Unix Interview Questions and Answers for 2 years experience

1. What is the difference between hard links and symbolic links?

   • Answer: A hard link creates another directory entry that points to the same inode as the original file. Deleting one hard link doesn't affect others. Hard links cannot point to directories. Symbolic links (or soft links) are files that contain the path to another file. Deleting a symbolic link doesn't affect the target file, and symbolic links can point to directories, files, and even other symbolic links. Hard links share the same data blocks; symbolic links do not.

2. Explain the significance of the inode.

   • Answer: An inode (index node) is a data structure in a Unix-like file system that stores metadata about a file, such as its permissions, owner, size, timestamps, and pointers to the data blocks. It's crucial because it's how the file system keeps track of files, regardless of their filenames. The filename is only a pointer to the inode.

3. What are the different file permissions in Unix?

   • Answer: Unix file permissions control access to files and are expressed in three sets of three characters (e.g., `-rwxr-xr-x`). Each set represents permissions for the owner, group, and others respectively. `r` (read), `w` (write), and `x` (execute) are the individual permissions. A hyphen indicates that the permission is not granted.

4. How do you change file permissions using chmod?

   • Answer: The `chmod` command changes file permissions. It takes two main forms: symbolic (e.g., `chmod u+x file.sh` adds execute permission for the owner) and octal (e.g., `chmod 755 file.sh` sets permissions to read, write, and execute for the owner, read and execute for the group and others).

5. What is the difference between `find` and `locate` commands?

   • Answer: `find` searches for files based on specified criteria (name, type, size, modification time, etc.) within a directory hierarchy. It's slower but more thorough. `locate` searches a database of file paths that is updated periodically. It is much faster, but might not reflect recent file changes.

6. Explain the purpose of the `grep` command.

   • Answer: `grep` (global regular expression print) searches for patterns in files. It can be used to find lines containing specific strings or more complex patterns defined using regular expressions. Common options include `-i` (case-insensitive), `-n` (display line numbers), `-r` (recursive search in directories).

7. How do you redirect standard output and standard error in Unix?

   • Answer: Standard output (stdout) is redirected using `>` (e.g., `command > output.txt`). Standard error (stderr) is redirected using `2>` (e.g., `command 2> error.txt`). Both can be redirected to the same file using `&>` (e.g., `command &> output.txt`).

8. What is the purpose of the `awk` command?

   • Answer: `awk` is a pattern-scanning and text-processing language. It's powerful for manipulating text files, extracting data, and performing calculations based on patterns in the input.

9. What is the `sed` command used for?

   • Answer: `sed` (stream editor) is used for text transformations. It operates on a stream of text, allowing for substitutions, deletions, insertions, and other manipulations of lines within a file.

10. Explain the use of pipes (`|`) in Unix.

    • Answer: Pipes connect the standard output of one command to the standard input of another. This enables chaining commands together to perform complex operations sequentially (e.g., `ls -l | grep txt` lists files and then filters for files ending in ".txt").

11. What is a shell script?

    • Answer: A shell script is a file containing a series of commands for a Unix shell. These commands are executed sequentially, automating tasks and improving efficiency.

12. How do you create and execute a shell script?

    • Answer: Create a file (e.g., using `vi my_script.sh`), add commands, and make it executable using `chmod +x my_script.sh`. Execute it with `./my_script.sh`.

13. What is the shebang line (`#!`)?

    • Answer: The shebang line (`#!`) at the beginning of a shell script specifies the interpreter (e.g., `#!/bin/bash`) used to execute the script.

14. How do you handle variables in shell scripts?

    • Answer: Variables are assigned using `variable_name=value` (no spaces around the equals sign). They're referenced using `${variable_name}`.

15. What are control flow statements in shell scripting?

    • Answer: Control flow statements like `if`, `else`, `elif`, `for`, `while`, `until`, and `case` control the order of execution in a shell script based on conditions or loops.

16. How do you comment in shell scripts?

    • Answer: Use `#` to add comments. Any text following `#` on a line is ignored by the interpreter.

17. What are some common shell scripting debugging techniques?

    • Answer: Techniques include using `echo` statements to print variable values, using `set -x` to trace script execution, and carefully reviewing error messages.

18. Explain the concept of process management in Unix.

    • Answer: Process management encompasses creating, controlling, and terminating processes. Tools like `ps`, `top`, `kill`, `jobs`, and `bg/fg` are essential for monitoring and managing processes.

19. What is the purpose of the `ps` command?

    • Answer: `ps` (process status) displays information about running processes. Options such as `aux` provide detailed information.

20. How do you kill a process using the `kill` command?

    • Answer: Use `kill [signal] [PID]`. Signal 15 (SIGTERM) is a graceful termination request, and 9 (SIGKILL) forces immediate termination.

21. What is the difference between `kill` and `pkill`?

    • Answer: `kill` requires the process ID (PID), while `pkill` kills processes based on their name.

22. What is the `top` command used for?

    • Answer: `top` displays dynamic real-time information about running processes, showing CPU usage, memory usage, etc.

23. Explain the concept of background processes.

    • Answer: Background processes run independently of the terminal and are not directly associated with a specific user session. They're started using `&`.

24. How do you bring a background process to the foreground?

    • Answer: Use `fg` to bring the most recently backgrounded process to the foreground or `fg %job_number` for a specific background job.

25. What is the purpose of the `jobs` command?

    • Answer: `jobs` lists currently running background jobs.

