For this assignment, you will use several tools you’ve learned throughout this lesson to find a file and filter it to see the message it contains.

Prepare

On a Linux machine (preferably the supercomputer), download treasure_hunt.tar.gz to a location where you have write permissions such as your home directory; you can copy the link and use wget <link> to download directly on the supercomputer. Extract the tarball using tar xf treasure_hunt.tar.gz.

Once you have done so, navigate into treasure_hunt and modify the permissions of which-file to allow you to execute it.

You will need to load a Julia environment module to successfully run which-file. This can be done by running module load julia.

Find the Hash Seed

which-file will fail if the environment variable TH_SALT isn’t set, or yield an incorrect result if it isn’t set correctly. The correct salt is determined by totaling the byte counts of all files in the directory noisy_files ending with “.gz”

You can find this total using wc (word count), giving the glob representing all “.gz” files in noisy_files as the last argument; search wc’s man page for “bytes” to see which flag to use to count bytes. The output of wc will end with something like “1234 total“–this is the total byte count. As a quick check, the last 4 digits of the total form a palindrome.

Set TH_SALT to this total, making sure it is available to child processes.

Get the File Name

With TH_SALT set, which-file will print the correct filename to stdout, but the result will be muddied by random characters printed to stderr interspersed with the correct filename. To see the unsullied filename, run which-file, sending stderr to /dev/null.

Make sure not to modify which-file-–it hashes itself to get the answer, so changing even a character will yield the wrong filename.

Check Your Work

To ensure that you found the correct file, run zcat on the file (located in noisy_files) and filter out all lines containing the word “noise”. This can be done by piping the output of zcat to grep and selecting lines not matching “noise;” search the man page on grep for ‘invert” to see which flag to use. This will result in a message affirming that you have selected the right file. If it doesn’t, you probably have the wrong hash seed–it may be worth re-downloading which-file, since even an extra space will invalidate its result.

Grading

Submit the name of the file and the message affirming its correctness. You’ll get 10 points for a correct answer, 5 points for the correct file but an incorrect message, and 0 points otherwise.