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.
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.
- Brent Laster's Professional Git (Wrox)
- Smoothwall Express
- Returning Values from Bash Functions
- Bash Shell Script: Building a Better March Madness Bracket
- Learning to Program the Arduino
- Own Your DNS Data
- Machine Learning Everywhere
- Simple Server Hardening
- Tech Tip: Really Simple HTTP Server with Python
- SSH Tunneling - Poor Techie's VPN
Pick up any e-commerce web or mobile app today, and you’ll be holding a mashup of interconnected applications and services from a variety of different providers. For instance, when you connect to Amazon’s e-commerce app, cookies, tags and pixels that are monitored by solutions like Exact Target, BazaarVoice, Bing, Shopzilla, Liveramp and Google Tag Manager track every action you take. You’re presented with special offers and coupons based on your viewing and buying patterns. If you find something you want for your birthday, a third party manages your wish list, which you can share through multiple social- media outlets or email to a friend. When you select something to buy, you find yourself presented with similar items as kind suggestions. And when you finally check out, you’re offered the ability to pay with promo codes, gifts cards, PayPal or a variety of credit cards.Get the Guide