How to add a string to a specific line

Question

I have a file which contains 50 lines. How can i add a string "-----" to a specific line say line 20 using python/linux ?

split over carriage return (\n) and loop through all lines incrementing a var. every 20 lines, insert whatever string you need. you could also have a regex with 20 carriage return — Youn Elan, Mar 06 '13 at 03:32

Artsiom Rudzenka · Accepted Answer · 2013-03-06T03:57:10.807

5

Have you tried something like this?:

exp = 20 # the line where text need to be added or exp that calculates it for ex %2

with open(filename, 'r') as f:
    lines = f.readlines()

with open(filename, 'w') as f:
    for i,line in enumerate(lines):
        if i == exp:
            f.write('------')
        f.write(line)

If you need to edit diff number of lines you can update code above this way:

def update_file(filename, ln):
    with open(filename, 'r') as f:
        lines = f.readlines()

    with open(filename, 'w') as f:
        for idx,line in enumerate(lines):
            (idx in ln and f.write('------'))
            f.write(line)

edited Mar 06 '13 at 03:57

answered Mar 06 '13 at 03:25

Artsiom Rudzenka

27,895
4
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What is `exp`? I'm assuming you mean 20. – squiguy Mar 06 '13 at 03:27
@squiguy yes) i mean a line where text need to be added – Artsiom Rudzenka Mar 06 '13 at 03:28
`exp` is some expression that would match what OP wants. – askewchan Mar 06 '13 at 03:28
Thanks Artsiom it worked for me, but what to do if i have more than 1 line to edit, i.e. i have a list called ln=(3,20,45) which is generated dynamically by the user ? – Ramesh Raithatha Mar 06 '13 at 03:54
@RameshRaithatha (3,20,45) is a lines to be modified? – Artsiom Rudzenka Mar 06 '13 at 03:55

score 3 · Answer 2 · answered Mar 06 '13 at 03:27

3

$ head -n 20 input.txt > output.txt
$ echo "---" >> output.txt
$ tail -n 30 input.txt >> output.txt

answered Mar 06 '13 at 03:27

Roozbeh Zabihollahi

7,207
45
39

score 0 · Answer 3 · edited May 23 '17 at 11:51

If the file to read is big, and you don't want to read the whole file in memory at once:

from tempfile import mkstemp
from shutil import move
from os import remove, close

line_number = 20

file_path = "myfile.txt"

fh_r = open(file_path)
fh, abs_path = mkstemp()
fh_w = open(abs_path, 'w')

for i, line in enumerate(fh_r):
    if i == line_number - 1:
        fh_w.write('-----' + line)
    else:
        fh_w.write(line)

fh_r.close()
close(fh)
fh_w.close()

remove(file_path)
move(abs_path, file_path)

Note: I used Alok's answer here as a reference.

How to add a string to a specific line

3 Answers3