Large file with NULL characters

[Michael D'Aulerio]

I'm puzzled by file sizes of files with NULL characters. I have a file with 2264512 bytes, most of which are NULL characters (\000). When I run ls -l, it shows 2264512 for the file size. When I run du -s, it only shows 64 blocks. If I copy the file or tar the file, the new file has 4448 blocks. How can a file with that many bytes only take up 64 blocks?

[Sundar_7]

Hi Michael,

You are basically looking at something called "Sparse" files.

Try this

# echo | dd bs=1024 seek=100000 of=/tmp/large

# ll /tmp/large

# du â sk /tmp/large

ls will tell you the size is 102 MB but du will tell you the size is only 8 KB.

I dont believe tar can even begin to understand a sparse file. use fbackup to backup/restore sparse files.

A typical sparse file content will look like something like this

data data NULL NULL ......... NULL data data

Now what you see from the ls -l output is the size of the file including the NULLs.

-- Sund

[Michael D'Aulerio]

Thanks Sund,
You explain the sparse files very well. I should have given you a 10. I only gave you 7 because I couldn't get the command you gave me to create a sparse file. I figured out 2 things:
1) I couldn't create the sparse file from csh. I had to be in sh or ksh
2) It wouldn't work on an NFS mounted file system.
It took me a while to realize that my home directory is mounted from another host. I didn't use the /tmp directory as in your example. Once I got into ksh and wrote to a local file system, it worked exactly as you said.
Mike

[Michael D'Aulerio]

The pax command can also be used to archive & restore sparse files. I had been using cp or tar to copy files from one directory to another and was baffled because the target directory took up much more space than the source.