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Is it possible in Linux (and/or on other Unix) 'shrink' file from beginning? I'd like to use it for persistent queue (no existing implementation suits my needs). From end of file I guess it's possible with truncate().

woky
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  • I started googling 'truncate beginning' after I wrote this post and it seems it isn't possible. – woky Feb 23 '11 at 23:07

3 Answers3

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If you are using ext4, xfs or some other modern file system, since Linux Kernel 3.15 you can use:

#include <fcntl.h>

int fallocate(int fd, int mode, off_t offset, off_t len);

with the FALLOC_FL_COLLAPSE_RANGE flag.

http://manpages.ubuntu.com/manpages/disco/en/man2/fallocate.2.html

Francesquini
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You can try dropping half of logs using ex, but it is not as fast as I would like (5GB of logs takes ages):

ex -s -c "1d$(( $(wc -l /var/log/messages | awk '{ print $1 }') / 2 ))|x" /var/log/messages
Miroslav Cibulka
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Yes, you can use cut or tail to remove portions of a file.

cut -b 17- input_file
tail -c +17 input_file

This will output the contents of input_file starting at the 17th byte, effectively removing the first 16 bytes of the file. Note that the cut example will also add a newline to the output.

MikeK
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  • I was interested in efficient in-place update of file through some system call. What you suggest is not it. – woky Aug 26 '15 at 08:19
  • In that case, you could look at the system calls that the above commands are doing via `strace` and only do the 'meat' of the operation and not all the command line parsing, etc. – MikeK Aug 26 '15 at 16:06