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Friday, November 19, 2010

Using rpm2cpio

Maximum RPM: Taking the Red Hat Package Manager to the Limit
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Using rpm2cpio

From time to time, you might find it necessary to extract one or more files from a package file. One way to do this would be to:

Install the package
Make a copy of the file(s) you need
Erase the package

An easier way would be to use rpm2cpio.

rpm2cpio — What does it do?

As the name implies, rpm2cpio takes an RPM package file and converts it to a cpio archive. Because it's written to be used primarily as a filter, there's not much to be specified. rpm2cpio takes only only one argument, and even that's optional!

The optional argument is the name of the package file to be converted. If there is no filename specified on the command line,rpm2cpio will simply read from standard input and convert that to a cpio archive. Let's give it a try:

# rpm2cpio logrotate-1.0-1.i386.rpm 0707020001a86a000081a4000000000000000000000001313118bb000002c200000008000 000030000000000000000000000190000e73eusr/man/man8/logrotate.8." logrotate  - log fi le rotator .TH rpm 8 "28 November 1995" "Red Hat Software" "Red Hat Linux" .SH NAME

(We've just shown the first few lines of output.)

What on earth is all that stuff? Remember, rpm2cpio is written as a filter. It writes the cpio archive contained in the package file to standard output, which, if you've not redirected it somehow, is your screen. Here's a more reasonable example:

# rpm2cpio logrotate-1.0-1.i386.rpm > blah.cpio # file blah.cpio blah.cpio: ASCII cpio archive (SVR4 with CRC) #

Here we've directed rpm2cpio to convert the logrotate package file. We've also redirected rpm2cpio's output to a file called blah.cpio. Next, using the file command, we find that the resulting file is indeed a true-blue cpio archive file. The following command is entirely equivalent to the one above and shows rpm2cpio's ability to read the package file from its standard input:

# cat logrotate-1.0-1.i386.rpm | rpm2cpio > blah.cpio #

A more real-world example — Listing the files in a package file

While there's nothing wrong with using rpm2cpio to actually create a cpio archive file, it's takes a few more steps and uses a bit more disk space than is strictly necessary. A somewhat cleaner approach would be to pipe rpm2cpio's output directly intocpio:

# rpm2cpio logrotate-1.0-1.i386.rpm  | cpio -t usr/man/man8/logrotate.8 usr/sbin/logrotate 14 blocks #

In this example, we used the -t option to direct cpio to produce a "table of contents" of the archive created by rpm2cpio. This can make it much easier to get the right filename and path when you want to extract a file.

Extracting one or more files from a package file

Continuing the example above, let's extract the man page from the logrotate package. In the table of contents, we see that the full path is usr/man/man8/logrotate.8. All we need to do is to use the filename and path as shown below:

# rpm2cpio logrotate-1.0-1.i386.rpm |cpio -ivd usr/man/man8/logrotate.8 usr/man/man8/logrotate.8 14 blocks #

In this case, the cpio options -i, -v, and -d direct cpio to:

Extract one or more files from an archive.
Display the names of any files processed, along with the size of the archive file, in 512-byte blocks. [1]
Create any directories that precede the filename specified in the cpio command.

So where did the file end up? The last option (-d) to cpio provides a hint. Let's take a look:

# ls -al total 5 -rw-rw-r--   1 root     root         3918 May 30 11:02 logrotate-1.0-1.i386.rpm drwx------   3 root     root         1024 Jul 14 12:42 usr # cd usr # ls -al total 1 drwx------   3 root     root         1024 Jul 14 12:42 man # cd man # ls -al total 1 drwx------   2 root     root         1024 Jul 14 12:42 man8 # cd man8 # ls -al total 1 -rw-r--r--   1 root     root          706 Jul 14 12:42 logrotate.8 # cat logrotate.8 .\" logrotate - log file rotator .TH rpm 8 "28 November 1995" "Red Hat Software" "Red Hat Linux" .SH NAME logrotate \- log file rotator .SH SYNOPSIS \fBlogrotate\fP [configfiles]  .SH DESCRIPTION \fBlogrotate\fP is a tool to prevent log files from growing without … #

Since the current directory didn't have a usr/man/man8/ path in it, the -d option caused cpio to create all the directories leading up to the logrotate.8 file in the current directory. Based on this, it's probably safest to use cpio outside the normal system directories unless you're comfortable with cpio, and you know what you're doing!