How to check if pipe is empty

Question

I have piped line in bash script and want to check if pipe has data, before feeding program

Searching I found about test -t 0 but it doesn't work here. Always returns false. So how to be sure that pipe has data?

---

I'm providing example:

echo "string" | [ -t 0 ] && echo "empty" || echo "fill"
output: fill

echo "string" | tail -n+2 | [ -t 0 ] && echo "empty" || echo "fill"
output: fill

Answer 1

There's no way to peek at the content of a pipe, nor is there a way to read a character to the pipe then put it back. The only way to know that a pipe has data is to read a byte, and then you have to get that byte to its destination.

So do just that: read one byte; if you detect an end of file, then do what you want to do when the input is empty; if you do read a byte then fork what you want to do when the input is not empty, pipe that byte into it, and pipe the rest of the data.

first_byte=$(dd bs=1 count=1 2>/dev/null | od -t o1 -A n)
if [ -z "$first_byte" ]; then
  # stuff to do if the input is empty
else
  {
    printf "\\${first_byte# }"
    cat
  } | {
    # stuff to do if the input is not empty
  }      
fi

test -t 0 has nothing to do with this; it tests whether standard input is a terminal. It doesn't say anything one way or the other as to whether any input is available.