I sit corrected, most of my scripting tends to be work related where
we use a unix emulator. Apparantly Linux dealings with the pipe is
asyncronous rather than the sequential execution that I would be used to . .
> From: Kenn Humborg[SMTP:kenn at bluetree.ie]
> Sent: 14 February 2000 11:28
> To: Stephen_Reilly at exchange.dell.com> Cc: ilug at linux.ie> Subject: RE: sed people ?
>> > > On Fri, 11 Feb 2000 20:02:33 -0000 "Kenn Humborg" <kenn at bluetree.ie>
> > > wrote:
> > > | > sed 's/ *| */|/g' file | cat > file
> > >
> > > | > surely ??
> > >
> > > | Is that _guaranteed_ not to truncate 'file' before sed
> > > | reads it?
> > >
> > well, yes it should do, because its handing the output to cat before
> > piped back into itself . . . now of course thats down to how much info
> > can hold . . . but you certainly shouldn't see sed overwrite the file
> > trying to parse
>> I'd have to disagree. I'd see it working like this:
>> 1. Shell breaks up pipeline into two commands:
>> sed -e '...' file
>> cat > file
>> 2. It creates a pipe.
>> 3a. To execute the first command, it creates an argv array,
> hooks fd 1 up to the pipe's input and forks/execs.
>> 3b. To execute the second command, it opens 'file' for writing
> on fd 1, which should truncate the file. Then it forks
> and execs cat.
>> So, depending on the order in which 3a and 3b are done, sed
> might get enough time to read all of file (if it is short and
> in-cache) before the shell truncates it.
>> Of course, I might be wrong, and there might be some obscure
> trick which makes this work the way you want, but I can't see
> it from the bash manpage.
>> And let's say file is 100MB and not in-cache. I really doubt this
> would work.
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