How to manually modify a .deb package

[I originally published this post on Blogspot.]

Every now and then you might need to tweak a .deb package for testing purposes. For instance you just want to change the behavior of the postinst script. Here are the steps to extract, modify and repackage the .deb package.

Step 1. Unpack with ar:

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ar x package.deb

You obtain three files:

• debian-binary: ASCII text
• control.tar.gz: gzip compressed data. This archive contains the files that actually get installed on the system (in /etc, /opt or /usr for example).
• data.tar.gz: gzip compressed data. This archive usually contains:
  • control: ASCII text
  • md5sums: contains the MD5 sums for the files in data.tar.gz
  • postinst: a script that is executed after installing the package
  • postrm: a script that is executed after removing the package
  • preinst: a script that is executed before installing the package
  • prerm: a script that is executed before removing the package

Some packages do not have pre/post install/removal scripts.

Step 2. Unpack the archive(s) depending on which files you need to modify. To unpack both archives, run:

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mkdir extras-control extras-data
tar -C extras-control/ -zxf control.tar.gz
tar -C extras-data/ -zxf data.tar.gz

Step 3. Modify the file(s) you need to change. If you modified files that are included in data.tar.gz, you need to update their MD5 sum in extras-control/md5sums.

Step 4. Repack control.tar.gz and/or data.tar.gz and remove the temporary directories:

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cd extras-control
tar cfz control.tar.gz *
mv control.tar.gz ..
cd ../extras-data
tar cfz data.tar.gz *
mv data.tar.gz ..
cd ..
rm -fr extras*

Step 5. Repackage the .deb package:

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ar r new_package.deb debian-binary control.tar.gz data.tar.gz

Note: I haven’t had the chance to test this recently, but it would appear that the order of files in the last step (step 5) is important.