In this month's column, Eric moves beyond find to cover duplicating files and directory trees using the versatile cpio command. cpio uses space on tape more efficiently than tar and is an excellent alternative for creating archives on platforms that do not have the GNU utilities available. Read on for a thorough discussion of cpio and its three modes of operation: Pass-through, Create and Extract.

In an earlier article, I suggested using the following command for duplicating files and directory trees:

$ find . -depth | cpio -pdmv dest_dir

Since the focus of that tutorial was the find utility, I didn't discuss cpio in depth.

cpio can create and extract archives on diskette, tape or in files using eight different archive formats, including tar. It can also create an almost perfect duplicate of a directory tree, preserving file ownership, modes, and access times. Since cpio is designed to accept a list of files such as the output of ls or find, it is more suited for comprehensive backup systems than tools like conventional tar; the set of files processed can be easily controlled programmatically.

cpio also has some less obvious advantages. The default cpio format uses the space on tape much more efficiently than the conventional tar format, and it can also skip damaged sections of archives and continue during restore operations, instead of quitting completely.

GNU tar, which will be covered in an upcoming tutorial, addresses many of these issues. However, when creating archives for other platforms that do not have the GNU utilities available, cpio is an excellent alternative.

Two of the reasons why cpio is not used are readily apparent. The list of possible command line switches fills nearly half a typewritten page, and since it does not accept file names or wildcards as arguments, novices can find it pretty intimidating. But cpio can be worth the extra effort.

cpio has three modes of operation. Pass-through mode, which is what I used in the example above for duplication, create mode, which is used for creating archives, and extract mode, which is used for extracting files from archives.

Pass-Through Mode

As its name implies, in pass-through mode cpio acts as a conduit for copying lists of files from one destination to another. The ability to do this while creating subdirectories as needed and handling special files makes it a crucial tool for any system administrator to be familiar with.

For example, one common situation on a multi-user system is the need for more drive space for user directories. The administrator will need to perform the following steps: add an additional drive to the system, create one or more file systems, copy the user directories from the old file system to the new one, and then, depending upon the circumstances, change the file system mount points in order to make the transition unobtrusive.

There are three methods available for copying the users' files over to the new disk. One is to use tar to archive the files and extract them to the new area. This requires the time necessary to archive and extract the files.

Another is to use cp's recursive mode to copy them directly. This mode copies only regular files and links. It also follows symbolic links, which can duplicate a lot of files when used carelessly.

Of course, few system administrators know exactly what is in their users' directories. A developer may have special files such as sockets or pipes. Any user may have files with special permissions in order to prevent unwanted access. Administrators do not have time to inspect home directories that carefully, and many users do not want them to anyway.

$ find . -depth | cpio --pass-through \
 --preserve-modification-times \
 --make-directories --verbose /mnt/export

This command causes find to output the name of every file under the present directory. (The -depth option insures that directory names are output before the names of the files in them.) cpio reads these file names in and copies them to /mnt/export.

The switches passed to cpio are:

--pass-throughOperate in pass-through mode.

--preserve-modification-timesSet the modification times of the new files to that of the old ones.

--make-directoriesCreate directories when necessary. (This option works when restoring archives, also.)

--verboseVerbose mode. This mode will produce output for all files. An alternative is the -dot option which only produces a . for each file processed. (These options work in all modes.)

The command above creates an exact duplicate of the original directory, regardless of the types of files or any special file modes that were set.

If the files are being copied to the same file system, the --link option can be used to hard link files when necessary.



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Useful article

Anonymous's picture

Just discovered this article while trying to diagnose some cpio problems... no surprises, it turned out to be the non-writable directories issue discussed...

Meaning of -depth option backwards, use -print0 and -0 options!

John Keith Hohm's picture

The -depth option to find ensures that directory names are output after the names of the files in them, not before. In combination with the --make-directories (or just -d) and --preserve-modification-times (or just -p) options to cpio, this results in cpio preserving the original modification time of both files and directories.

This works because cpio will create a directory automatically while writing the files inside it; only after it is done writing all the directory contents does it visit the directory itself to set its attributes, which includes resetting the modification time.

You are missing a couple other important options, though: the -print0 option to find and the --null (or just -0) option to cpio cause find and cpio to write and read the list of filenames terminated by a null character instead of a newline. Since most Linux filesystems allow names to contain nulls, this is important to properly archive such files and avoids doing something very bad with a file named like:


So the full recommended command is:

$ find . -depth -print0 | cpio -pdm0v dest_dir

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