exists
between exactly two processes, a fifo can be accessed from the file
system by multiple processes, but but under the hood, it uses the same
kernel pipe handling code).  It has the drawback of being non-seekable
in comparison to temporary regular files.  It has the advantage of
atomic operations not guaranteed by disk files, provided you stick to
transactions below the size guaranteed by your kernel.  And if you use a
ramdisk backing store for /tmp, there is little difference in speed
(either way, a ramdisk or a fifo does not have to do disk I/O); but
since you can't guarantee that /tmp is a ramdisk, yes, a fifo can be
faster for interprocess communication.  And if used incorrectly, it has
the potential to deadlock your shell script (something that won't happen
with regular files).

> 
> $ cat a.txt
> In a.txt
> $ cat b.txt
> In b.txt
> $ cat main.sh
> #!/usr/bin/env bash
> 
> mkfifo a.suffix
> cat a.txt >a.suffix &
> mkfifo b.suffix
> cat b.txt >b.suffix &
> cat <a.suffix <b.suffix

This is redirecting stdin twice, with the net effect that cat only sees
the contents of the fifo b.suffix.  You probably meant to do:

cat a.suffix b.suffix

> wait
> rm a.suffix b.suffix
> $ ./main.sh
> In b.txt
> 

-- 
Eric Blake   eblake <at> redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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