Shell Programming and Scripting

Here is an example code that shows the issue I have:

Code:

#!/bin/bash
counter() {
  seq 1000 | while read NUM; do
    echo $NUM
    echo "debug: $NUM" >&2 
    sleep 0.1 # slow it down so we know when this loop really ends
  done
}

counter | grep --line-buffered "[27]" | head -n1

Code:

debug: 1
debug: 2
2
debug: 3
debug: 4
debug: 5
debug: 6
debug: 7

If I understand it correctly, "head" finishes on the first match (as expected), but "grep" is not aware of it until it tries to write the next line (the second match). When it does, it finds out the pipe is closed so it also finishes.

That's normally not a problem, but if you have an infinite input stream containing only one match, it won't never stop. Any solution?