How to use sed to remove the last n lines of a file

Question

I want to remove some n lines from the end of a file. Can this be done using sed?

For example, to remove lines from 2 to 4, I can use

$ sed          


        

Answer 1

I came up with this, where n is the number of lines you want to delete:

count=`wc -l file`
lines=`expr "$count" - n`
head -n "$lines" file > temp.txt
mv temp.txt file
rm -f temp.txt

It's a little roundabout, but I think it's easy to follow.

1. Count up the number of lines in the main file
2. Subtract the number of lines you want to remove from the count
3. Print out the number of lines you want to keep and store in a temp file
4. Replace the main file with the temp file
5. Remove the temp file

Answer 2

If hardcoding n is an option, you can use sequential calls to sed. For instance, to delete the last three lines, delete the last one line thrice:

sed '$d' file | sed '$d' | sed '$d'

Answer 3

To truncate very large files truly in-place we have truncate command. It doesn't know about lines, but tail + wc can convert lines to bytes:

file=bigone.log
lines=3
truncate -s -$(tail -$lines $file | wc -c) $file

There is an obvious race condition if the file is written at the same time. In this case it may be better to use head - it counts bytes from the beginning of file (mind disk IO), so we will always truncate on line boundary (possibly more lines than expected if file is actively written):

truncate -s $(head -n -$lines $file | wc -c) $file

Handy one-liner if you fail login attempt putting password in place of username:

truncate -s $(head -n -5 /var/log/secure | wc -c) /var/log/secure

Answer 4

Just for completeness I would like to add my solution. I ended up doing this with the standard ed:

ed -s sometextfile <<< $'-2,$d\nwq'

This deletes the last 2 lines using in-place editing (although it does use a temporary file in /tmp !!)