why bash read command create a loop-local variable?

Question

I wrote following bash script. The read command creates variable var. if $var==5 then break loop.

    seq 10 | while read -r var
    do
      echo "read: $var"
      if [[ $var == 5 ]]; then
        echo "break loop: $var"
        break
      fi
    done
    
    echo "after loop: $var"

Here is the output

read: 1
read: 2
read: 3
read: 4
read: 5
break loop: 5
after loop: 

My question is: why after loop, $var is empty?

Answer 1

The variable isn't loop-local, and everything related to read and loops here are just red herrings. The behavior you're seeing is because the pipe starts a subshell. If you get rid of the pipe and run this code instead:

    while read -r var
    do
      echo "read: $var"
      if [[ $var == 5 ]]; then
        echo "break loop: $var"
        break
      fi
    done < <(seq 10)
    
    echo "after loop: $var"

Then it will print this instead:

read: 1
read: 2
read: 3
read: 4
read: 5
break loop: 5
after loop: 5