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I usually prefer to fix a Linux system than to reinstall from scratch. My computers have seen many distribution upgrades and a list of PPAs or third-party repositories. APT usually makes sure that everything works in the end. However, the fact that the package manager thinks that all required packages are 'installed' does not guarantee that all the files are present on the file system.

Such a such situation may occur if you have to work-around dependency problems with dpkg --force-*. One could also reproduce such a situation by deleting a file from /usr as root.

Is there a simple way to verify whether all files belonging to an installed package are present?

If a such a problem package is found, aptitude reinstall fixes the problem.

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From the 'debsums' man page:

apt-get install --reinstall $(dpkg -S $(debsums -c) | cut -d : -f 1 | sort -u)
       Reinstalls packages with changed files.

I just ran this on my system because the disk content was randomly corrupted while I was experimenting with a beta OS. Here's what I did (and it seems to have worked well):

First, I installed 'debsums' and ran it to see if I had any corrupt files on my system:

peniwize@penibox-2:~$ sudo apt-get install debsums
peniwize@penibox-2:~$ sudo apt-get clean
peniwize@penibox-2:~$ sudo debsums_init
peniwize@penibox-2:~$ sudo debsums -cs

As you can see, I have five corrupt files so I need to reinstall them. This is how I found which packages contain the corrupt files:

peniwize@penibox-2:~$ sudo dpkg -S $(sudo debsums -c) | cut -d : -f 1 | sort -u

If you have no corrupt packages the you will see this (or something like it):

peniwize@penibox-2:~$ sudo dpkg -S $(sudo debsums -c) | cut -d : -f 1 | sort -u
dpkg-query: error: --search needs at least one file name pattern argument

Use --help for help about querying packages.

Then I repaired the corruption by reinstalling the damaged packages:

peniwize@penibox-2:~$ sudo apt-get install --reinstall $(sudo dpkg -S $(sudo debsums -c) | cut -d : -f 1 | sort -u)
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following package was automatically installed and is no longer required:
Use 'apt-get autoremove' to remove it.
0 upgraded, 0 newly installed, 5 reinstalled, 0 to remove and 0 not upgraded.
Need to get 43.9 MB of archives.
After this operation, 0 B of additional disk space will be used.
Get:1 saucy/main bash-completion all 1:2.0-1ubuntu3 [173 kB]
Get:2 saucy/main cheese-common all 3.8.3-0ubuntu1 [2,929 kB]
Get:3 saucy/universe gnumeric-doc all 1.12.6-1 [7,295 kB]     
Get:4 saucy/main linux-image-extra-3.11.0-12-generic i386 3.11.0-12.19 [33.5 MB]
Get:5 saucy/main indicator-sound i386 12.10.2+13.10.20131011-0ubuntu1 [55.7 kB]
Fetched 43.9 MB in 10min 23s (70.4 kB/s)                                                           
(Reading database ... 174913 files and directories currently installed.)
Preparing to replace bash-completion 1:2.0-1ubuntu3 (using .../bash-completion_1%3a2.0-1ubuntu3_all.deb) ...
Unpacking replacement bash-completion ...
Preparing to replace cheese-common 3.8.3-0ubuntu1 (using .../cheese-common_3.8.3-0ubuntu1_all.deb) ...
Unpacking replacement cheese-common ...
Preparing to replace gnumeric-doc 1.12.6-1 (using .../gnumeric-doc_1.12.6-1_all.deb) ...
Unpacking replacement gnumeric-doc ...
Preparing to replace linux-image-extra-3.11.0-12-generic 3.11.0-12.19 (using .../linux-image-extra-3.11.0-12-generic_3.11.0-12.19_i386.deb) ...
Unpacking replacement linux-image-extra-3.11.0-12-generic ...
Examining /etc/kernel/postrm.d .
run-parts: executing /etc/kernel/postrm.d/initramfs-tools 3.11.0-12-generic /boot/vmlinuz-3.11.0-12-generic
run-parts: executing /etc/kernel/postrm.d/zz-update-grub 3.11.0-12-generic /boot/vmlinuz-3.11.0-12-generic
Preparing to replace indicator-sound 12.10.2+13.10.20131011-0ubuntu1 (using .../indicator-sound_12.10.2+13.10.20131011-0ubuntu1_i386.deb) ...
Unpacking replacement indicator-sound ...
Processing triggers for man-db ...
Processing triggers for libglib2.0-0:i386 ...
No such key 'auto-launch' in schema 'com.ubuntu.update-notifier' as specified in override file '/usr/share/glib-2.0/schemas/20_xubuntu-default-settings.gschema.override'; ignoring override for this key.
Processing triggers for hicolor-icon-theme ...
Setting up bash-completion (1:2.0-1ubuntu3) ...
Setting up cheese-common (3.8.3-0ubuntu1) ...
Setting up gnumeric-doc (1.12.6-1) ...
Setting up linux-image-extra-3.11.0-12-generic (3.11.0-12.19) ...
Running depmod.
update-initramfs: deferring update (hook will be called later)
Not updating initrd symbolic links since we are being updated/reinstalled 
(3.11.0-12.19 was configured last, according to dpkg)
Not updating image symbolic links since we are being updated/reinstalled 
(3.11.0-12.19 was configured last, according to dpkg)
Examining /etc/kernel/postinst.d.
run-parts: executing /etc/kernel/postinst.d/apt-auto-removal 3.11.0-12-generic /boot/vmlinuz-3.11.0-12-generic
run-parts: executing /etc/kernel/postinst.d/dkms 3.11.0-12-generic /boot/vmlinuz-3.11.0-12-generic
run-parts: executing /etc/kernel/postinst.d/initramfs-tools 3.11.0-12-generic /boot/vmlinuz-3.11.0-12-generic
update-initramfs: Generating /boot/initrd.img-3.11.0-12-generic
run-parts: executing /etc/kernel/postinst.d/pm-utils 3.11.0-12-generic /boot/vmlinuz-3.11.0-12-generic
run-parts: executing /etc/kernel/postinst.d/update-notifier 3.11.0-12-generic /boot/vmlinuz-3.11.0-12-generic
run-parts: executing /etc/kernel/postinst.d/zz-update-grub 3.11.0-12-generic /boot/vmlinuz-3.11.0-12-generic
Generating grub.cfg ...
Found linux image: /boot/vmlinuz-3.11.0-14-generic
Found initrd image: /boot/initrd.img-3.11.0-14-generic
Found linux image: /boot/vmlinuz-3.11.0-12-generic
Found initrd image: /boot/initrd.img-3.11.0-12-generic
Found memtest86+ image: /boot/memtest86+.bin
Found Windows 7 (loader) on /dev/sda1
Setting up indicator-sound (12.10.2+13.10.20131011-0ubuntu1) ...

