while read -r line; do
echo $line
done < myfile.txt

while read -r line; do
echo $line
sed '1d' myfile.txt
done < myfile.txt

unset n
while read -r line; do
  echo $line
  : $((n++))
done < myfile.txt
sed "1,$n d" myfile.txt

sed -n '$d;w file2' >file1 <<IN
$(cat file1; echo .)
IN

sed -n '$d;/pattern1/w file2
           /pattern2/p' >file1 <<IN
$(cat file1; echo .)
IN

while read; do
  if some_command_worked_correctly; then
    sed -i '1,1 d' file-to-read
  fi
done < file-to-read