Unix Interview Questions and Answers for 5 years experience

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

   • Answer: A hard link is a direct pointer to an inode, while a symbolic link (symlink) is a file that contains the path to another file. Hard links cannot point to directories, while symlinks can. Deleting a file with hard links doesn't delete the data until all hard links are removed. Deleting a symlink only removes the symlink, not the target file.

2. Explain the concept of an 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 size, permissions, ownership, and location of the data blocks on the disk. It doesn't store the file's name or content.

3. How do you find the largest file in a directory?

   • Answer: `find . -type f -printf '%s %p\n' | sort -nr | head -n 1` This command finds all files (-type f), prints their size and path, sorts them numerically in reverse order (-nr), and displays the top one (head -n 1).

4. How would you find all files modified in the last 24 hours?

   • Answer: `find . -type f -mtime -1` This uses the `-mtime` option with `-1` to find files modified within the last 24 hours. `-mtime +1` would find files modified more than 24 hours ago.

5. What is the purpose of the `cron` daemon?

   • Answer: The `cron` daemon is a time-based job scheduler in Unix-like operating systems. It allows you to schedule commands or scripts to run automatically at specified times or intervals.

6. Explain the difference between `grep`, `egrep`, and `fgrep`.

   • Answer: `grep` uses basic regular expressions. `egrep` (or `grep -E`) uses extended regular expressions, offering more features and a more concise syntax. `fgrep` (or `grep -F`) performs fast fixed-string searches, without regular expression interpretation, which is faster for simple searches.

7. How do you redirect both standard output and standard error to a file?

   • Answer: `command > output.txt 2>&1` This redirects standard output (>) to `output.txt` and then redirects standard error (2) to the same file as standard output (&1).

8. What are file permissions in Unix and how are they represented?

   • Answer: File permissions control access to files. They are represented as a nine-character string, with three sets of three characters each representing permissions for owner, group, and others (read, write, execute). `rwxr-xr-x` for example, means read, write, and execute for the owner, read and execute for the group, and read and execute for others.

9. Explain the use of `awk`.

   • Answer: `awk` is a powerful text processing tool that's useful for pattern scanning and text manipulation. It's particularly good at processing structured data, such as CSV files, by parsing lines and applying actions based on patterns.

10. What is the purpose of the `sed` command?

    • Answer: `sed` (stream editor) is used for text transformations, such as substitutions, deletions, and insertions on a line-by-line basis.

11. How do you kill a process by its PID?

    • Answer: `kill ` or `kill -9 ` (the -9 signal forces termination).

12. What is the difference between `ps` and `top` commands?

    • Answer: `ps` shows a snapshot of currently running processes, while `top` displays a dynamic, real-time view of system processes, updating continuously.

13. Explain the significance of the shell environment variables.

    • Answer: Shell environment variables store settings and information that affect how the shell and programs behave. Examples include `PATH` (for executable search paths), `HOME` (for the user's home directory), and `USER` (for the current user's name).

14. How do you find the number of lines in a file?

    • Answer: `wc -l `

15. What is the purpose of the `chmod` command?

    • Answer: `chmod` is used to change the permissions of a file or directory.

16. What is the difference between absolute and relative paths?

    • Answer: Absolute paths begin with the root directory ("/") and specify the complete path from the root to the file or directory. Relative paths are relative to the current working directory.

17. How do you search for a string within a file?

    • Answer: `grep "string" `

18. What is the `umask` command and how is it used?

    • Answer: `umask` sets the default file creation permissions mask. It specifies which permissions are *removed* from the default permissions when a new file or directory is created.

19. Explain the use of pipes in Unix.

    • Answer: Pipes (`|`) connect the standard output of one command to the standard input of another, allowing you to chain commands together to process data sequentially.

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

    • Answer: `tar` (tape archiver) is used for archiving and compressing files.

21. How do you list all files and directories in a directory, including hidden ones?

    • Answer: `ls -a`

22. What is the purpose of the `find` command?

    • Answer: `find` searches for files and directories based on various criteria, such as name, size, modification time, permissions, etc.

23. Explain the use of wildcards in Unix.

    • Answer: Wildcards, such as `*` (matches any number of characters) and `?` (matches any single character), are used in file names to match multiple files at once.

24. How do you copy files in Unix?

    • Answer: `cp `

25. How do you move or rename files in Unix?

    • Answer: `mv `

26. How do you create a directory in Unix?

    • Answer: `mkdir `

27. How do you remove a directory in Unix?

    • Answer: `rmdir ` (for empty directories) or `rm -r ` (for non-empty directories).

28. How do you remove files in Unix?

    • Answer: `rm `

29. What is the significance of the `.` and `..` directories?

    • Answer: `.` represents the current directory, and `..` represents the parent directory.

30. Explain the concept of permissions inheritance in Unix.

    • Answer: When a new file or directory is created, it typically inherits permissions from its parent directory, unless modified using `umask` or `chmod`.

31. What are shell scripts and how are they useful?

    • Answer: Shell scripts are programs written in a shell scripting language (like Bash, Zsh) that automate tasks by executing a series of commands. They are useful for automating repetitive tasks, streamlining workflows, and improving efficiency.

