while read, and the pipe that eats your variable

Every stage of a pipeline runs in a subshell — a copy of the shell that is thrown away when the pipeline ends. Anything a stage assigns is assigned in that copy.

So echo x | read var sets var in a shell that no longer exists, and nothing reports an error. It is the most reliable surprise in the language.

The fix is to move the pipe, not the read. A redirection keeps the loop in this shell.

The same applies to a while read loop: redirect the file into the loop rather than piping it in, and the variables survive.

read -r is almost always what you want: without it, a backslash in the input is treated as an escape.