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Without -d
dupemap will take action when it sees the same checksum repeated more than once, i.e. it simply finds duplicates recursively. The action depends on operation:
WARNING: use the report operation first to see what will be deleted.
WARNING: You are advised to make a backup of the target first, e.g. with
"cp -al" (for GNU cp) to create hard links recursively.
With -d
The database argument to -d will denote a database file (see the ``DATABASE'' section in this manual for details) to read from or write to. In this mode, the scan operation should be run on one path, followed by the report or delete operation on another (not the same!) path.
WARNING: if you run dupemap delete on the same path you just ran dupemap scan on, it will delete every file! The idea of these options is to scan one path and delete files in a second path.
WARNING: use the report operation first to see what will be deleted.
WARNING: You are advised to make a backup of the target first, e.g. with "cp -al" (for GNU cp) to create hard links recursively.
The paths given here will NOT be scanned recursively. Directories will be ignored and symlinks will be followed.
General usage
The easiest operations to understand is when the -d option is not given. To delete all duplicate files in /tmp/recovered-files, do:
$ dupemap delete /tmp/recovered-files
Often, dupemap scan is run to produce a checksum database of all files in a directory tree. Then dupemap delete is run on another directory, possibly following dupemap report. For example, to delete all files in /tmp/recovered-files that already exist in $HOME, do this:
$ dupemap -d homedir.map scan $HOME
$ dupemap -d homedir.map delete,report /tmp/recovered-files
Usage with magicrescue
The main application for dupemap is to take some pain out of performing undelete operations with magicrescue(1). The reason is that magicrescue will extract every single file of the specified type on the block device, so undeleting files requires you to find a few files out of hundreds, which can take a long time if done manually. What we want to do is to only extract the documents that don't exist on the file system already.
In the following scenario, you have accidentally deleted some important Word documents in Windows. If this were a real-world scenario, then by all means use The Sleuth Kit. However, magicrescue will work even when the directory entries were overwritten, i.e. more files were stored in the same folder later.
You boot into Linux and change to a directory with lots of space. Mount the Windows partition, preferably read-only (especially with NTFS), and create the directories we will use.
$ mount -o ro /dev/hda1 /mnt/windows
$ mkdir healthy_docs rescued_docs
Extract all the healthy Word documents with magicrescue and build a database of their checksums. It may seem a little redundant to send all the documents through magicrescue first, but the reason is that this process may modify them (e.g. stripping trailing garbage), and therefore their checksum will not be the same as the original documents. Also, it will find documents embedded inside other files, such as uncompressed zip archives or files with the wrong extension.
$ find /mnt/windows -type f \
|magicrescue -I- -r msoffice -d healthy_docs
$ dupemap -d healthy_docs.map scan healthy_docs
$ rm -rf healthy_docs
Now rescue all "msoffice" documents from the block device and get rid of everything that's not a *.doc.
$ magicrescue -Mo -r msoffice -d rescued_docs /dev/hda1 \
|grep -v '\.doc$'|xargs rm -f
Remove all the rescued documents that also appear on the file system, and remove duplicates.
$ dupemap -d healthy_docs.map delete,report rescued_docs
$ dupemap delete,report rescued_docs
The rescued_docs folder should now contain only a few files. This will be
the undeleted files and some documents that were not stored in contiguous
blocks (use that defragger ;-)).
Usage with fsck
In this scenario (based on a true story), you have a hard disk that's gone bad. You have managed to dd about 80% of the contents into the file diskimage, and you have an old backup from a few months ago. The disk is using reiserfs on Linux.
First, use fsck to make the file system usable again. It will find many nameless files and put them in lost+found. You need to make sure there is some free space on the disk image, so fsck has something to work with.
$ cp diskimage diskimage.bak
$ dd if=/dev/zero bs=1M count=2048 >> diskimage
$ reiserfsck --rebuild-tree diskimage
$ mount -o loop diskimage /mnt
$ ls /mnt/lost+found
(tons of files)
Our strategy will be to restore the system with the old backup as a base and merge the two other sets of files (/mnt/lost+found and /mnt) into the backup after eliminating duplicates. Therefore we create a checksum database of the directory we have unpacked the backup in.
$ dupemap -d backup.map scan ~/backup
Next, we eliminate all the files from the rescued image that are also present in the backup.
$ dupemap -d backup.map delete,report /mnt
We also want to remove duplicates from lost+found, and we want to get rid of any files that are also present in the other directories in /mnt.
$ dupemap delete,report /mnt/lost+found
$ ls /mnt|grep -v lost+found|xargs dupemap -d mnt.map scan
$ dupemap -d mnt.map delete,report /mnt/lost+found
This should leave only the files in /mnt that have changed since the last
backup or got corrupted. Particularly, the contents of /mnt/lost+found
should now be reduced enough to manually sort through them (or perhaps use
magicsort(1)).
Primitive intrusion detection
You can use dupemap to see what files change on your system. This is one of the more exotic uses, and it's only included for inspiration.
First, you map the whole file system.
$ dupemap -d old.map scan /
Then you come back a few days/weeks later and run dupemap report. This will give you a view of what has not changed. To see what has changed, you need a list of the whole file system. You can get this list along with preparing a new map easily. Both lists need to be sorted to be compared.
$ dupemap -d old.map report /|sort > unchanged_files
$ dupemap -d current.map scan /|sort > current_files
All that's left to do is comparing these files and preparing for next week. This assumes that the dbm appends the ".db" extension to database files.
$ diff unchanged_files current_files > changed_files
$ mv current.map.db old.map.db
dupecheck depends on a database library for storing the checksums. It currently requires the POSIX-standardized ndbm library, which must be present on XSI-compliant UNIXes. Implementations are not required to handle hash key collisions, and a faliure to do that could make dupecheck delete too many files. I haven't heard of such an implementation, though.
The current checksum algorithm is the file's CRC32 combined with its size. Both values are stored in native byte order, and because of varying type sizes the database is not portable across architectures, compilers and operating systems.
This tool does the same thing weeder does, except that weeder cannot seem to handle many files without crashing, and it has no largefile support.
It is possible to craft a file to have a known CRC32. You need to keep this in mind if you use dupemap on untrusted data. A solution to this could be to implement an option for using MD5 checksums instead.
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