Re: Why can redirection be combined in an assignment statement?

[Andreas Kusalananda Kähäri]

On Mon, May 10, 2021 at 10:40:47AM -0500, Peng Yu wrote:
> I can not see why redirection is allowed in an assignment statement.
> For example, redirection in the following command does not do anything
> useful.
> 
> x=10 < /tmp/1.txt
> 
> Is there a useful situation in which redirection does do something
> useful in an assignment statement? If not, why not prompt an error for
> this case?
> 
> I can think of this, which basically truncate or create a new file
> /tmp/1.txt, and perform the assignment. But I don't see a point to
> combine both in a single line. Therefore, I would not consider this is
> as a useful situation.
> 
> >/tmp/1.txt x=10
> 
> -- 
> Regards,
> Peng


A redirection is allowed even though the redirected data is not read.
A redirection is allowed even though no data is transferred across the
stream.

A hypothetical scenario where you actually *want* redirection into an
assigment is when the assigment involves a command substitution (or
something similar) that reads the data:

        $ reverse=false; if "$reverse"; then a=$(rev); else a=$(cat); fi 
<<<"hello"; echo "$a"
        hello
        $ reverse=true; if "$reverse"; then a=$(rev); else a=$(cat); fi 
<<<"hello"; echo "$a"
        olleh


-- 
Andreas (Kusalananda) Kähäri
SciLifeLab, NBIS, ICM
Uppsala University, Sweden

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