Understanding CPIO

In the previous article, I demonstrated the usage of the tar archiver utility. This week I'll continue by introducing the cpio archiver utility.

While both tar and cpio will achieve the same results, the cpio utility approaches things a little bit differently. The tar utility assumes that you want to recursively archive everything under the specified directory or directories, meaning that you have to explicitly tell tar if you want to exclude certain portions of that directory structure. In contrast, the cpio utility expects to be explicitly told which files or directories you wish to archive; this behavior is commonly referred to as "receiving from standard input." In other words, cpio expects to receive a list that contains one file per line, and if you remember from the "Finding Things in Unix" and "Find: Part Two" articles, that is exactly the type of list that the find utility creates. The ls utility can also create this type of list, meaning that you will see either of the ls or the find utilities used in conjunction with cpio. And since cpio archives a list of files it receives from standard input, you usually use a pipe (|) whenever you create an archive with the cpio utility.

The tar utility also assumes that you want to write the archive to your first SCSI tape drive, unless you explicitly specify a file using the f switch. In contrast, the cpio utility writes to what is known as standard output. This means that you will be using a redirector (either < or >) whenever you are creating, listing, or extracting a cpio archive file. Again, that file may be an actual file, or it may be your floppy, or it may be a tape device, since in Unix everything is a file.

This may sound a bit more complicated at first, but a few examples should convince you that it really isn't.

Let's start by creating a cpio archive. In the last article, I created a test user account and created a directory structure named www in this user's home directory so I would have some files on which to practice using the archiving utilities. I'll log in as the test user, cd into the www directory, and see what happens if I use the ls command with the cpio utility:

cd www
ls | cpio -ov > backup.cpio

You'll note that I first cded into the directory that contained the files I wished to archive. I used the ls utility to make a list of the files in the current directory and used a pipe (|) to send that list to the cpio utility. The o switch invokes what is known as "copy out mode," which tells cpio to create an archive. The v switch tells cpio to be verbose, meaning it will list each file as it archives it. Finally, I used the > redirector to write the results (the archive) to a file called backup.cpio. I can call this file anything I like; I chose to give it a cpio extension to remind me that it is a cpio backup file. I can verify the file type using the file utility:

file backup.cpio
backup.cpio: cpio archive

Instead of using the redirector, I could have also used the F switch to specify which file to write the archive to. So the following command will achieve the same results:

ls | cpio -ovF backup.cpio

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Once the archive was created, cpio told me how many blocks it wrote to the archive; in my case, it was 48 blocks.

So to create an archive, use the o switch or copy-out mode. To either view or extract the contents of the archive, use what is known as "copy-in mode." You invoke this mode by using the i switch. If you just want to view the contents of the archive, also include the t switch, which will list the contents of the archive without extracting them:

cpio -it < backup.cpio

You'll note that this time I used the other redirector (<), as I wanted the contents of the backup.cpio file to be sent to the cpio utility. I can also include the v switch, if I want to see a verbose listing of the backup:

cpio -itv < backup.cpio

Remember that it is important to view the contents of an archive before attempting to restore it, as you want to ensure that the files don't begin with a /.

To restore this archive, I simply cd into the directory to which I'd like to restore the archive, and repeat the above command without the t switch. I'll cd back into my home directory and create a directory named backupand do the restore there:

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

You'll note something interesting if you try this exercise yourself; if you use the ls -F command, you'll see that you did indeed restore all of the files and directories that were in the www directory. But if you cd into any of those subdirectories, you'll note that they are empty. Even more interestingly, if you try to remove any of those subdirectories, you still have to use the R switch, as they are still valid directories.

What happened here? Since the cpio utility received its file list from the ls utility (and the ls utility can only list the files in the current directory), cpio was unaware of all of the files that existed below the current directory. Remember, cpio will only archive the files that are sent to it in a list. This may seem odd at first, but it is an ideal way to archive just the files in the current directory. In order to do this with the tar utility, you would have to create an exclude file, as tar wants to recursively copy everything in and below the current directory.

This doesn't mean that cpio can't archive recursively; it simply means that if you want to just archive the current directory, you use ls and if you want to archive recursively, you use find instead.

Let's try that backup and restore again, this time using the find utility. First, I'll remove the old backup and empty out the backup directory:

rm www/backup.cpio
rm -R backup/*

Then I'll cd into the directory I wish to back up (www) and archive its contents:

cd www
find -d . -print | cpio -ov > backup.cpio

When using the find utility with cpio, it is always a good idea to include either the d or the depth switch. Remember from the find article that this switch prevented permissions from interfering with a backup. When using this switch, either put -d right after the word find and before the directory to search (in this case, "."), or put the word -depth after the directory to search, like so:

find . -depth -print | cpio -ov > backup.cpio

So as a recap on the find command, I told find to search the current directory (".") and to "print" its contents; the | was used to send those contents to the cpio utility, which created an archive (-o) and wrote that archive to a file called backup.cpio. When I created this archive, I noted that cpio wrote 43097 blocks, which is many more than the 48 I received with the ls command.