drills

1. Append 'done' to log.txt without overwriting

2. Count lines in /etc/passwd

3. Follow the tail of /var/log/syslog live

4. Replace all 'foo' with 'bar' in f.txt in place

5. Find all SUID files system-wide

6. Symlink 'cur' → /opt/app/v2

7. Recursively grep 'ERROR' under /var/log

8. Print 1st & 3rd colon-fields of /etc/passwd

9. Show 5 largest dirs under /var

10. Extract archive.tar.gz into /opt