OverTheWire Bandit: Level 8 to Level 9

The Goal

Find the one line in data.txt that appears only once. Every other line is a duplicate.

What I Did

Listed the file and opened it with cat. Hundreds of lines of what looked like random passwords, most of them repeated multiple times. Reading through it manually wasn’t an option.

The level page pointed to a piping and redirection tutorial. I read through it, understood how pipes work, then figured out the solution from there.

The key insight: I needed two commands chained together.

bandit8@bandit:~$ sort data.txt | uniq -u
EjmOSvuAu7sGAHqHVcBDPirRe9T03kxl

One line. That’s the password.

What Was Actually Happening

uniq only removes consecutive duplicate lines — it checks each line against the one directly above it. If duplicates are scattered throughout the file in different positions, uniq won’t catch them.

sort fixes this by rearranging all lines alphabetically first. After sorting, every duplicate ends up next to its copies. Now uniq can see them.

uniq -u prints only the lines that are unique — lines that appear exactly once. Without -u, uniq collapses duplicates into one but still prints them. With -u it filters them out entirely, leaving only what appeared once in the whole file.

The pipe | connects the two — it takes the sorted output from sort and feeds it directly into uniq -u without writing anything to disk.

Why sort + uniq is a Classic Combination

These two commands almost always appear together. sort alone just reorders lines. uniq alone only works on adjacent lines. Together they solve a whole class of problems:

sort file.txt | uniq          # remove all duplicates
sort file.txt | uniq -u       # show only unique lines
sort file.txt | uniq -d       # show only duplicate lines
sort file.txt | uniq -c       # count how many times each line appears

The -c flag is particularly useful — it shows you exactly how many times each line appears, sorted output looking like:

   3 duplicated_line
   1 unique_line
   5 another_duplicate

What I Learned

uniq only compares adjacent lines — always sort first unless the file is already sorted.

uniq -u filters for unique lines — the opposite of -d which shows only duplicates.

Pipes connect commands without temporary files — sort data.txt | uniq -u processes everything in memory. No intermediate file needed.

Reading the manual before solving pays off — understanding piping from the tutorial made it obvious which tools to combine and why.

Commands Used

Command	What it did
cat data.txt	Showed the file — hundreds of repeated lines
sort data.txt	Sorted all lines alphabetically, grouping duplicates together
sort data.txt \| uniq	Removed consecutive duplicates after sorting
sort data.txt \| uniq -u	Printed only the line that appeared exactly once