From users.csv, write the unique city names (no header) into /tmp/cities.txt.
Cut the 3rd CSV column from `/home/user/data/users.csv`, drop the header line, sort uniquely, and save to `/tmp/cities.txt`.
Hint: pipe through `grep -v "^city$"` to remove the header row.
Expected content:
```
berlin
paris
vienna
```
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