Section 3.12. Backing Up and Restoring with the ditto Utility

3.12. Backing Up and Restoring with the ditto Utility

ditto is a Mac OS X recursive copying utility, which can also create archive files (like tar or cpio ). What makes it interesting is that it's the one native tool with the ability to create full backups on all versions of Mac OS X since support for HFS+ features such as resource forks was added when the tool was brought forward from NEXTSTEP. (See the section "How Mac OS Filesystems Are Different" earlier in this chapter for more on HFS+.)

ditto can copy files and directories to one of three types of destinations: a directory, a ZIP archive file, or a cpio archive file. It does not support copying directly to tape. On the other hand, it doesn't come with Yet Another Archive Format, so you won't get stuck with backup archives in some format that might not be easily readable in a few years .

3.12.1. Syntax of ditto When Backing Up

The most common use of ditto is to make recursive copies of files and directories, like so:

$

ditto V --rsrc




src... dest_dir

The V flag shows everything that ditto is copying. The -rsrc flag ensures that HFS+ attributes and resource forks are copied (which is the default from Mac OS X 10.4 onwards). Extra HFS+ information is stored in AppleDouble format, where the data for a file named filename is kept in ._ filename .

Using ditto like this is a lot like using cp R , with one big difference. Let's say you want to make a copy of a directory. Using cp R src_dir dest_dir , you'd end up with the contents of src_dir under dest_dir/src_dir/ . With ditto src_dir dest_dir , the contents of src_dir end up directly under dest_dir/ , which can be somewhat confusing if you don't expect it. Also, ditto creates dest_dir/ if it doesn't already exist.

In most cases, ditto makes an exact duplicate of the source. However, there are a few things that ditto won't copy, in which case you'll be missing some information:

• Named sockets; see the socket(2) and bind(2) manpages (which don't appear to exist in Mac OS X 10.4 for some reason, although they do in earlier versions). However, sockets should be created dynamically by programs that use them.

• Named pipes (or FIFOs); see the mkfifo manpage . Fortunately, Mac OS X itself doesn't employ any named pipes.

• BSD flags; see the chflags manpage. Again, Mac OS X doesn't come with BSD flags set on any files.

• Extended ACLs; see the chmod and fsaclctl manpages. By default, filesystems don't have extended ACL functionality enabled.

These are apparently limitations of the underlying bill-of-materials (or BOM) framework employed by ditto . (See the bom , mkbom , and lsbom manpages.) mkbom also doesn't get named sockets or pipes, and the BOM file format doesn't include fields for BSD flags or extended ACLs.

In addition to making straight copies of files and directories, ditto can copy them into an archive file. To create a cpio file (with optional gzip compression), use the command:

$

ditto V -rsrc c -z




src_dir dest




.cpgz

To create a ZIP file, use the command:

$

ditto V -rsrc c -k




src_dir dest




.zip

When creating a ZIP file, using the -sequesterRsrc flag stores extra HFS+ data in a directory named _ _MACOSX ; PKZIP-compatible utilities (other than ditto itself) may handle this better than AppleDouble.

As when making recursive copies, src_dir is lost from pathnames stored in an archive file. To retain src_dir in the archived pathnames, use the -keepParent flag.

One thing you can't do with ditto is selectively archive only part of a directory's contentsfor example, you can't use a filename pattern or make incremental backups. ditto is suitable only for archiving entire directory trees.

You can use ssh and dd to make backups to remote systems, the same way you can with tar or cpio . For example:

$

ditto V -rsrc c -k




src_dir




-  ( ssh




remote_host




dd of=




dest




.zip )

Note that in this example, ditto can archive to standard output; it can also accept standard input as the source. It's possible this functionality could be used for tape-based backup and restore (if suitable tape device drivers are available), but this hasn't been tested .

$

ditto V -rsrc x




src




.cpgz




dest_dir

$

ditto V -rsrc x k




src




.zip




dest_dir

$

cpio itvz <




src




.cpgz

$

unzip lv




src




.zip