cat text.txt
echo '123' > text.txt
echo 'abc' >>text.txt
echo '456' >>text.txt

cat top.txt /dev/stdin bottom.txt > text.txt
# with cat, - works the same as /dev/stdin

{
  echo 123 #top.txt
  cat
  echo 456 #bottom.txt
} > text.txt

cat top.txt /dev/stdin bottom.txt > text.txt
gcc whatever

#!/bin/bash
 cat top.txt /dev/stdin bottom.txt > text.txt
 gcc whatever