32. How do you create a symbolic link in Unix?

    • Answer: `ln -s `

33. How do you create a hard link in Unix?

    • Answer: `ln `

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

    • Answer: `chown` changes the owner and/or group of a file or directory.

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

    • Answer: `chgrp` changes the group ownership of a file or directory.

36. How do you display the contents of a file?

    • Answer: `cat ` or `less ` (for large files).

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

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

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

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

39. How do you redirect the output of a command to a file?

    • Answer: `command > filename`

40. How do you append the output of a command to a file?

    • Answer: `command >> filename`

41. What are process signals in Unix?

    • Answer: Process signals are software interrupts sent to processes to control their behavior (e.g., termination, pausing).

42. Explain the concept of background processes.

    • Answer: Background processes run independently of the shell, allowing you to continue working while they execute (using `&`).

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

    • Answer: `fg` (or `fg %job_number`)

44. How do you list all running background processes?

    • Answer: `jobs`

45. What is the shell's `PATH` variable?

    • Answer: The `PATH` variable tells the shell where to look for executable files when you type a command.

46. What is the `export` command used for?

    • Answer: `export` makes a shell variable available to child processes.

47. What is the difference between `=` and `==` in shell scripting?

    • Answer: `=` is used for variable assignment. `==` is used for string comparison (in some shells like Bash).

48. How do you comment in a shell script?

    • Answer: Using `#` at the beginning of a line.

49. What are regular expressions and their use in Unix?

    • Answer: Regular expressions are patterns used to match text strings. They're used extensively in Unix tools like `grep`, `sed`, and `awk` for powerful text searching and manipulation.

50. Explain the concept of input/output redirection in Unix.

    • Answer: Input/output redirection changes where a command gets its input from and where it sends its output to (using `>` , `<`, `>>`, `2>`, `2>&1`, etc.).

51. How do you check disk space usage?

    • Answer: `df -h` (for human-readable output) or `du -sh ` (for disk usage of a specific directory).

52. What is a process ID (PID)?

    • Answer: A unique identifier assigned to each running process by the operating system.

53. How do you determine the PID of a running process?

    • Answer: `ps aux | grep ` (or `pgrep `)

54. What are system calls in Unix?

    • Answer: System calls are functions that allow programs to interact with the operating system kernel.

55. Explain the significance of the `/etc/passwd` and `/etc/shadow` files.

    • Answer: `/etc/passwd` stores user account information (username, UID, GID, home directory, etc.), while `/etc/shadow` stores user password information (for security reasons, it's not directly readable by users).

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

    • Answer: `sudo` allows a user to execute a command with the privileges of another user, typically the root user.

57. What is a shell?

    • Answer: A shell is a command-line interpreter that allows users to interact with the operating system.

58. Name some common Unix shells.

    • Answer: Bash, Zsh, Ksh, Csh, Tcsh

59. How do you customize your shell environment?

    • Answer: By editing shell configuration files like `.bashrc`, `.bash_profile`, `.zshrc`, etc.

60. What is the difference between a process and a thread?

    • Answer: A process is an independent execution environment with its own memory space, while a thread is a lightweight unit of execution within a process, sharing the same memory space.

61. What is the role of the init process?

    • Answer: The init process (PID 1) is the first process started during system boot. It's responsible for starting other system processes and managing the system's runlevels.

62. Explain the concept of daemons in Unix.

    • Answer: Daemons are background processes that run continuously, providing services to the system (e.g., `cron`, `httpd`, `sshd`).

63. What is a socket?

    • Answer: A socket is an endpoint for communication between processes, typically used for network programming.

64. Explain the concept of inter-process communication (IPC).

    • Answer: IPC is a mechanism for processes to communicate and exchange data with each other.

65. How do you monitor system performance in Unix?

    • Answer: Using tools like `top`, `htop`, `vmstat`, `iostat`, `iotop`, and system monitoring utilities.

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

    • Answer: `uname` displays system information, such as operating system name, kernel version, etc.

67. Explain the use of the `ssh` command.

    • Answer: `ssh` (secure shell) is used to securely connect to remote machines.

68. What is the `scp` command used for?

    • Answer: `scp` (secure copy) is used to securely copy files between remote machines.

69. How do you manage users and groups in Unix?

    • Answer: Using commands like `useradd`, `userdel`, `groupadd`, `groupdel`, `usermod`, `groupmod`.

70. What is the `passwd` command used for?

    • Answer: `passwd` is used to change a user's password.

71. What is the difference between a regular file and a directory?

    • Answer: A regular file contains data, while a directory contains information about files and other directories.

72. What is a file system?

    • Answer: A file system is a method of organizing and storing files and directories on a storage device.

73. Name some common Unix file systems.

    • Answer: ext2, ext3, ext4, XFS, Btrfs, NTFS, FAT32

74. What is the `quota` command used for?

    • Answer: `quota` manages disk space usage limits for users.

75. Explain the concept of symbolic links.

    • Answer: Symbolic links (symlinks) are like shortcuts, pointing to other files or directories.

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