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Re: Directing into a variable doesn't work

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  Re: Directing into a variable doesn't work Martijn Dekker <martijn@inlv.org> - 2018-06-24 12:38 +0200

#14271 — Re: Directing into a variable doesn't work

Op 24-06-18 om 05:08 schreef Peter Passchier:
> With memory being abundant and filesystem access expensive, I want to
> put stdout and stderr of a command into variables (without needing to
> write to a file):
> 
> output=$($command 2>>>errors)

This would not work even if the feature is implemented. The $(command 
substitution) forks a subshell process to execute the command in, 
because the main shell process needs a process it can set up a pipe 
with. So the 'errors' variable would only exist in that subshell process 
and would be lost as soon as the command substitution completes.

> Or:
> 
> $command >>>output 2>>>errors

This form seems conceivable to me.

However, note that here-documents and here-strings internally use 
temporary files, so they do involve file system access. I'm not Chet, 
but I don't think that would be different for your proposed feature. So 
while this might be some nice syntactic sugar, I'm afraid you would be 
disappointed about the performance.

I still kind of like the idea, though. As far as I know, there's 
currently no way to capture more than one output stream into separate 
variables without involving rather laborious handling of temporary 
files. Your proposal would probably still involve that, but the shell 
would do the hard work for you which seems like an improvement to me.

BTW, 'reverse here-document' doesn't sound quite right. You're not 
specifying any document or string containing input, you're specifying a 
variable in which to store output. So, here-variable?

- M.

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