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

[Koichi Murase]

2020/3/25 9:22 Peng Yu
> `empty` means as if it is read from /dev/null.
> I end up with something like this. Is it robust? I am not sure if can
> cover all the corner cases.

If `empty' means /dev/null like behavior, your function fails with the
following case.

$ printf '\0' | f
No input from stdin

Also, `-r' option is missing.

$ echo '\a' | f
a

I don't know if this is the best solution, but this is an improved version:

f2() {
  local REPLY
  if read -r -d '' -n 1; then
    if [[ $REPLY ]]; then
      printf '%s' "$REPLY"
    else
      printf '\0'
    fi
    cat
  else
    echo 'No input from stdin'
  fi
}

2020/3/25 9:43 João Eiras
> https://www.gnu.org/software/bash/manual/html_node/Bash-Builtins.html
>
> if read -t 0 -u 0; then #  -u 0 is optional and just here as example

$ { sleep 1; echo A; } | { read -t 0 || echo No input; }

If `empty' means /dev/null, `read -t 0' doesn't work with the above
case. One needs to wait until EOF when there is no input.