How to append a string at end of a specific line in a file in bash

Question

I want to append an alias to the end of a certain line of my hosts file. For example

I have

192.168.1.1    www.address1.com
192.168.1.2    www.address2.com
192.168.1.3    www.address3.com

I want to it to look like

192.168.1.1    www.address1.com
192.168.1.2    www.address2.com myalias
192.168.1.3    www.address3.com

I want to find the line that contains 19.2.68.1.2 and append myalias at the end of it. The line is not necessarily the second line in the file like I've shown here. It could be anywhere.

The question is not precisely identical but some of the answers are. — tripleee, Mar 03 '14 at 22:44

Steve · Accepted Answer · 2014-03-03T22:44:45.660

64

Using sed and the pattern described:

sed '/192.168.1.2/s/$/ myalias/' file

Using sed and a specific line number:

sed '2s/$/ myalias/' file

edited Mar 03 '14 at 22:44

answered Mar 03 '14 at 22:39

Steve

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`sed '2s/$/ myalias/' file > file ` make the file empty, is there any way we can save the output in the same file ? – Chauhan Jul 01 '20 at 11:10
@Chauhan: With in-place editing: `sed -i.bak '2s/$/ myalias/' file` or just `sed '2s/$/ myalias/' file > file.tmp && mv file.tmp file` – Steve Jul 01 '20 at 14:10
How about to prepend to a line? – Muhammad Dyas Yaskur Aug 26 '22 at 01:31
1

to prepend just replace `$` to `^` ref https://stackoverflow.com/a/13588498/2671470 – Muhammad Dyas Yaskur Aug 26 '22 at 01:33

John1024 · Answer 2 · 2014-03-03T23:10:14.707

2

An awk solution is also possible:

awk '{if (/^192\.168\.1\.2 /) {$0=$0 " myalias"}; print}' hosts

The above reads lines from the hosts file one by one. If the line has our IP address at the beginning, then myalias is appended to the line. The line is then printed to stdout.

Note two things. There is a space after the IP address in the if condition. Otherwise, the regex could match 192.168.1.20 etc. Also, the periods in the IP address are escaped with backslashes. Otherwise, they could match any character.

A pithier form of the solution is:

awk '/^192\.168\.1\.2 /{$0=$0 " myalias"}1' hosts

edited Mar 03 '14 at 23:10

answered Mar 03 '14 at 22:48

John1024

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`awk '/^192\.168\.1\.2 /{$0=$0 " myalias"}7' hosts` – Kent Mar 03 '14 at 22:52
How to make this change in-place? Is there a better solution than my dirty hack? `awk '{if (/^After\=/) {$0=$0 " postgresql.service"}; print}' jetty.service > jetty.service.tmp && mv -f jetty.service.tmp jetty.service` – gouessej Apr 28 '21 at 20:45
1

@gouessej If your `awk` is BSD (MacOS), then your hack is the correct solution. If your `awk` is GNU (Linux, homebrew, etc), then can use the `-i inplace` option instead. – John1024 Apr 29 '21 at 05:51

Scrutinizer · Answer 3 · 2014-03-04T06:05:51.777

I would go with awk, since you can use strings and do not have to use regex, where dots would need to be escaped and anchors and/or word boundaries would need to be used. Also you can make sure the string matches a value in column 1.

awk '$1==s{$0=$0 OFS alias}1' s=192.168.1.2 alias=myalias file

Also when it is part of a larger script, it is nice to be able to use variable strings. With sed you would need shell variables and quote trickery..

jaypal singh · Answer 4 · 2014-03-03T23:24:13.950

0

Here is another way using awk

awk '/search pattern/{print $0 " myalias"; next}1'  file

edited Mar 03 '14 at 23:24

answered Mar 03 '14 at 22:52

jaypal singh

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OP doesn't want the `\n` :) also this cmd will only print the matched line, which may not what OP wants – Kent Mar 03 '14 at 22:53
Sorry about that Kent. Was answering from phone and the way it was formatted on phone it appeared OP wanted newline. Thanks for pointing it out. `:)` – jaypal singh Mar 03 '14 at 22:58
Lesson learnt. Don't answer from phone! – jaypal singh Mar 03 '14 at 22:59
2

jaypal, you do need a "smart" phone! I guess you are using iphone right? not smart enough... :) – Kent Mar 03 '14 at 23:05

How to append a string at end of a specific line in a file in bash

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