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Re: bash uses tmp files for inter-process communication instead of pipes


From: Pierre Gaston
Subject: Re: bash uses tmp files for inter-process communication instead of pipes?
Date: Mon, 6 Oct 2014 22:51:38 +0300

On Mon, Oct 6, 2014 at 10:38 PM, Linda Walsh <address@hidden> wrote:

> Greg Wooledge wrote:
>
>> On Mon, Oct 06, 2014 at 12:14:57PM -0700, Linda Walsh wrote:
>>
>>>    done <<<"$(get_net_IFnames_hwaddrs)"
>>>
>>
>>  Where am I using a HERE doc?
>>>
>>
>> <<< and << both create temporary files.
>>
>
>
> According to Chet , only way to do a multi-var assignment in bash is
>
> read a b c d  <<<$(echo address@hidden)
>
> Forcing a simple assignment into using a tmp file seems Machiavellian --
> as it does exactly the thing the user is trying to avoid through
> unexpected means.
>
> The point of grouping assignments is to save space (in the code) and have
> the group initialized at the same time -- and more quickly than using
> separate assignments.
>
> So why would someone use a tmp file to do an assignment.
>
> Even the gcc chain is able to use "pipe" to send the results of one stage
> of the compiler to the next without using a tmp.
>
> That's been around for at least 10 years.
>
> So why would a temp file be used?
>
>
> Creating a tmp file to do an assignment, I assert is a bug.
>
> It is entirely counter-intuitive that such wouldn't use the same mechanism
> as LtR ordered pipes.
>
> I.e.
>
> cmd1 | cmd2 -- that hasn't used tmp files on modern *nix systems for
> probably 20 years or more (I think DOS was the last shell I knew that used
> tmp files...)
>
> so why would "cmd2 < <(cmd1 [|])" not use the same paradigm -- worse, is
>
> cmd1 >& MEMVAR   -- output is already in memory...
>
> so why would read a b c <<<${MEMVAR} need a tmp file if the text to be
> read is already in memory?
>
>
> Because it's not a simple assignment, it's using a mechanism to send data
to an external program and another one to read from a stream of data.

Some shell use the buffer of a pipe as an optimization when the amount of
data is small (which is probably the case of most heredocs/string).


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