How to cut a file to a given size under Linux?

You may want to use the truncate command:

truncate --size=1G test.txt

SIZE can be specified as bytes, KB, K, MB, M, etc. I assume you can calculate the desired size by hand; if not, you could probably use the stat command to get information about the file's current size.

perl -we 'open( FILE, "< ./test.txt" ) && truncate( FILE, 8 ) && close(FILE);'

opens the file for reading. However, to truncate the file you need to modify it, so a read-only file handle isn't going to work. You need to use the "modify" mode ("+>").

As a side issue, it always amazes me when people let system calls fail silently and then ask what went wrong. An essential part of diagnosing a problem is looking at the error message produced; even if you don't understand it, it makes life much easier for those you ask for help.

The following would have been somewhat more helpful:

perl -we 'open(FILE, "<", "./test.txt") or die "open: $!";
          truncate(FILE, 8) or die "truncate: $!";
          close(FILE);'

although admittedly that would only have reported "invalid argument". Still, that is useful information and might well have led you to the conclusion that the open mode was wrong (as it did for me).

You can use tail to cut last 1000 bytes, example:

tail -c 1000 file > file2

the -c outputs final 1000 bytes of the file, for more options:

man tail

To replace original file with the file you just generated:

mv file2 file

there is a completely different way to do this, with bash, using the ed program. the following script will retain only the last 5000 lines of all files sitting in the specified directory. this can easily be modified to loop over several directories, change the number of lines, etc.

#!/bin/bash

LOGDIR=/opt/log
SAVELINES=5000

dirs="$LOGDIR"
for dir in $dirs ; do
    files=${dir}/*
    for f in $files ; do
        echo -e "1,-${SAVELINES}d\nwq" | ed $f 1>/dev/null 2>&1
    done
done