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How can I apply this sed command to a specific line in a text file instead of to the entire file?

I found a solution here for replacing any IP addresses in a file with a specific address. I need to apply this command to a specific line in the file so that it only replaces one single unknown IP address. I saw that sed uses -n to filter but I literally know nothing about how to apply this to achieve my goal.

This code works for every IP in the file:

sed -e 's/[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}/x.x.x.x/g' test.txt

How can I apply it to the only line in the file that also includes the string, "ipv4" so that other lines containing IP addresses are left unmodified?

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  • The -n flag will suppress the output. Usually it's used with the 'p' command to print specific lines. Commented Jun 10, 2019 at 2:51
  • To state the obvious, that only works for IPv4 addresses. It breaks for IPv6 addresses. Commented Jun 10, 2019 at 4:29
  • edit your question to include a minimal reproducible example with concise, testable sample input and expected output to get the best help. Commented Jun 10, 2019 at 14:59

3 Answers 3

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With , you can use a regex as an address like this:

sed -re '/ipv4/s/[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}/x.x.x.x/g' test.txt

If you don't specify -r, you'll need to escape the braces, i.e.:

sed -e '/ipv4/s/[0-9]\{1,3\}.[0-9]\{1,3\}.[0-9]\{1,3\}.[0-9]\{1,3\}/x.x.x.x/g' test.txt
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Thank you for that. So adding /ipv4/ at the beginning of the command will only replace the IP on the one line that also includes the string ipv4? The full line is: "bind-address-ipv4": "x.x.x.x",
@DocCaliban exactly! Just adapt this pattern to fulfil your need
@DocCaliban: Does that work for you? Do you use -e or -E or -r?
That's not working for me. I am using -e If I leave out the /ipv4/ the overall command works by changing all IP addresses in the file. Still trying to target just the one line.
@DocCaliban: Sorry missed that one. You'll need to specify the -r flag to use extended regular expressions. Alternatively, you'll need to escape the braces.
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You can skip over uninteresting lines:

sed '/ipv4/!b;s/YOUR PATTERN HERE/YOUR IP HERE/' input
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If I get it right you want to find the first occurrence of the ip address on a line that also contains ipv4, and skip all other occurrences.

With GNU , try 

sed -i -E '/ipv4/{s/[0-9]{1,3}(\.[0-9]{1,3}){3}/x.x.x.x/g;T;:a;n;ba}' test.txt

The -E will enable POSIX ERE syntax, no need to escape {} and (). The -i will replace inline in the file directly. /ipv4/ will find a line with ipv4 on it, and then {s/[0-9]{1,3}(\.[0-9]{1,3}){3}/x.x.x.x/g;T;:a;n;ba} will do the replacement only on that line. See potong's explanation of the T;:a;n;ba here

If you have a POSIX sed, try

sed -e '/ipv4/ {' -e 's/[0-9]\{1,3\}\(\.[0-9]\{1,3\}\)\{3\}/x.x.x.x/g' -e ':a' -e n -e 'ba' -e '}' file > newfile
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Thanks! When I try that I get an unterminated { error, but I don't see a problem in the code.
@DocCaliban Interesting, I just tested this code and it looks working, i.e. it replaces the first occurrence of the IP on a line having ipv4 substring. Do you have a GNU sed? Are you using variables?
@DocCaliban As it has been pointed out by Wiktor, the code is not causing errors, kindly please check your code. Share the exact code you are using if possible.
Hello. I must be using a version of sed that is not cooperating. It's whatever comes with my QNAP NAS.
@DocCaliban Sorry, no idea what version of sed is used there. Try these two: 1) sed '/ipv4/{s/[0-9]\{1,3\}\(\.[0-9]\{1,3\}\)\{3\}/x.x.x.x/g;T;:a;n;ba}' file or 2) sed -e '/ipv4/ {' -e 's/[0-9]\{1,3\}\(\.[0-9]\{1,3\}\)\{3\}/x.x.x.x/g' -e T -e ':a' -e n -e 'ba' -e '}' file. If neither works, try sed -e '/ipv4/ {' -e 's/[0-9]\{1,3\}\(\.[0-9]\{1,3\}\)\{3\}/x.x.x.x/g' -e ':a' -e n -e 'ba' -e '}' file which is a bit less precise, but seems to work in POSIX mode.

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