[SOLVED] How can I insert a new line into a text file into every other 2 lines?

Issue

Starting with this,

example.txt

1.qwer
2.asdf
3.xzcv
4.cbvn
5.erty

Going to this,

apendedtext.txt

1.append
2.qwer
3.asdf
4.append
5.xzcv
6.cbvn
7.append

Solution

Assuming you added the line numbers for simplicity and that the output is missing the 8th line "erty" you can get around with a simple awk one-liner:

#                                     ┌─ input      ┌─ output
awk 'NR % 2 {print "append"} {print}' < example.txt > apendedtext.txt
#     │                      └─ Print the original line
#     └─ Append if line has even index          

If you want to manipulate the line numbers too you could remove and add them back:

( sed -E 's/[0-9]*\.//g'| awk 'NR % 2 {print (++i) "." "append"} {print (++i) "." $0}' ) < example.txt > apendedtext.txt
#           └─ Remove line number               └─   Prepend a counter  ─┘                  

Answered By - lucaslugao