Delete lines from file with SED or AWK

Question

Ive seen many variations, very confused on how to solve these 3 problems.

1. deleting all rows except the first from a file
2. deleting a row from file with

Answer 1

you can just use bash if your system has it. The basic idea behind is to set a count and incrementing this count while iterating the file.

1) deleting all rows except the first from a file

read -r line < file; echo "$line" > temp && mv temp file

2) deleting a row from file with a line number

declare -i count=0
while read -r line
do
  ((count++))
  case "$count" in
    10) continue;;
    * ) echo "$line";;
  esac
done < file > temp && mv temp file

3) deleting rows from a file with a range of line numbers eg from 10 to 20

declare -i count=0
while read -r line
do
  ((count++))
  if (( $c < 10 && $c > 20 ));then
    echo "$line";;
  fi
done < file > temp && mv temp file