26. What is the `bg` command used for?

    • Answer: `bg` puts a stopped job into the background.

27. What are environment variables?

    • Answer: Environment variables store system and user configuration information. They are accessible to all processes launched by a shell. Examples include `PATH`, `HOME`, `SHELL`.

28. How do you view and set environment variables?

    • Answer: View using `printenv` or `env`. Set using `export variable_name=value`.

29. What is the significance of the `PATH` environment variable?

    • Answer: `PATH` tells the shell where to look for executable files when you type a command. It's a colon-separated list of directories.

30. Explain the concept of input/output redirection.

    • Answer: Input/output redirection changes the default input or output of a command. `<` redirects input, `>` redirects output, `>>` appends to output, `2>` redirects standard error.

31. What are regular expressions?

    • Answer: Regular expressions are patterns used to match strings of text. They're powerful tools for searching and manipulating text.

32. Give some examples of regular expression metacharacters.

    • Answer: `.` (matches any character), `*` (matches zero or more occurrences), `+` (matches one or more occurrences), `?` (matches zero or one occurrence), `[]` (matches a character within the brackets), `^` (matches the beginning of a line), `$` (matches the end of a line).

33. How do you use regular expressions with `grep`?

    • Answer: Use the `-E` option for extended regular expressions (e.g., `grep -E "pattern"`).

34. What is the `tar` command used for?

    • Answer: `tar` (tape archiver) is used for creating and extracting archive files (e.g., `.tar`, `.tar.gz`, `.tgz`).

35. How do you create a compressed archive using `tar`?

    • Answer: `tar -czvf archive_name.tar.gz file1 file2` creates a compressed archive using gzip.

36. How do you extract a `tar` archive?

    • Answer: `tar -xzvf archive_name.tar.gz` extracts a gzip-compressed archive.

37. What is the `zip` command used for?

    • Answer: `zip` creates and manages zip archives.

38. How do you create and extract a zip archive?

    • Answer: `zip archive_name.zip file1 file2`; `unzip archive_name.zip`

39. What is the `gzip` command used for?

    • Answer: `gzip` compresses and decompresses files using the gzip algorithm.

40. How do you compress and decompress a file using `gzip`?

    • Answer: `gzip file.txt` (compresses); `gunzip file.txt.gz` (decompresses)

41. What is the `bzip2` command used for?

    • Answer: `bzip2` compresses and decompresses files using the bzip2 algorithm, often providing higher compression than `gzip`.

42. How do you compress and decompress a file using `bzip2`?

    • Answer: `bzip2 file.txt` (compresses); `bunzip2 file.txt.bz2` (decompresses)

43. What is the `wc` command used for?

    • Answer: `wc` (word count) counts lines, words, and characters in a file.

44. How do you use `wc` to count lines in a file?

    • Answer: `wc -l file.txt`

45. What is the `head` command used for?

    • Answer: `head` displays the first few lines of a file (default is 10).

46. How do you use `head` to display the first 5 lines of a file?

    • Answer: `head -n 5 file.txt`

47. What is the `tail` command used for?

    • Answer: `tail` displays the last few lines of a file (default is 10).

48. How do you use `tail` to display the last 20 lines of a file?

    • Answer: `tail -n 20 file.txt`

49. What is the `cut` command used for?

    • Answer: `cut` extracts sections from each line of files.

50. How do you use `cut` to extract the first 5 characters of each line?

    • Answer: `cut -c -5 file.txt`

51. What is the `sort` command used for?

    • Answer: `sort` sorts lines of text files.

52. How do you sort a file numerically?

    • Answer: `sort -n file.txt`

53. What is the `uniq` command used for?

    • Answer: `uniq` reports or omits repeated lines.

54. How do you remove duplicate lines from a sorted file?

    • Answer: `sort file.txt | uniq`

55. What is the `diff` command used for?

    • Answer: `diff` compares files line by line, showing the differences.

56. What is the `patch` command used for?

    • Answer: `patch` applies changes (diffs) to a file.

57. What is the `cp` command used for?

    • Answer: `cp` copies files and directories.

58. What is the `mv` command used for?

    • Answer: `mv` moves or renames files and directories.

59. What is the `rm` command used for?

    • Answer: `rm` removes files and directories.

60. What is the `mkdir` command used for?

    • Answer: `mkdir` creates directories.

61. What is the `rmdir` command used for?

    • Answer: `rmdir` removes empty directories.

62. What is the `ln` command used for?

    • Answer: `ln` creates hard and symbolic links.

63. What is the `chown` command used for?

    • Answer: `chown` changes file ownership.

64. What is the `chgrp` command used for?

    • Answer: `chgrp` changes file group ownership.

65. What is the `df` command used for?

    • Answer: `df` (disk free) reports file system disk space usage.

66. What is the `du` command used for?

    • Answer: `du` (disk usage) estimates file space usage.

67. What is a shell?

    • Answer: A shell is a command-line interpreter that provides an interface for interacting with the operating system.

68. Name some common Unix shells.

    • Answer: Bash, Zsh, Csh, Ksh.

69. What is a process?

    • Answer: A process is an instance of a program in execution.

70. What is a signal in Unix?

    • Answer: A signal is an asynchronous notification sent to a process. It can be used to terminate, pause, or otherwise affect the process's behavior.

71. Explain the concept of pipes and filters.

    • Answer: Pipes connect the standard output of one command to the standard input of another, creating a chain of commands (filters) that process data sequentially.

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