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Bruce Korb | 22 Dec 2011 23:09
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Re: '>;' redirection operator [was: [1003.1(2008)/Issue 7 0000530]: Support in-place editing in sed (-iEXTENSION)]

On 12/22/11 13:03, Eric Blake wrote:
> I assume on the ksh implementation that the temp file is discarded if
> the command (simple or compound) feeding the redirection failed?

One would hope!

>  If the
> redirection is used on a simple command, is there any shorthand for
> specifying that the destination name on success also be fed as an
> argument to the command, to avoid the redundancy of having to type
> 'file' both before and after the'>;' operator?

Doesn't the shell already have enough hieroglyphs?  It is what
intimidates many folks from figuring it out.

>  I assume that this is
> like any other redirection operator, where an optional fd number can be
> prepended, as in '2>; file' to collect stderr and overwrite file on
> success?

When the exact opposite is the useful variation?  I.e. keep-on-failure.
"-i" for sed is simple, understandable and implemented a lot.
Please don't add another glyph to the standardized shell.  Let us not
slide on slippery slopes.  Shells can always add some useful builtins:

    sh_move_if_changed
    sh_save_on_success
    sh_save_on_failure

to cope with this stuff.  Or you can write your own such library for
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David Korn | 22 Dec 2011 23:06
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Re: '>;' redirection operator [was: [1003.1(2008)/Issue 7 0000530]: Support in-place editing in sed (-iEXTENSION)]

cc: eblake <at> redhat.com bug-bash <at> gnu.org dash <at> vger.kernel.org miros-discuss <at> mirbsd.org
Subject: Re: '>;' redirection operator [was: [1003.1(2008)/Issue 7 0000530]: Support  in-place editing
in sed  (-iEXTENSION)]
--------

> On 12/22/2011 08:39 AM, David Korn wrote:
> > Subject: Re: Re: [1003.1(2008)/Issue 7 0000530]: Support in-place editi=
> ng in sed  (-iEXTENSION)
> > --------
> >=20
> > There are many commands other than sed that want the output to replace
> > an input file.  That is why I added the >; redirection operator to ksh9=
> 3.
> >=20
> > With >; you can do
> > 	sed -e s/foo/bar/ file >; file
> > to do in place sed.  The >; operator generates the output in a temporar=
> y file
> > and moves the file to the original file only if the command terminates
> > with 0 exit status.
> 
> On 12/22/2011 16:04 AM,Eric Blake  wrote:
> I agree that engineering a single fix into the shell that can apply to
> multiple situations, rather than chasing down a set of applications to
> add an in-place editing option to each, is a much more flexible and
> powerful approach.  Can we get buy-in from other shell developers to
> support '>;' as an atomic temp-file replacement-on-success idiom, if
> POSIX were to standardize the existing practice of ksh93 as the basis?
> 
> I assume on the ksh implementation that the temp file is discarded if
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