Essential Linux Command Cheat Sheet: Master Files, Processes, and Shell Basics

Question 1: How are absolute paths, current directory, parent directory, and home directory represented, and what command changes directories?

Answer: Absolute path example: /etc/init.d. Current and parent directories: ./ and ../. Home directory: ~/. Change directory command: cd.

Question 2: How to view current processes, exit the shell, and display the current path?

Answer: View processes: ps. Exit: exit. Show current path: pwd.

Question 3: How to clear the screen, abort a command, suspend a process, view user ID, and get help for a command?

Answer: Clear screen: clear. Abort command: Ctrl+C. Suspend process: Ctrl+Z (resume with fg). Show user ID: id. Get command help: man adduser, adduser --help, or info adduser.

Question 4: What does the ls command do and which options are common?

Answer: ls lists directory contents. Common options: -a (show all files) and -l (detailed list).

Question 5: How to create symbolic and hard links?

Answer: Symbolic link: ln -s slink source. Hard link: ln link source.

Question 6: Commands for creating directories, creating files, copying files, and changing file permissions?

Answer: Create directory: mkdir. Create file: touch (or vi). Copy file: cp. Change permissions: chmod (e.g., chmod u+x file, chmod 751 file, chmod -R u+r directory).

Question 8: Commands to view file contents?

Answer: vi filename, cat filename, more filename, less filename, tail filename, head filename.

Question 9: How to write to a file and echo a string with spaces?

Answer: Write file: vi. Echo with spaces: echo hello world.

Question 10: Which files represent the terminal and the null device?

Answer: Terminal: /dev/tty. Null device: /dev/null.

Question 11: Commands to move and rename files?

Answer: Move/rename: mv.

Question 12: Command to copy files and directories, with prompts?

Answer: Copy files: cp. Copy directories recursively: cp -r. (Prompt option not specified.)

Question 13: Commands to delete files, directories, and empty directories?

Answer: Delete files: rm. Delete recursively: rm -r. Remove empty directory: rmdir.

Question 14: Common Linux wildcard characters and their meanings?

Answer: ? matches a single character. * matches any number of characters. [charset] matches any single character in the set (e.g., [a-z], [abABC]).

Question 15: Command to count lines, words, and bytes?

Answer: wc with options -l (lines), -w (words), -c (bytes).

Question 16: What is grep used for and how to ignore case or find non-matching lines?

Answer: grep searches text using regular expressions. Use -i to ignore case, and -v to select non-matching lines (e.g., grep -i pattern file, grep -v pattern file).

Question 17: Linux process states and their symbols in ps output?

Answer: Uninterruptible (D), Running (R), Sleeping (S), Stopped/Traced (T), Zombie (Z), Swapped (W, obsolete), Dead (X).

Question 18: How to run a command in the background?

Answer: Append & to the command (no space required).

Question 19: How to list all processes and filter by PID?

Answer: List all: ps -ef (System V) or ps -aux (BSD). Filter: ps -ef | grep pid.

Question 20: Command to view background jobs?

Answer: jobs -l.

Question 21: Commands to bring a background job to the foreground or resume a stopped job?

Answer: Foreground: fg. Resume in background: bg.

Question 22: Command to terminate a process with options?

Answer: kill (e.g., kill -9 pid or kill -s SIGNAL pid).

Question 23: How to list all supported signals?

Answer: kill -l.

Question 24: Commands to search for files?

Answer: find <directory> <conditions> <actions>. Also whereis and locate for faster searches.

Question 25: How to see who is logged in and which terminal they are using?

Answer: Current terminal: who am i. All logged-in users: who.

Question 26: Command to view command history?

Answer: history.

Question 27: Commands to check disk usage and free space?

Answer: df -hl shows filesystem size, used, available, and mount points.

Question 28: Command to test network connectivity?

Answer: netstat.

Question 29: Command to display IP address and interface information?

Answer: ifconfig.

Question 30: Command to view environment variables?

Answer: Show all: env. Show specific: env $HOME.

Question 31: How to customize the Bash prompt?

Answer: Use escape sequences such as \u (user), \h (hostname), \W (current directory), \w (full path), \$ (prompt symbol), etc., in the PS1 variable.

Question 32: How does the shell locate executable files and how to modify the search path?

Answer: whereis searches for binaries, sources, and manuals with options like -b, -m, -s. The which command searches the PATH for the first matching executable.

Question 33: How to find the location of an executable?

Answer: Use which for executables; whereis for binaries, manuals, and source files.

Question 34: How to create an alias for a command?

Answer: alias la='ls -a'.

Question 35: Difference between du and df?

Answer: du reports disk usage of files and directories (user‑level). df reports filesystem‑level statistics, including metadata.

Question 36: Overview of awk usage.

Answer: awk '{pattern + action}' {filenames}. Example: cat /etc/passwd | awk -F ':' '{print $1"\t"$7}' prints username and shell.

Question 37: How to bind a macro or key sequence in Bash?

Answer: Use the bind command, e.g., bind '"\e[24~":"date"'. Obtain key sequences with Ctrl+V or showkey -a.

Question 38: How to list all commands supported by the system?

Answer: compgen -c prints all available commands.

Question 39: How to display the current directory stack?

Answer: dirs (managed with pushd and popd).

Question 40: How to detach all running processes without rebooting?

Answer: disown -r removes all jobs from the shell’s job table.

Question 41: What does the Bash hash command do?

Answer: It maintains a hash table of command paths, showing how many times each command has been executed.

Question 42: Which Bash built‑in performs arithmetic?

Answer: let evaluates integer expressions.

Question 43: How to view a large file page by page?

Answer: Pipe cat filename to more (e.g., cat file_name.txt | more).

Question 44: Which user owns the data dictionary?

Answer: The SYS user (system‑created).

Question 45: How to get a brief description of an unknown command?

Answer: Use whatis, e.g., whatis zcat.