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I often have to edit a large file by removing a few lines from the middle of it. I know which lines I wish to remove and I typically do the following:

sed "linenum1,linenum2 d" input.txt > input.temp

or in-line by adding the -i option. Since I know the line numbers, is there a command to avoid stream-editing and just remove the particular lines? input.txt can be as large as 50 GB.

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the ed line editor may be faster. –  jordanm Mar 3 '13 at 17:01
Somehow uglier, but may perform better: head -$((linenum1-1)) input.txt > input.temp; tail -n +$((linenum2+1)) input.txt >> input.temp. –  manatwork Mar 3 '13 at 17:38
@jordanm, it definitely is much slower (it has to gulp down the whole file, and set up the data structures it uses to represent the file in memory). –  vonbrand Mar 3 '13 at 18:40
@manatwork, I believe the processing is mostly inconsequential against reading/writing 50GiB... –  vonbrand Mar 3 '13 at 18:41
sed reads through the data once. With head|tail the whole data stream must be copied twice (once by each process). –  alexis Mar 4 '13 at 17:51

7 Answers 7

up vote 4 down vote accepted

What you could do to avoid writing a copy of the file is to write the file over itself like:

  sed "$l1,$l2 d" < file
  perl -le 'truncate STDOUT, tell STDOUT'
} 1<> file

Dangerous as you've no backup copy there.

Or avoiding sed, stealing part of manatwork's idea:

  head -n "$(($l1 - 1))"
  head -n "$(($l2 - $l1 + 1))" > /dev/null
  perl -le 'truncate STDOUT, tell STDOUT'
} < file 1<> file

That could still be improved because you're overwriting the first l1 - 1 lines over themselves while you don't need to, but avoiding it would mean a bit more involved programming, and for instance do everything in perl which may end up less efficient:

perl -ne 'BEGIN{($l1,$l2) = ($ENV{"l1"}, $ENV{"l2"})}
    if ($. == $l1) {$s = tell(STDIN) - length; next}
    if ($. == $l2) {seek STDOUT, $s, 0; $/ = \32768; next}
    if ($. > $l2) {print}
    END {truncate STDOUT, tell STDOUT}' < file 1<> file

Some timings for removing lines 1000000 to 1000050 from the output of seq 1e7:

  • sed -i "$l1,$l2 d" file: 16.2s
  • 1st solution: 1.25s
  • 2nd solution: 0.057s
  • 3rd solution: 0.48s

They all work on the same principle: we open two file descriptors to the file, one in read-only mode (0) using < file short for 0< file and one in read-write mode (1) using 1<> file (<> file would be 0<> file). Those file descriptors point to two open file descriptions that will have each a current cursor position within the file associated with them.

In the second solution for instance, the first head -n "$(($l1 - 1))" will read $l1 - 1 lines worth of data from fd 0 and write that data to fd 1. So at the end of that command, the cursor on both open file descriptions associated with fds 0 and 1 will be at the start of the $l1th line.

Then, in head -n "$(($l2 - $l1 + 1))" > /dev/null, head will read $l2 - $l1 + 1 lines from the same open file description through its fd 0 which is still associated to it, so the cursor on fd 0 will move to the beginning of the line after the $l2 one.

But its fd 1 has been redirected to /dev/null, so upon writing to fd 1, it will not move the cursor in the open file description pointed to by {...}'s fd 1.

So, upon starting cat, the cursor on the open file description pointed to by fd 0 will be at the start of the next line after $l2, while the cursor on fd 1 will still be at the beginning of the $l1th line. Or said otherwise, that second head will have skipped those lines to remove on input but not on output. Now cat will overwrite the $l1th line with the next line after $l2 and so on.

cat will return when it reaches the end of file on fd 0. But fd 1 will point to somewhere in the file that has not been overwritten yet. That part has to go away, it corresponds to the space occupied by the deleted lines now shifted to the end of the file. What we need is to truncate the file at the exact location where that fd 1 points to now.

That's done with the ftruncate system call. Unfortunately, there's no standard Unix utility to do that, so we resort on perl. tell STDOUT gives us the current cursor position associated with fd 1. And we truncate the file at that offset using perl's interface to the ftruncate system call: truncate.

In the third solution, we replace the writing to fd 1 of the first head command with one lseek system call.

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I don't get nearly as dramatic an improvement as you do. I get about 2 times better performance with your second solution. Good enough for the answer. –  sturgman Mar 4 '13 at 2:06
Off topic, can you explain solution 2 briefly? Or point to where I can learn? Is file fed to each command and each command operates from where the previous one ended? –  sturgman Mar 4 '13 at 2:09
@sturgman, the reason why I'm getting such improvements is probably because all the data is in cache (memory) for me. I've added some details to my answer. –  Stéphane Chazelas Mar 4 '13 at 18:14
Nice! I seem to be able to replace perl with GNU truncate, for example { head -n "$(($l1 - 1))"; c=$(head -n "$(($l2 - $l1 + 1))" | wc -m); cat; truncate -s -$c file; } <file 1<>file. I can't see a way to avoid passing the actual file name to truncate though. Do you see any issues with this? Thanks –  1_CR Sep 22 '14 at 12:59

Using sed is a good approach: It is clear, it streams the file (no problem with long files), and can easily be generalized to do more. But if you want a simple way to edit the file in-place, the easiest thing is to use ed or ex:

(echo 10,31d; echo wq) | ed input.txt

A better approach, guaranteed to work with files of unlimited size (and for lines as long as your RAM allows) is the following perl one-liner which edits the file in place:

perl -n -i -e 'print if $. < 10 || $. > 31' input.txt


-n: Apply the script to each line. Produce no other output.
-i: Edit the file in-place (use -i.bck to make a backup).
-e ...: Print each line, except lines 10 to 31.

