sed adding replacement text twice

Question

I've never seen sed behave like this but I'm trying to replace all instances of "nameserver" within /etc/resolv.conf with my own 3 entries, including the "nameserver". Every combo I've tried adds my 3 entries twice, so I have 6 entries.

sed -i 's/.*nameserver.*/nameserver 10.1.1.1\nnameserver 10.1.1.2\nnameserver 10.1.1.3/g' /etc/resolv.conf

which outputs

nameserver 10.1.1.1
nameserver 10.1.1.2
nameserver 10.1.1.3
nameserver 10.1.1.1
nameserver 10.1.1.2
nameserver 10.1.1.3

So, the correct order and entries but adding it twice. I've also tried narrowing it down to nameserver and ip but still get the same output printed twice.

sed -i "s/^nameserver [0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}/nameserver 10.1.1.1\nnameserver 10.1.1.2\nnameserver 10.1.1.3/g"

I'm really confused here and can't seem to get past this. Any help appreciated.

ETA: current resolv.conf

search domain1.net. domain2.net. domaine.com.
nameserver 192.168.1.1
nameserver 192.168.1.2

desired output

search domain1.net. domain2.net. domaine.com.
nameserver 10.1.1.1
nameserver 10.1.1.2
nameserver 10.1.1.3

Answer 1

You have two occurrences of "nameserver" in the original, so sed replaces each of them.

You can do multiline replacements with sed, but it's simpler to delete the lines you don't want, then append the ones you do. Though I'm not familiar with /etc/resolv.conf so I don't know if order matters.

sed '/nameserver.*/d' /etc/resolv.conf
cat <<'EOF'
nameserver 10.1.1.1
nameserver 10.1.1.2
nameserver 10.1.1.3
EOF

If the output looks good from this, run the below to actually make the changes:

sudo sed -i '/nameserver.*/d' /etc/resolv.conf
sudo tee -a /etc/resolv.conf >/dev/null <<'EOF'
nameserver 10.1.1.1
nameserver 10.1.1.2
nameserver 10.1.1.3
EOF