Unit 3: Searching and pipes · Lesson 3 of 6 · 16 min
Redirection and find
Save output to files, silence errors, read exit codes and search the disk by name and size.
mission
- You are
- You are hunting for data that should not be public.
- Situation
- The scanner asked for backups. The web root is large, and you want a saved list of anything suspicious.
- Goal
- Search the web root by size and name and save the results.
- Hand in
- The path of any oversized file in the web root.
Where output goes
- 0 standard inputkeyboard, or < file
- 1 standard outputscreen, > file, >> file, | next
- 2 standard errorscreen, or 2>/dev/null
Write and append
echo hello > t.txt echo again >> t.txt cat t.txt hello again
you should see > replaces the file, >> adds to the end.
echo- Prints its arguments. Built into Linux and macOS.
cat- Prints the contents of one or more files. Built into Linux and macOS.
Silence errors and read the exit code
ls /nonexistent ls: cannot access '/nonexistent': No such file or directory ls /nonexistent 2>/dev/null echo $? 2
you should see No message the second time, but the exit code still says it failed. 0 means success.
defender tip Monitoring checks and scripts trust exit codes, not messages.
ls- Lists the files in a folder. Built into Linux and macOS.
Find big and named files
find /var/www -size +5M /var/www/html/assets/site-backup.tar.gz find /var/www -name '*.css' /var/www/html/assets/css/site.css find /var/www -size +5M > ~/big-files.txt
you should see A 6 MB backup inside the public web root. Anyone who guesses the name can download it.
find- Searches folders for files by name, type, owner, size or age. Built into Linux and macOS.
| Command | What it does |
|---|---|
cmd > file | Save output, replacing the file |
cmd >> file | Append output |
cmd 2>/dev/null | Discard errors |
echo $? | Exit code of the last command |
find dir -name 'x*' | Find by name |
find dir -size +5M | Find by size |
What a defender takes away
- 1>, >> and 2> decide where output goes.
- 2Exit codes tell scripts what happened.
- 3find searches by name, size, owner and permission.
Questions
0 of 2 solved
Answer each one from what you find on your lab machine.