UNIX and SQL Questions with Answers

UNIX Questions with Answers

1. What are the key features of the UNIX operating system?

  • Multi-user, multitasking, portability, hierarchical file system, built-in networking, and robust security.

2. Explain the file structure in UNIX. How is it organized?

  • It is a hierarchical tree structure starting with the root (/). Directories branch off to organize files logically.

3. What is the difference between an absolute path and a relative path?

  • Absolute path: Specifies the complete path from the root (e.g., /home/user/file).
  • Relative path: Specifies the path from the current directory (e.g., ../file).

4. How do you change file permissions using chmod?

  • Command: chmod 755 file
  • Gives the owner read, write, execute permissions, and others read and execute permissions.

5. What is the difference between chown and chmod?

  • chown: Changes the owner of a file.
  • chmod: Changes the file’s permissions.

6. How do you view hidden files in UNIX?

  • Command: ls -a

7. What is the purpose of the grep command? Provide an example.

  • Searches for patterns in a file.
  • Example: grep "error" logfile.txt

8. How do you check disk usage in UNIX?

  • Command: df -h

9. What is the difference between > and >> in file redirection?

  • >: Overwrites the file.
  • >>: Appends to the file.

10. How do you display the first 10 lines of a file?

  • Command: head filename

11. What is the purpose of the tail command?

  • Displays the last 10 lines of a file.
  • Example: tail logfile.txt

12. How do you copy a file in UNIX?

  • Command: cp sourcefile destinationfile

13. What is the purpose of the find command?

  • Finds files and directories.
  • Example: find /path-name -name "filename"

14. How do you kill a process in UNIX?

  • Command: kill -9 PID

15. What is the difference between a soft link and a hard link?

  • Soft Link: Points to a file path; breaks if the file is deleted.
  • Hard Link: Points to the file’s inode; doesn’t break if the file is deleted.

16. How can you check currently running processes?

  • Commands: ps or top
  • Example: ps -ef

17. What does the awk command do?

  • Used for pattern scanning and text processing.
  • Example: awk '{print $1}' filename

18. How do you schedule tasks in UNIX?

  • Command: crontab -e

19. What is the difference between df and du?

  • df: Shows disk space usage of filesystems.
  • du: Shows disk usage of directories/files.

20. How do you compress files in UNIX?

  • Command: gzip filename

21. What is the difference between vi and nano editors?

  • vi: A powerful, mode-based editor.
  • nano: Simpler and user-friendly.

22. How do you list only directories in UNIX?

  • Command: ls -d */

23. How do you display the current working directory?

  • Command: pwd

24. What is the purpose of the cut command?

  • Extracts specific columns or fields from a file.
  • Example: cut -d',' -f1 filename

25. How do you display a file’s contents page by page?

  • Commands: more or less
  • Example: more filename

26. How do you find out the shell you’re using?

  • Command: echo $SHELL

27. How can you view a file’s permissions?

  • Command: ls -l

28. What is the purpose of the env command?

  • Displays environment variables.

29. How do you terminate all processes owned by a specific user?

  • Command: pkill -u username

30. What does the nohup command do?

  • Runs a command immune to hang-ups.
  • Example: nohup ./script.sh &

SQL Questions with Answers

1. What is a primary key?

  • A unique identifier for each row in a table, ensuring no duplicate values.

2. What is a foreign key?

  • A column that references the primary key of another table to establish a relationship.

3. What is the difference between WHERE and HAVING?

  • WHERE: Filters rows before grouping.
  • HAVING: Filters groups after grouping.

4. Explain normalization and its benefits.

  • Organizes data to reduce redundancy and improve consistency.
  • Examples: 1NF, 2NF, 3NF.

5. What is the purpose of JOIN in SQL?

  • Combines rows from two or more tables.
  • Types: INNER, LEFT, RIGHT, FULL.

6. What is the difference between DELETE and TRUNCATE?

  • DELETE: Removes specific rows; can have a WHERE clause.
  • TRUNCATE: Removes all rows; faster and cannot be rolled back.

7. What are aggregate functions?

  • Functions like SUM, AVG, MAX, MIN, and COUNT perform calculations on a set of rows.

8. What is the difference between CHAR and VARCHAR?

  • CHAR: Fixed-length storage.
  • VARCHAR: Variable-length storage.

9. How do you retrieve unique values from a column?

  • Command: SELECT DISTINCT column_name FROM table_name;

10. What is a subquery?

  • A query inside another query.
  • Example: SELECT * FROM employees WHERE salary > (SELECT AVG(salary) FROM employees);

11. What is the difference between UNION and UNION ALL?

  • UNION: Removes duplicates.
  • UNION ALL: Keeps duplicates.

12. What is indexing in SQL?

  • Improves query performance by allowing faster data retrieval.

13. How do you create a view in SQL?

  • Command: CREATE VIEW view_name AS SELECT column1, column2 FROM table_name;

14. What is a stored procedure?

  • A reusable block of SQL code stored in the database.

15. What is the difference between GROUP BY and ORDER BY?

  • GROUP BY: Groups rows based on a column.
  • ORDER BY: Sorts rows in ascending or descending order.

16. What is a transaction in SQL?

  • A sequence of operations performed as a single unit, ensuring ACID properties.

17. How do you handle NULL values in SQL?

  • Use functions like IS NULL, IS NOT NULL, or COALESCE.

18. What is the use of CASE in SQL?

  • Implements conditional logic in a query.
  • Example: SELECT column, CASE WHEN condition THEN result END FROM table;

19. What is a self-join?

  • A table joined with itself.
  • Example: SELECT A.column, B.column FROM table A, table B WHERE A.id = B.parent_id;