Finally, I checked to make sure that no corrupt files remain:

peniwize@penibox-2:~$ sudo debsums -c

There was no output from this command, which means that no errors were found. :-)

One final note: you should also check your packages' config files to make sure they're OK. This can be more difficult because config files often change, and the changes are legitimate, so you'll need to manually inspect each changed config file to determine whether or not it's actually corrupt. This is how you get a list of changed config files:

peniwize@penibox-2:~$ sudo debsums -as
debsums: changed file /etc/gnome/defaults.list (from desktop-file-utils package)
debsums: changed file /etc/default/rcS (from initscripts package)
debsums: changed file /etc/subuid (from login package)
debsums: changed file /etc/subgid (from login package)
debsums: changed file /etc/sudoers (from sudo package)
share|improve this answer

The script given by PeniWize works great for corrupted files, but does not take care of packages with missing files, because debsums reports them to stderr. To reinstall packages with missing files, this worked for me:

sudo apt-get install --reinstall $(sudo dpkg -S $(sudo debsums -c 2>&1 | cut -d " " -f 4 | sort -u) | cut -d : -f 1 | sort -u)
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up vote 1 down vote accepted

The question has been answered elsewhere:

Is there a Ubuntu sanity check?: package debsums can compute MD5 hashes and compare against the deb package.

Is there a safe way to reinstall via the package manager: Yes, but not recommended.

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Using the dpkg program along with some Bash scripting should be able to this for you. The only caveat would be if someone replace "clean" versions of the files with "malicious" ones. For that you would need to get valid MD5 checksums from a pristine package. Anyways, here is the shell code to achieve what you want:

for i in `sudo dpkg -L ${PACKAGE_NAME}`
    if ! [ -e $i ]; then
            echo "$i is a missing file in the $PACKAGE_NAME package."

The script would only print out if a file or directory that was defined in the package was missing. Also you would need to replace the 'PACKAGE_NAME' variable with the package that you would want to inspect. Hope this helps.

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I'm not sure whether this would work in all cases. What does dpkg -L list exactly? debsums as answered elsewhere looks like a good alternative. – Jan Aug 19 '11 at 8:01

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