Understanding CPIO
Pages: 1, 2

Now let's see what happens when I try to restore this archive:

cd ../backup
cpio -iv < ~/www/backup.cpio

I received an interesting message on my screen when I did this restore:

<snip>
cpio: mod_tsunami/Makefile: No such file or directory
cpio: mod_tsunami/distinfo: No such file or directory
cpio: mod_tsunami/pkg-comment: No such file or directory
cpio: mod_tsunami/pkg-descr: No such file or directory
cpio: mod_tsunami/pkg-plist: No such file or directory
mod_tsunami
Makefile
.
43097 blocks

It looks like cpio read all 43097 blocks but complained about missing files or directories. Indeed, if I do an ls on any of the restored subdirectories, I'll discover that they are once again empty! Don't worry, all of those files and directories are in that archive file; I've simply demonstrated the default extraction behaviour of cpio. Unlike tar, the cpio utility does not recreate any directories during the restore unless you specifically ask it to with the d switch. And, unlike tar, the cpio utility will not overwrite any existing files unless you specifically ask it to with the u switch.

So let's try that restore again, this time using the d switch to create the directories and the u switch to overwrite the files I've already restored:

cpio -ivdu < ~/www/backup.cpio

This time I don't receive any error messages and I've successfully restored all of the subdirectories and their files.

There're a few more switches you may consider using when backing up and restoring with cpio. If I compare the modification times of a file before it was archived and after it was restored, I will see this:

ls -l www/zope/Makefile	
-rw-r--r--  1 test  wheel  4308 May 11 09:53 www/zope/Makefile

ls -l backup/zope/Makefile
-rw-r--r--  1 test  wheel  4308 Jun  2 11:38 backup/zope/Makefile

ls -l www/backup.cpio
-rw-r--r--  1 test  wheel  22065664 Jun  2 10:39 www/backup.cpio

You'll note that the original file was created on May 11, that it was backed up on June 2 at 10:39, and that it was restored on June 2 at 11:38. If you want to preserve the file's original time, include the a switch when creating the archive, and the m switch when restoring the archive:

cd www
find -d . -print | cpio -ova > backup.cpio
cd ../backup
cpio -ivdm < ~/www/backup.cpio

If you try this and repeat the ls -l command, you'll see that the original times of the archived files were kept intact.

The nice thing about using the find utility with cpio is that you have all of find's switches available to you, to fine-tune which files you would like to back up. For example, if you'd like to do an incremental backup, use find's -newer switch. In this example, I'll back up all of the files in my home directory that have changed since 11 PM on June 1st:

cd
touch -t 06012300 June1
find -d . -newer June1 -print | cpio -ova > backup.cpio

Here I used the touch utility to create an empty file with a timestamp of month 06 day 01 time 2300, then I told find to use the time on that file as the reference point when searching the current directory. Alternatively, if I wasn't concerned so much about the time as the date, I could have used find's atime, ctime, or mtime switches. And if I only want to archive files of a certain size, I can use find's size switch.

Before ending today's article, I'd also like to demonstrate cpio's third mode, which is known as "copy-pass mode." This is an interesting mode, as it archives and extracts in the same command, making it ideal for copying one directory structure and recreating it in another location.

Let's say I want to copy the www directory structure from the home directory of the test user to the home directory of the user genisis. I'll have to become the superuser, as I'll be creating the archive in one user's home directory and recreating it in another user's home directory:

su
Password:
cd ~test/www
find -d . -print | cpio -pvd ~genisis/www

Note that I first cded into the directory I wanted to archive, in this case the www subdirectory of the test user's home directory. Then, with the cpio command, I invoked copy-pass mode with the p switch and specified that I wanted the archive recreated in the www subdirectory of the home directory of the user genisis.

If I run this command and then do an ls -l of genisis' home directory, I'll see that I've successfully recreated the entire www directory structure. However, I'll want to fine-tune that above command as those restored files still belong to the user "test." I'll repeat that command using the u switch so it will overwrite that last restore, and I'll include the R switch, which tells cpio to change the ownership of the files as it recreates them:

find -d . -print | cpio -pvdu -R genisis ~genisis/www

When using the R switch, follow it by the name of the user you wish to become the owner of the files, then follow that by the name of the directory to restore the files to.

Finally, if I want to keep the original times of the files instead of having them changed to the time the files were restored, I'd also add the a and m switches:

find -d . -print | cpio -pvduam -R genisis ~genisis/www

This should get you started with the cpio command. If you're planning on using cpio to copy between different computers, you'll want to read its manpage first, as there may be considerations, especially if the computers are running different versions of Unix or different architectures.

In next week's article, I'll continue the archiver series by introducing the pax command and, if space permits, the dd command.

Dru Lavigne is a network and systems administrator, IT instructor, author and international speaker. She has over a decade of experience administering and teaching Netware, Microsoft, Cisco, Checkpoint, SCO, Solaris, Linux, and BSD systems. A prolific author, she pens the popular FreeBSD Basics column for O'Reilly and is author of BSD Hacks and The Best of FreeBSD Basics.

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