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

[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
  echo stdin has input waiting
else
  echo stdin is empty
fi

You can play with the timeout parameter to retrieve incomplete lines of
input.