Let’s take an plain text file, input.txt, that looks like this

PATTERN-1
First line of unimportant text
Second line of unimportant text
PATTERN-2
Some more texts (may/ mayn't be important!)

We want to delete some of the lines from the file using the command line stream editor, sed.

1. Use the following command to delete the lines lying between PATTERN-1 and PATTERN-2 , excluding the lines containing these patterns:
sed '/PATTERN-1/,/PATTERN-2/{//!d}' input.txt

If you want to modify the file itself, instead of just the file stream, include the “-i” flag after sed.

2. Use the following command to delete the lines lying between PATTERN-1 and PATTERN-2 , including the lines containing these patterns:
sed '/PATTERN-1/,/PATTERN-2/d' input.txt

3. To delete all the lines after PATTERN-2, use this
sed '/PATTERN-1/,$d' input.txt

4. To delete lines, say 2 through 4 (if you know the correct line numbers, of course!), use this

sed '2,4d' input.txt

Here is a good sed reference.