level-9.md

Level 8 🡒 9: Finding the One Line That Doesn't Repeat

🎯 Level Goal / Objective

The password for the next level is stored in the file data.txt, and it is the only line in that file that occurs exactly once. Every other line appears more than once.

🛠️ Commands You May Need

• sort - arranges the lines of a file so that identical lines end up next to each other
• uniq - collapses or filters adjacent duplicate lines (only works correctly if the input is already sorted)
• uniq -u - flag that prints only the lines with no duplicates, instead of collapsing repeats

📝 Steps Taken

1. Initial exploration:

   • Looked at the file with cat data.txt
   • Too many lines to spot the odd one out by eye, so scrolling through manually wasn't practical

2. Sorting the data:

   • sort on data.txt groups every repeated phrase together, so identical lines sit next to each other instead of being scattered

3. Filtering out the duplicates:

   • The sorted output needs to be piped (|) into uniq
   • uniq only detects duplicates that are directly adjacent, which is why the sort step has to come first
   • Adding the -u flag tells uniq to print only the lines that have no duplicate at all, instead of just collapsing repeats down to one copy each

4. Reading the result:

   • The single line left over after this is the passphrase for bandit9

💡 Explanation

Key Concepts Learned:

• Piping: sending the output of one command straight into another with |, so the two work together as a pipeline
• uniq needs sorted input: it doesn't scan the whole file for duplicates, it only compares each line to the one before it
• Filtering vs collapsing: plain uniq reduces every group of duplicates down to a single line, while uniq -u throws those groups away entirely and keeps only lines that never repeated

Why This Solution Works:

Since the passphrase is defined as the only line that occurs once, sorting the file first guarantees every duplicate phrase lands next to its copies. Once that's true, uniq -u can do exactly what it's built for: drop every line that has a neighbor matching it, leaving just the one line with no match.

Command / Flag Reference:

Command / Flag	Purpose
sort	Reorders lines alphabetically so duplicates become adjacent
uniq	Compares adjacent lines, collapsing consecutive duplicates
uniq -u	Prints only lines that have no adjacent duplicate
|	Pipes the output of one command into the next

Alternative Approaches:

• sort also has a -u flag, but that suppresses duplicates entirely rather than isolating the one line that never repeated, so it doesn't answer the same question on its own
• grep combined with -c (count) could work, but sorting the file for adjacent duplicates is the more direct route the level hints at

Pro Tip:

Whenever a task says "find the line that doesn't repeat" (or the one that does), think sort first. Almost every duplicate-detection tool, uniq included, only compares neighboring lines rather than scanning the whole file, so getting matching lines next to each other is usually the real first step.

---

Next: Level 9 🡒 10