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

Answer 2

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

Answer 3

Using sed:

Delete 1st line:

sed '1d' file-name

Delete 10th line:

sed '10d' file-name

Delete line # 5 to 10

sed '5,10d' file-name

All above sed commands will write output on stdout that you can redirect to another file if you want or use -i flag of sed to inline edit the file.