Comment by LukeShu

Scanning input just is unrelated to the "modified while running" problem. The "modified while running" problem is a read-buffering problem.

For example, consider the following change:

    -echo $x; rm -rf /n/foobar/
    +rm -rf /n/foobar/
     ^^^^^^^^^^^^^^^^

If the shell's first read() reads 16 bytes (indicated above with "^"), then the file is changed, then the shell reads the rest; then the shell will see "echo $x; rm -rf /" regardless of whether or not it scans the input multiple times.

I am unfamiliar with the read-buffering done by either of the 2 main implementations of rc, and so am unable to comment on whether it does things to avoid this problem. But if it does do things to avoid it, those things are orthogonal to the "not a macro processor / input is never scanned more than once" thing.