How do I pair every two lines of a text file with Bash?

Question

With a simple bash script I generate a text file with many lines like this:

192.168.1.1
hostname1
192.168.1.2
hostname2
192.168.1.3
hostname3

Now I want to reformat this file so it looks like this:

192.168.1.1 hostname1
192.168.1.2 hostname2
192.168.1.3 hostname3

How would I reformat it this way? Perhaps sed?

why don't you generate you file in the desirable format to begin with? — SilentGhost, Oct 03 '09 at 14:27

score 59 · Accepted Answer · answered Oct 03 '09 at 14:44

59

$ sed '$!N;s/\n/ /' infile
192.168.1.1 hostname1
192.168.1.2 hostname2
192.168.1.3 hostname3

answered Oct 03 '09 at 14:44

Tim

1

Would you please explain how it come to be? – NawaMan Oct 03 '09 at 14:57
12

@NawaMan: If you mean "how does it work?" then: If not (!) the last line ($) then append the next line (N) and replace the newline between them with a space (s/\n/ /) and repeat starting with the next line (which will be the 3rd, 5th, etc.). – Dennis Williamson Oct 03 '09 at 17:07

score 47 · Answer 2 · answered Sep 23 '15 at 12:27

47

I love the simplicity of this solution

cat infile | paste -sd ' \n'

192.168.1.1 hostname1
192.168.1.2 hostname2
192.168.1.3 hostname3

or make it comma separated instead of space separated

cat infile | paste -sd ',\n'

and if your input file had a third line like timestamp

192.168.1.1
hostname1
14423289909
192.168.1.2
hostname2
14423289910
192.168.1.3
hostname3
14423289911

then the only change is to add another space in to the delimiter list

cat infile | paste -sd '  \n'

192.168.1.1 hostname1 14423289909
192.168.1.2 hostname2 14423289910
192.168.1.3 hostname3 14423289911

answered Sep 23 '15 at 12:27

rob

What if the lines themselves have spaces in them? – Mihai Danila Jul 08 '19 at 17:30
1

@MihaiDanila just tried it, works fine with no changes. – rob Jul 09 '19 at 11:22

score 39 · Answer 3 · edited Oct 03 '22 at 17:13

39

Here's a shell-only alternative:

while read -r first; do read second; echo "$first $second"; done < filname

-r - Do not allow backslashes to escape any characters

edited Oct 03 '22 at 17:13

HappyTown

answered Oct 03 '09 at 19:20

Jukka Matilainen

2

But you should [always](http://wiki.bash-hackers.org/commands/builtin/read#read_without_-r) use ``-r`` flag with ``read``. – Aleks-Daniel Jakimenko-A. Jan 25 '14 at 00:32
3

does the `done` line need to be `done < somefile.txt` ? – Brad Parks Apr 22 '16 at 17:34

3 Answers3