create file

rguy · 06-10-2010, 02:57 PM

Hi,

I want to create a file, say example.txt of 5MB.

I use dd to create a file of 5MB, but when I start
pushing more data into the file, the file size keeps increasing.

I want to have file of 5MB, and when I direct data to it should overwrite
the existing data and the file size should be fixed to 5MB.

Thanks

nuwen52 · 06-10-2010, 03:33 PM

I have not seen something like this for normal/generic files. But, under FreeBSD, there is a circular log file implementation for syslogd. Something like this might exist for other purposes, so it might be a place to start looking. Google "freebsd clog" for more info.

colucix · 06-10-2010, 04:21 PM

Hi and welcome to LinuxQuestions!

You may want to create a sparse file of fixed (apparent) size, for example using the seek operand of the dd command:

Code:

dd if=/dev/zero of=this_is_a_sparse_file bs=1K count=0 seek=5120

In this way you can allocate a fixed amount of disk space, without actually store any data on it. See this page on Arch wiki for an introduction to sparse files (plenty of them out there). The problem could be how to write data into the sparse file: you need a software that is able to write data at specific positions. For example a C program or Python can do that.

pixellany · 06-10-2010, 04:43 PM

Quote:

you need a software that is able to write data at specific positions. For example a C program or Python can do that.

As can dd, n'est-ce pas?

colucix · 06-10-2010, 05:08 PM

Quote:

Originally Posted by pixellany

As can dd, n'est-ce pas?

Oui!