Using sed to delete all lines between two matching patterns

I have a file something like:

# ID 1
blah blah
blah blah
$ description 1
blah blah
# ID 2
blah
$ description 2
blah blah
blah blah

How can I use a sed command to delete all lines between the # and $ line? So the result will become:

# ID 1
$ description 1
blah blah
# ID 2
$ description 2
blah blah
blah blah

Can you please kindly give an explanation as well?

Use this sed command to achieve that:

sed '/^#/,/^\$/{/^#/!{/^\$/!d}}' file.txt

Mac users (to prevent extra characters at the end of d command error) need to add semicolons before the closing brackets

sed '/^#/,/^\$/{/^#/!{/^\$/!d;};}' file.txt

OUTPUT

# ID 1
$ description 1
blah blah
# ID 2
$ description 2
blah blah
blah blah

Explanation:

• /^#/,/^\$/ will match all the text between lines starting with # to lines starting with $. ^ is used for start of line character. $ is a special character so needs to be escaped.
• /^#/! means do following if start of line is not #
• /^$/! means do following if start of line is not $
• d means delete

So overall it is first matching all the lines from ^# to ^\$ then from those matched lines finding lines that don't match ^# and don't match ^\$ and deleting them using d.