Re: How to test if stdin is empty, if not empty print to stdout, otherwise do something else.

[Eli Schwartz]

On 3/24/20 7:43 PM, Peng Yu wrote:
> Hi,
> 
> ifne from moreutils can work similar to this goal. But it is an
> external program.
> 
> I'd like to test if stdin is not empty, then print to stdout of the input.
> 
> But the following code is not robust for this goal. Is there a robust
> way to do so in bash? Thanks.
> 
> $ function f {
>       if read -n1; then
>               echo "$REPLY"
>               cat
>       else
>               echo 'No input from stdin'
>       fi
> }
> 
> $ { echo; seq 3; } | f
> 
> 1
> 2
> 3
> $ seq 10 13 | f
> 1
> 0
> 11
> 12
> 13
> $ f < /dev/null
> No input from stdin

The problem you are having here is, how do you define "empty"?

$ printf '%s\n' '' | { read; echo ret=$?; declare -p REPLY; }
ret=0
declare -- REPLY=""
$ printf '%s' '' | { read; echo ret=$?; declare -p REPLY; }
ret=1
declare -- REPLY=""
$ help read
[...]
    Exit Status:
    The return code is zero, unless end-of-file is encountered, read
times out
    (in which case it's greater than 128), a variable assignment error
occurs,
    or an invalid file descriptor is supplied as the argument to -u.
$

And the thing is, that you're currently using the return value of read
to determine that you *do* have input, that input is of length zero, and
that input is terminated by a newline of length 1 (before reaching an EOF).

If you wanted to check whether you have "interesting" input, then don't
check the return value of `read`, instead, analyze $REPLY to see whether
you consider it interesting, for example, if [ -n "$REPLY" ] to see if
it is zero-length. If you don't consider whitespace to be interesting,
then you can do similar checks if $REPLY contains characters other than
[:space:]

-- 
Eli Schwartz
Arch Linux Bug Wrangler and Trusted User

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