Trim a file in place without copying.

Narcopolypse · 08-11-2009, 02:02 AM

I have a large file that needs to be trimmed by a shell script periodically. Normally I would just use the head command or dd but this is running on an embedded system and won't always have enough free space to make a copy of the file. Does anyone know of a way I can trim off part of the end of a file without making a copy of it?

neonsignal · 08-11-2009, 02:19 AM

Have a look at the 'truncate' and 'ftruncate' posix calls.

There are some tools in the embutils package that may help (including 'truncate'), although I have not tried these.

chrism01 · 08-11-2009, 02:21 AM

If you don't mind using a more powerful lang like Perl or C, you can use the seek() fn to move to your new endpoint and write an EOF there.

http://perldoc.perl.org/functions/seek.html
http://linux.die.net/man/3/fseek

Narcopolypse · 08-12-2009, 01:58 AM

Truncate seems to be as close to a perfect solution as I can get. Unfortunately I must use the device's SDK to compile it. Oh well, now I know what I'm doing on my day off.

Narcopolypse · 08-18-2009, 11:54 PM

Never mind, I actually did find THE perfect solution!

Code:

dd if=/dev/null bs=1k seek=new_file_size_in_KB of=filename

gnashley · 08-19-2009, 06:41 AM

Hey that is a neat solution!