Question

In Bash, I would like to use /dev/stdin. As with many commands, stdin is optional though. So, how would I know if I had stdin to read? If there is no standard input, a command like cat /dev/stdin hangs waiting for input.

Here is my first attempt:

#Always says "no stdin"
if test -s /dev/stdin
then 
  echo stdin
else
  echo no stdin
fi

man test => -s FILE exists and has a size greater than zero

I guess the device file does not keep up with the available bytes. It would be nice to do this in bash, ssh implements this functionality (in another language no doubt).

Answer 1

I think you are looking for Asynchronous IO.

Try read with -t flag. It will return after specified time even if there was no input to be read.