Bash: insert a line after each line

Question

So, I have a file called "burpfile". I want to add a specific line "peter" after every second line. An example burpfil can contain:

line1
line2
line3
line4
line5

I want the bashscript to modify it to become:

line1
peter
line2
peter
line3
peter
line4
peter
line5
peter

To realise this I have tried sed, since that was the choice in this post: https://stackoverflow.com/a/45964337/11155582

This is what I have got so far. However, since I don't understand sed the script only inserts "peter" at the end of the file. And I don't know why.


nr=$(wc -l burpfil | awk '{print $1}')

for i in $(seq 0 2 $nr)
do
   sed -e '$i peter' -i burpfil
done

As seen in the codesnippet the command doesn't resemble what is in the thread I linked to. That is because having a variable in the command doesnt work... And I don't know how to get around it.

Your desired result adds the line after every line, not every other line. — Barmar, Jul 22 '20 at 19:51
you say you want to add the line _after_ every second line, but you've got it _before_ the first line too. Please clarify — Bill Jetzer, Jul 22 '20 at 19:51
@MrKrisey, your samples were saying that to you want to insert line after each line so I changed your question's subject now. — RavinderSingh13, Jul 22 '20 at 20:07

Bill Jetzer · Accepted Answer · 2020-07-22T20:09:48.467

To add before each line:

$ echo "line1
line2
line3
line4
line5" | sed 's/^/peter\n/'
peter
line1
peter
line2
peter
line3
peter
line4
peter
line5

The sed script is fairly simple. It's a substitution command s/regex/replacement/. In this case, the regex is just ^, meaning "beginning of the line". And the replacement is peter plus \n, which is a newline special character. So it reads like this:

before each line, insert "peter" plus a newline

To add after each line:

$ echo "line1
line2
line3
line4
line5" | sed 's/$/\npeter/'
line1
peter
line2
peter
line3
peter
line4
peter
line5
peter

Here, the script is similar except that the regex is $, which means "end of the line", and the newline is substituted before peter instead of after. It reads as follows:

after each line, add a newline and the word "peter"

That does it. However, I don't understand the sed command completely. Could you explain abit? — Mr Krisey, Jul 22 '20 at 19:59

RavinderSingh13 · Answer 2 · 2020-07-22T20:02:37.323

4

Could you please try following.

awk '{print $0 ORS "peter"}' Input_file

Explanation: looks like OP changed samples so I changed it as per samples now. Simply printing current line with new line and string value peter then for each line of Input_file.

edited Jul 22 '20 at 20:02

answered Jul 22 '20 at 19:54

RavinderSingh13

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score 2 · Answer 3 · answered Jul 22 '20 at 20:55

2

although there is an accepted answer, for the record the right way to do it in sed is not regex substitution, but using the append command

$ sed 'a peter' file

answered Jul 22 '20 at 20:55

karakfa

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No, the "correct" was is to use `i`. But correctness is in the eye of the beholder. :) – William Pursell Jul 22 '20 at 23:08
And (reader beware), not all `sed` accept that syntax. – William Pursell Jul 22 '20 at 23:10
1

`i` will insert text before the line though. Here it's after. – karakfa Jul 22 '20 at 23:23
1

Argh....the question was edited! It used to be inserted before. Yes, with the edit, `a` is the correct approach. – William Pursell Jul 23 '20 at 01:17

William Pursell · Answer 4 · 2020-07-23T01:17:58.227

1

If you want to insert data before each line (as the question was originally), use i:

$ yes | nl | sed 4q | sed -e 'i\
peter'
peter
     1  y
peter
     2  y
peter
     3  y
peter
     4  y

It's sometimes painful to keep track of which sed allow you to skip the literal newline, but if you're using bash you can also do: sed -e $'i\\\npeter'

edited Jul 23 '20 at 01:17

answered Jul 22 '20 at 19:53

William Pursell

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score 1 · Answer 5 · answered Jul 23 '20 at 07:53

1

Here is a simpler awk that uses a custom ORS and works without matching any regex:

awk -v ORS='\npeter\n' '1' file

line1
peter
line2
peter
line3
peter
line4
peter
line5
peter

answered Jul 23 '20 at 07:53

anubhava

761,203
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score 0 · Answer 6 · answered Jul 24 '20 at 06:15

0

This might work for you (GNU sed):

sed 'p;c peter' file

or if you prefer:

sed 'p;s/.*/peter/' file

Just for completeness, of course:

sed 'a peter' file

Is the obvious answer (see karakfa)!

answered Jul 24 '20 at 06:15

potong

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Bash: insert a line after each line

6 Answers6