bash/sed - adding multiple lines at specific location in file - linux

Question

I want to automate adding new backends to a configuration file and can't figure out how to do it. The relevant part of the file looks like this:
[backend]
backends = backend-1 backend-2
timeout = 10
connectionsperhost = 100
[backend-1]
url = https://somedomain.com
secret = 6135bcb7849ca1886e2de193
[backend-2]
url = https://anotherdomain.com
secret = 75048ad646a5af787cbbaaa3
I'm looking for a simple solution to add another backend entry with a specific url and secret.
[backend-3]
url = https://adiffrentdomain.com
secret = 42349234238423424
and at the same time append the name "backend-3" to the line "backends = " at the "[backend]" section.
Is there a way to make this work with sed or bash scripting in genereal?
kindly appreciate any help or hints!

Answer 1

With awk for an arbitrary new backend entry with newbackendname, url and secret being the only provided inputs. Simply adds the new backend to the backends =-list and the corresponging block directly after the general [backend]-block.
awk -vnewentry="newbackendname" -vurl="http://new.url" -vsecret="secret123" '
#in backends= line: add new entry (at first position)
#and set `add` marker
/^backends =/{sub(/^backends = /,"&"newentry" ",$0) ; add=1}
#marker exists and empty line (end of header block) is found:
#print new entry and unset marker `add`
add && /^$/{ print "\n["newentry"]"
print "url = "url
print "secret = "secret
add=0}
#print by default
1' infile.txt
Hopefully the comments make it easy to follow. With this code, you may as well just add url and secret via shell variables as awk -vurl="$URL" -vsecret="$SECRET" -vnewentry="$NEWBACKEND" '...' infile.txt

Answer 2

You can try an approach with awk. It adds a backends entry to lines starting with backends and attaches a [backend-x] section with the given info after the last line of the [backend] block (after connectionsperhost).Obviously it relies on the entries of the backends being correct since it doesn't count the [backend] sections individually.
url="www.domain.com"
s=9283283h4923h42934
% awk -v url="${url}" -v s="${s}" '/^backends/{x=NF-1; $0=$0" backend-"x}
/^connectionsperhost/{ print; $0="\n[backend-"x"]\nurl = "url"\nsecret = "s}
{print}' file
[backend]
backends = backend-1 backend-2 backend-3
timeout = 10
connectionsperhost = 100
[backend-3]
url = www.domain.com
secret = 9283283h4923h42934
[backend-1]
url = https://somedomain.com
secret = 6135bcb7849ca1886e2de193
[backend-2]
url = https://anotherdomain.com
secret = 75048ad646a5af787cbbaaa3
Data
cat file
[backend]
backends = backend-1 backend-2
timeout = 10
connectionsperhost = 100
[backend-1]
url = https://somedomain.com
secret = 6135bcb7849ca1886e2de193
[backend-2]
url = https://anotherdomain.com
secret = 75048ad646a5af787cbbaaa3

Answer 3

Using sed
$ sed '/backends =/s/$/ backend-3/;/^\[backend-2/{N;N;s|secret.*|&\n\n[backend-3] \
url = https://adifferentdomain.com \
secret = 42349234238423424|}' input_file
[backend]
backends = backend-1 backend-2 backend-3
timeout = 10
connectionsperhost = 100
[backend-1]
url = https://somedomain.com
secret = 6135bcb7849ca1886e2de193
[backend-2]
url = https://anotherdomain.com
secret = 75048ad646a5af787cbbaaa3
[backend-3]
url = https://adiffrentdomain.com
secret = 42349234238423424
If the input is coming from variables, you would want to use double quotes instead to expand the variables. E.g;
sed "/backends =/s/$/ backend-3/;$a\\n[backend-3] \
url = $url_input \
secret = $secret_input" input_file