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Nice trick! It's also possible to count lines from the end, for instance (echo '$-4,$d'; echo wq ) | ed input.txt deletes the last 5 lines. See also the line addressing section of the manual. –  Nemo Mar 10 at 6:48
However, I think ex (elvis 2.2.0) is better for large files: CPU-bound, few KB of memory used. ed (1.6) run out of memory (over 2 GB memory used) over my 9 GB file. Which explains why you said to use sed or perl for files of unlimited size. ;-) –  Nemo Mar 10 at 12:20

If you need to read and write 50GiB, that will take a long time, regardless what you do. And unless the lines are of fixed length, or you have some other way to know where the lines to be deleted are, there is no way around reading the file up to the last line to be deleted. Maybe a custom program that just counts newlines and later copies full blocks is a bit faster than sed(1), but I believe that is not your bottleneck. Try using time(1) to find out how the time is aportioned.

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If you want to edit the file in place, most shell tools won't help you because when you open a file for writing, you only have a choice of truncating it (>) or appending to it (>>), not overwriting existing contents. dd is a notable exception. See Is there a way to modify a file in-place?

export LC_ALL=C
lines_to_keep=$((linenum1 - 1))
lines_to_skip=$((linenum2 - linenum1 + 1))
deleted_bytes=$({ { head -n "$lines_to_keep"
                    head -n "$lines_to_skip" >&3;
                  } <big_file | dd of=big_file conv=notrunc;
                } 3>&1 | wc -c)
dd if=/dev/null of=big_file bs=1 seek="$(($(wc -c <big_file) - $deleted_bytes))"

(Warning: untested!)

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Is this bash? It will probably be very different if I am working in zsh. I should probably switch to bash to test all of these suggestions out. –  sturgman Mar 4 '13 at 0:39
@sturgman This snippet is POSIX. It'll work in zsh just as well as in *ash, bash or *ksh. –  Gilles Mar 4 '13 at 0:46
You're forgetting about the <> Bourne/POSIX shell redirection operator (see my answer) –  Stéphane Chazelas Mar 4 '13 at 18:16

Would this help?

perl -e '
           $num1 = 5;
           $num2= 10000;
           open IN,"<","input_file.txt";
           open OUT,">","output_file.txt";
           print OUT <IN> for (1 .. $num1-1)
           <IN> for ($num1 .. $num2);
           undef $/ and print OUT <IN>;
           close IN;
           close OUT;

This removes any lines between 5 and 10000 inclusive. Change the numbers to fit your needs. Can't see an efficient way of doing it in situ, though (i.e. this approach will have to print to a different output file).

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undef $/ means the rest of the file will be slurped in memory which for a 50 GB file is probably not a good idea, and probably won't be much more efficient than say using $/ = \32768 and a loop. –  Stéphane Chazelas Mar 3 '13 at 20:33
Run perl with the -i flag and it will do it in situ for you! –  alexis Mar 4 '13 at 17:59
@alexis: not sure how that would go. –  Joseph R. Mar 4 '13 at 18:03
See my answer for an example. –  alexis Mar 4 '13 at 18:10

This is nice and simple:

perl -ine 'print unless $.==13' /path/to/your/file

to remove e.g. line 13 from /path/to/your/file

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like GNU sed (GNU sed borrowed -i from perl), that writes the output into a second file (by the same name) so is not going to be significantly faster. –  Stéphane Chazelas Nov 24 '14 at 14:25

you could add a *q*uit instruction to your sed command whenn linenum2 is reached, so sed stops processing the file.

sed 'linenum1,linenum2d;linenum2q' file
share|improve this answer
This will improve things proportionally to how close is linenum2 to the beginning of the file. Thanks! –  sturgman Mar 3 '13 at 17:05
This improves nothing: “d Delete pattern space. Start next cycle.” – man sed. That means, the q will never be executed. Which is good, because otherwise the entire remaining part would be removed. –  manatwork Mar 3 '13 at 17:23
@manatwork you are right. Tried it out with no improvement in performance. –  sturgman Mar 3 '13 at 17:32
yes, of course. silly me. –  watael Mar 3 '13 at 19:09
Note that you can delete your own answer if you realise it's a mistake. Nothing wrong with that, quite the contrary. –  Stéphane Chazelas Mar 3 '13 at 20:29

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