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Comparison of tar and cpio

The following information may be obsolete or inaccurate. Please take it with a grain of salt (or even two :-) for the time being.

Here is a summary of differences between tar and cpio. The accuracy of the following information has not been verified. The following people contributed to this section, mainly through a survey conducted in 1991. The remainder of this section does not otherwise try to relate topics to people.

Bent Bertelsen          
David Hoopes            talgras!david
Guy Harris              
Kai Petzke              -berlin.de
Kristen Nielsen         
Leslie Mikesell         

tar handles symbolic links in the form in which it comes in BSD; cpio doesn't handle symbolic links in the form in which it comes in System V prior to S5R4, and some vendors may have added symlinks to their system without enhancing cpio to know about them. Others may have enhanced it in a way other than the way I did it at Sun, and which was adopted by AT&T (and which is, I think, also present in the cpio that Berkeley picked up from AT&T and put into a later BSD release - I think I gave them my changes).

(S5R4 does some funny stuff with tar; basically, its cpio can handle tar format input, and write it on output, and it probably handles symbolic links. They may not have bothered doing anything to enhance tar as a result.)

cpio handles special files; tar, unless you're talking about a POSIXish version, doesn't.

tar comes with V7, System III, System V, and BSD source; cpio comes only with System III, System V, and later BSD (4.3-tahoe and later).

tar's way of handling multiple hard links to a file can handle file systems that support 32-bit inumbers (e.g., the BSD file system); cpios way requires you to play some games (in its "binary" format, i-numbers are only 16 bits, and in its "portable ASCII" format, they're 18 bits - it would have to play games with the "file system ID" field of the header to make sure that the file system ID/i-number pairs of different files were always different), and I don't know which cpios, if any, play those games. Those that don't might get confused and think two files are the same file when they're not, and make hard links between them.

tars way of handling multiple hard links to a file places only one copy of the link on the tape, but the name attached to that copy is the *only* one you can use to retrieve the file; cpios way puts one copy for every link, but you can retrieve it using any of the names.

>What type of check sum (if any) is used, and how is this calculated.

See the attached manual pages for tar and cpio format. tar uses a checksum which is the sum of all the bytes in the tar header for a file; cpio uses no checksum.

>If anyone knows why cpio was made when tar was prasent >at the unix scene,

It wasn't. cpio first showed up in PWB/UNIX 1.0; no generally-available version of UNIX had tar at the time. I don't know whether any version that was generally available *within AT&T* had tar, or, if so, whether the people within AT&T who did cpio knew about it.

tar does not backup special files. I got bite by this once. After a system crash I did a total restore and the tty ports for my multi-port serrial card did not get restored. cpio does restore special files (I checked).

On restore if there is a coruption on then tape tar will stop at that point, while cpio will skip over it and try to restore the rest of the files.

cpio seems to do a better job of restoreing links.

Please post the results that you get.

The main difference is just in the command syntax and header format.

tar is a little more tape-oriented in that everything is blocked to start on a block boundary. cpio knows about special files (devices and FIFOS and is thus more suitable for complete backups on systems that don't have dump.

>Is there any differences between the ability to recover crashed >archives between the two of them. (Is there any chance of recovering >crashed archives at all.)

Theoretically it should be easier under tar since the blocking lets you find a header with some variation of "dd skip=nn". However, modern cpio's and variations have an option to just search for the next file header after an error with a reasonable chance of re-syncing. However, lots of tape driver software won't allow you to continue past a media error which should be the only reason for getting out of sync unless a file changed sizes while you were writing the archive.

>If anyone knows why cpio was made when tar was prasent >at the unix scene, please tell me about this too.

Probably because it is more media efficient (by not blocking everything and using only the space needed for the headers where tar always uses 512 bytes per file header) and it knows how to archive special files.

You might want to look at the freely available alternatives. The major ones are afio, GNU tar, and pax, each of which have their own extensions with some backwards compatibility.

Sparse files were tarred as sparse files (which you can easily test, because the resulting archive gets smaller, and GNU cpio can no longer read it).


 

 

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