Let's say I have a large file (several gigs) with n lines in it. I would like to add/insert a line after k bytes offset, from the beginning of the file, then what would be the fastest way to achieve that?

  • what did you try until now? – yael Mar 10 at 5:53
  • please let us know why you want to do it? – yael Mar 10 at 5:54
  • I was thinking of trying something like head -c k file > temp.log; echo 'some data\n' >> temp.log; tail -c +(k+1) file > temp.log; mv temp.log file; but not sure if its the best way to do this simple task.. – khan Mar 10 at 5:56
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    @yael it doesn't really matter why he wants to do it. – RonJohn Mar 10 at 6:38
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    For Very Large Files, your standard Unix text tools might not be the best solution. It certainly can't hurt to try head -c k file > temp.log; echo 'some data\n' >> temp.log; tail -c +(k+1) file > temp.log; mv temp.log file especially if it's a one-time operation. It might have already completed by now. – RonJohn Mar 10 at 7:32

Here's a Python solution:

#!/usr/bin/env python3
# -*- encoding: utf-8 -*-

import os
import sys

stdout = os.fdopen(sys.stdout.fileno(), 'wb')

path_to_file = sys.argv[1]
width_in_bytes = int(sys.argv[2])

with open(path_to_file, "rb") as f:
    byte = f.read(1)
    while byte:
        for i in range(width_in_bytes):
            byte = f.read(1)

You could execute it like this:

python split_bytes.py path/to/file offset > new_file

As a test, I generated a 1GB file of random data:

dd if=/dev/urandom of=data.bin bs=64M count=16 iflag=fullblock

Then ran the script on that file:

python split_lines.py data.bin 10 > split-data.bin
  • This looks like a promising approach..let me try some tests. – khan Mar 11 at 3:05

a bash only solution :

use split command :

split --lines=2 --suffix-length=6 /etc/passwd /tmp/split.passwd.part

reassemble the file into one new

  for F in /tmp/split.passwd.part* ; 
    cat $F ; 
    echo ; 
) > /tmp/passwd_emptyline_evrey_2

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