[SOLVED] Test if the input stream not empty without consuming data from it

Issue

Consider next snippet.

generate_build_specs(){
  echo ubuntu gcc9 debug
  echo macos clang debug
  echo macos gcc10 release
  echo macos clang release
}
generate_build_specs | awk -v IGNORECASE=1 '/macos/ && /release/' |
  if input_data_available ;then
      echo "FIXME. Cannot build 'macos release' until it is fixed. Buildspec dropped: $(cat)"
  fi

My idea is to echo message with contents of input stream only if data in stream available. Something like peek(char) in stream, not taking it from there.

Of course, I know a workaround. i.e

var="$(cat)"; if test -n "$var"; echo "blah:: $var" ; fi

Solution

During the discussion, I realized that

there is no way to check if a stream is ended without reading at least one char from it.

input_data_available could not be implemented in Bash. It is a conceptual question on streams.

So the current solution is to store input to variable, then work on it.

var="$(cat)"; if test -n "$var"; then echo "::ERROR:: $var" ; fi

A better (i.e. laconic) way to do the same is:

sed -zE 's,^.+$,::ERROR:: \0\n,'

It means prepend string and output only if input is not empty. Regexp ^.+$ checks if there is at least one symbol. But I think as all oneliners this one is harder to read.

Thank you everybody for participation.

Answered By - kyb