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

With awk:

# delete line 1
awk 'NR == 1 {next} {print}' file

# delete line number stored in shell variable $n
awk -v n=$n 'NR == n {next} {print}' file

# delete between lines $a and $b inclusive
awk -v m=$a -v n=$b 'm <= NR && NR <= n {next} {print}' file

To save a few chars, {print} can be replaced just with 1

To overwrite the original file, you have to do something like this

awk '...' file > tmpfile && mv tmpfile file