How do I remove certain lines (using line numbers) in a file?

There are specific lines that I want to remove from a file. Let’s say it’s line 20-37 and then line 45. How would I do that without specifying the content of those lines?

Answers:

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Method 1

With sed, like so:

sed '20,37d; 45d' < input.txt > output.txt

If you wanted to do this in-place:
sed --in-place '20,37d; 45d' file.txt

Method 2

If the file fits comfortably in memory, you could also use ed.
The commands are quite similar to the sed one above with one notable difference: you have to pass the list of line numbers/ranges to be deleted in descending order (from the highest line no/range to the lowest one). The reason is that when you delete/insert/split/join lines with ed, the text buffer is updated after each subcommand so if you delete some lines, the rest of the following lines will no longer be at the same position in the buffer when the next subcommand is executed. So you have to start backwards1.
In-place editing:

ed -s in_file <<IN
45d
20,37d
w
q
IN

or
ed -s in_file <<< $'45dn20,37dnwnqn'

or
printf '%sn' 45d 20,37d w q | ed -s in_file

Replace write with ,print if you want to print the resulting output instead of writing to file. If you want to keep the original file intact and write to another file you can pass the new file name to the write subcommand:
ed -s in_file <<IN
78,86d
65d
51d
20,37d
w out_file
q
IN

1
Unless you are willing to calculate the new line numbers after each delete, which is quite trivial for this particular case (after deleting lines 20-37, i.e. 18 lines, line 45 becomes line 27) so you could run:

ed -s in_file <<IN
20,37d
27d
w
q
IN

However, if you have to delete multiple line numbers/ranges, working backwards is a no-brainer.

Method 3

Just read it into memory, alter it, then write it back. You can do something like

filename = "foo"
f = open(filename, 'r+')                                                                                                                                 
linenums = [1, 3]                                                                                                                                            
s = [y for x, y in enumerate(f) if x not in [line-1 for line in linenums]]                                                                                                                                          
f.seek(0)
f.write(''.join(s))
f.truncate(f.tell())
f.close()

Tested with a 5 line file. Credits to http://pleac.sourceforge.net/pleac_python/fileaccess.html, see section “Modifying a File in Place Without a Temporary File”. See also https://stackoverflow.com/questions/125703/how-do-i-modify-a-text-file-in-python

Some notes:

  1. One could first truncate the file,
    then write to it, rather than write,
    then truncate, as above. However, I
    don’t know of a Python flag that
    allows one to read, and then do a
    truncated write. But maybe I’m
    missing something, as the document
    isn’t all that clear. Which brings
    me to
  2. Sometimes the Python docs really
    suck. See
    http://docs.python.org/library/functions.html#open

    Modes ‘r+’, ‘w+’ and ‘a+’ open the file for updating (note that ‘w+’
    truncates the file).

    Does this mean anything to you? What
    the hell is “open for updating”?

  3. I don’t know if doing this in python
    as opposed to something unixy like
    the stream editor is better. It
    might be more portable, but I don’t
    know how portable sed is. I just
    wrote it like that because I’m more
    comfortable with low level
    programming than using classic unix
    tools, which are good if they do
    exactly what you want, but (I think)
    are generally less flexible.
  4. This approach (manipulating the file in memory)
    trades memory for disk
    space. It should work Ok on machines
    with a few Gb of memory for files up
    to a few hundred Mb. Python doesn’t
    handle strings very efficiently, so
    switching to C/C++ for example would
    slightly increase performance and
    greatly reduce memory usage.

Method 4

You can use Vim in Ex mode:

ex -sc '20,37d|45d|x' file
  1. d delete
  2. x save and close


All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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