shell insert a line every n lines

Question

I have two files and I am trying to insert a line from file2 into file1 every other 4 lines starting at the beginning of file1. So for example:

file1:

line 1
line 2
line 3
line 4
line 5
line 6
line 7
line 8
line 9
line 10

file2:

50
43
21

output I am trying to get:

50
line 1
line 2
line 3
line 4
43
line 5
line 6
line 7
line 8
21
line 9
line 10

The code I have:

while read line
do
    sed '0~4 s/$/$line/g' < file1.txt > file2.txt
done < file1.txt

I am getting the following error:

sed: 1: "0~4 s/$/$line/g": invalid command code ~

its reading from another file. In this case file1 – jeabesli Mar 28 '16 at 19:30 — jeabesli, Mar 28 '16 at 19:30
Can you show sample data from `file1` as well? – anubhava Mar 28 '16 at 19:31 — anubhava, Mar 28 '16 at 19:31
I edited the question since my files were mixed up – jeabesli Mar 28 '16 at 19:47 — jeabesli, Mar 28 '16 at 19:47
Posted an answer below using awk – anubhava Mar 28 '16 at 19:53 — anubhava, Mar 28 '16 at 19:53

score 2 · Answer 1 · answered Mar 28 '16 at 19:48

It is easing to do this using awk:

awk 'FNR==NR{a[i++]=$0; next} !((FNR-1) % 4){print a[j++]} 1' file2 file1

50
line 1
line 2
line 3
line 4
43
line 5
line 6
line 7
line 8
21
line 9
line 10

While processing first file in input i.e. file2, we store each line in array with key as an incrementing number starting with 0.
While processing second file in input i.e. file1, we check if current record # is divisible by 4 using modulo arithmetic and if it is then insert a line from file2 and increment the index counter.
Finally using action 1, we print lines from file1.

score 2 · Accepted Answer · answered Mar 28 '16 at 19:59

The following steps through both files without loading either one into an array in memory:

awk '(NR-1)%4==0{getline this<"file2";print this} 1' file1

This might be preferable if your actual file2 is larger than what you want to hold in memory.

This breaks down as follows:

(NR-1)%4==0 - a condition which matches every 4th line starting at 0
getline this<"file2" - gets a line from "file2" and stores it in the variable this
print this - prints ... this.
1 - shorthand for "print the current line", which in this case comes from file1 (awk's normal input)

score 2 · Answer 3 · answered Mar 29 '16 at 00:24

2

This might work for you (GNU sed):

sed -e 'Rfile1' -e 'Rfile1' -e 'Rfile1' -e 'Rfile1' file2

or just use cat and paste:

cat file1 | paste -d\\n file2 - - - -

answered Mar 29 '16 at 00:24

potong

55,640
6
51
83

score 1 · Answer 4 · answered Mar 28 '16 at 20:09

1

another alternative with unix toolchain

$ paste file2 <(pr -4ats file1) | tr '\t' '\n'

50
line 1
line 2
line 3
line 4
43
line 5
line 6
line 7
line 8
21
line 9
line 10

answered Mar 28 '16 at 20:09

karakfa

66,216
7
41
56

score -1 · Answer 5 · answered Mar 28 '16 at 20:00

-1

Here's a goofy way to do it with paste and tr

paste file2 <(paste - - - - <file1) | tr '\t' '\n'

Assumes you don't have any actual tabs in your input files.

answered Mar 28 '16 at 20:00

glenn jackman

238,783
38
220
352

shell insert a line every n lines

5 Answers5

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