How can I remove all but the last 10 lines from a file?

Question

Is it possible to keep only the last 10 lines of a lines with a simple shell command?

tail -n 10 test.log

delivers the right result, but I don't know how to modify test.log itself. And

tail -n 10 test.log > test.log

doesn't work.

as a rule of thumb, never use a file simultaneously as input and output, you never know how it will end up. — tokland, Sep 23 '10 at 14:48
Good advice. I just did `tail -n 10 test.log > test.log` and ended up with nothing (a blank file). — kgf3JfUtW, Jan 16 '18 at 16:37

score 19 · Answer 1 · answered Sep 23 '10 at 04:11

19

You can do it using tempfile.

tail -n 10 test.log > test1.log

mv test1.log test.log

answered Sep 23 '10 at 04:11

Ankit Bansal

2

This works, thank you. However, **do NOT forget the `1`** because I did `tail -n 10 test.log > test.log` and am left with a blank file! – kgf3JfUtW Jan 16 '18 at 16:36

score 15 · Answer 2 · edited Jul 10 '15 at 02:39

15

echo "$(tail -n 10 test.log)" > test.log

Quotes are important. They preserve newline characters.

edited Jul 10 '15 at 02:39

Mike Laren

answered Jul 10 '15 at 02:29

Inna

hluk · Answer 3 · 2010-09-23T13:15:45.283

2

Invoke ed command (text editor):

 echo -e '1,-10d\nwq' | ed <filename>

This will send command to delete lines ('1,-10d'), save file ('w') and exit ('q').

Also note that ed fails (return code is 1) when the input file has less than 11 lines.

Edit: You can also use vi editor (or ex command):

vi - +'1,-10d|wq' <filename>

But if the input file has 10 or less lines vi editor will stay opened and you must type ':q' to exit (or 'q' with ex command).

edited Sep 23 '10 at 13:15

answered Sep 23 '10 at 12:36

hluk

Odd not to see this answer being liked more. It’s elegant and also does the job on BSD rather than just Linux. Thanks! – Xen Jul 31 '20 at 07:17

score 1 · Answer 4 · answered Apr 24 '15 at 17:26

1

Also you may use a variable:

LOG=$(tail -n 10 test.log)
echo "$LOG" > test.log

answered Apr 24 '15 at 17:26

David

score 1 · Answer 5 · answered Sep 24 '10 at 01:43

1

ruby -e 'a=File.readlines("file");puts a[-10..-1].join' > newfile

answered Sep 24 '10 at 01:43

ghostdog74

score 0 · Answer 6 · answered May 02 '23 at 09:48

I know this is old, but I wanted to demonstrate how process substitution can also help here:

cp <(tail -n 10 test.log) test.log

As process substitution results in a temporary file it doesn't overwrite test.log too soon. That's in contrast to the attempt in the question.

6 Answers6