How to insert new lines after a specific empty line in linux?

Question

I'm trying to find a way to insert a new line after a specific empty line.
for example I have a file contains firewall rules, and I want to insert new lines after "# ok icmp code for FORWARD" and first empty line. (I don't know the exact line number and it is different in multiple machines):

... # ok icmp code for FORWARD -A ufw-before-forward -p icmp --icmp-type destination-unreachable -j ACCEPT -A ufw-before-forward -p icmp --icmp-type time-exceeded -j ACCEPT -A ufw-before-forward -p icmp --icmp-type parameter-problem -j ACCEPT -A ufw-before-forward -p icmp --icmp-type echo-request -j ACCEPT [Insert new lines here] # new lines comment new line 1 new line 2 ...

here is my incomplete solution:
to find the specific line number:

cat file.rules | grep -n "ok icmp code for FORWARD" | cut -d':' -f 1

to show the lines after specific line and before the empty line:

awk '!NF{f=0} /ok icmp code for FORWARD/ {f=1} f' file.rules

Shawn · Accepted Answer · 2022-04-21 12:17:15Z

1

Using ed to edit the file:

ed -s file.rules <<'EOF' /^# ok icmp code for FORWARD/;/^$/a # new lines comment new line 1 new line 2 . w EOF

First sets the current line to the one matching # ok icmp code for FORWARD, then advances the current line to the first blank line following that, and appends text after it (A line with a single period marks the end of input), then writes the changed file back to disk.

answered Apr 21, 2022 at 12:17

Shawn

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3 Comments

Signor Over a year ago

Thank you. Can I read a user input with this solution? I'm using 'read ip_network' in my script to receive the input from user and I want to call that in inserted line. ($ip_network). but it doesn't work in this case. this is the line had been inserted: -A ufw-before-forward -d $ipv4_network/24 -j ACCEPT

Shawn Over a year ago

Sure; just use the version of a heredoc that does parameter expansion, and remember to escape the $ in the regex.

Signor Over a year ago

Thanks. I just removed quotes in 'EOF' and calling the variable worked.

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