Police officers recovered a laptop from a home that they believed was stolen. One roommate said the device had arrived in the home a few days earlier, but did not know how it got there. The remaining members of the household claimed to know nothing about the computer at all.

I booted the device with a Linux boot disc designed for forensic examination. The disc allows storage devices to be examined without making changes. I was lucky enough to find a user account that had been established a few years earlier, and files in that account that allowed me to identify and contact the computer’s owner. The owner reported the device had been stolen from him two weeks earlier. The owner had password protected his account, but there was a guest account available for use.

I could have stopped there, but the job would have been only half-done. I knew who owned the computer, but I didn’t know who’d stolen it. Fingerprints were not an option, so I decided to look for data in the computer that might identify who had used the computer since it had been stolen. A quick look in the guest account showed me I was not going to be as lucky identifying the suspect as I had the victim: there were no user created documents.

What I need to do was to find the files modified by the suspect and inspect those files for identifying information. The suspect may not have purposely created files, but browsing the Internet, etc, creates cache and history files that point out a person as surely than a witness in a suspect lineup (that is to say, not with 100 percent certainty, but often reliable none-the-less).

File systems are very helpful in examinations of this nature: they keep dates and times that files are created, accessed and modified, just to name a few date attributes. Modern operating systems are very helpful, too, because they usually auto-sync the computer’s clock with NTP (Network Time Protocol) servers. Simply stated, modern operating systems keep accurate time automatically.

With this knowledge in mind, I was looking for guest account files (and, ultimately, all files) that were modified in in the past two weeks. Files modified outside that range were changed by the owner and of no interest. Fortunately, the find command provides a solution:

The -mtime n option takes integer argument n. This is where a little explanation is in order. Had I passed the integer "14", I would have only returned files modified 14 days ago. Passing "-14" returns all files modified less than 14 days ago. Passing "+14" would cause find to return all files modified more that 14 days ago. It is possible to pass two -mtime options to create a narrow range, such as:

The command in the first example resulted in just over 1600 file names being returned. I saw that most of these were Google Chrome browser application data files. Both the "History" and "Login Data" SQLite databases contained data leading to the identity of the computer user since the date the laptop was stolen (a roommate) and the dates of the activity suggested the computer had been in that person’s possession since shortly after the theft.

The date command can really be your friend in figuring out dates and date ranges. It is easier to demonstrate than explain:

Note: The info date command will show you many, many more useful invocations of the date command.

You may recall that the find command takes an integer for its date range options, but none of the date commands I illustrated above yielded and integer show the number of days elapsed or until that date. If there is an option for date to yield such information, I have not discovered it. However, a simple shell script can be created to allow us to use the "plain language" of the date command to help us determine the integers required by find.

Now, we can figure out the number of days elapsed using the same plain language conventions accepted by the date command. Be sure to enclose each date in parenthesis if the date string is more than one word.

You get the idea. And I hope I’ve given you some ideas on how to use the find and date commands to your advantage in a preview or other forensic examination.