How can I write to the second line of a file from the command line?

Question

I have an external program that produces an output file (largish, 20K lines possible).

I need to insert a new line between the existing line 1 and line 2. I've been looking at awk and sed - I use one liners in each fairly regularly - but I haven't been able to come up with the right switches to do this.

Answer 1

awk 'NR==1{print; print "new line"} NR!=1'

Answer 2

For your specific case, this should be simpler:

sed '1 { P ; x }' your-file

Explanation: at line 1, do the following

1. Print the line
2. Exchange the pattern space with the holding space (basically empties the buffer)

Then, the line (empty now) is printed again as part of the cycle.

---

If you want to add a new line instead of a new line character (what I understood initially) then just use sed's command a\ (append):

sed '1 a\
appended line' your-file

or even

sed '1 aappended line' your-file

It appends "appended line" after line 1.

Answer 3

sed ':a;N;$!ba;s/\n/\n\n\n/' yourBigFile

Explanation.

Answer 4

I guess the sed approach would be:

sed '2 i whatever_line_of_text_you_wanted_to_INSERT' filename.txt

This will get the text into the second line of the file and then the the actual second line with in the file will become the third.

Note that, using append mode, if it had to be, it had to use the first line, since the append will happen after the line number noted.

sed '1 a whatever_line_of_text_you_wanted_to_INSERT' filename.txt

Answer 5

Using sed and (Bash-specific) ANSI escape codes:

# note: \ --> \\
printf '%s\n' 1 2 3 4 5 | sed -e $'1 { s/\\(.*\\)/\\1\\\ninserted line/; }'

Answer 6

I usually use ed for this:

(echo 1a; echo 'Line to insert'; echo .; echo w) | ed - filename

Similar to a sed solution, but without the clutter of escaped newlines.

Answer 7

This might work for you:

sed 1G file

If you want to insert something after the newline:

sed '1{G;s/$/something/}' file

Or if you sed handles \n:

sed '1s/$/\nsomething/' file

Of course a is even easier:

sed '1a something' file

Answer 8

i would use python for this

import fileinput

for linenum,line in enumerate(fileinput,FileInput("file",inplace=1)):
    if linenum ==1:
          print ""
          print line.rstrip() 
      else: 
          print line.rstrip()`

Answer 9

I just like using an explicit counter:

awk '(c==1) {print "new line"} (c!=1) {print $0} {c++}' file