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.

potong · Answer

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)!

anubhava · Answer

Here is a simpler awk that uses a custom ORS and works without matching any regex:
awk -v ORS='npetern' '1' file

line1
peter
line2
peter
line3
peter
line4
peter
line5
peter

karakfa · Answer

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

RavinderSingh13 · Answer

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.

William Pursell · Answer

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
peter'

Bill Jetzer · Answer

To add before each line:
$ echo "line1
line2
line3
line4
line5" | sed 's/^/petern/'
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"

Bash: insert a line after each line

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