Basics

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Commands

Informational

• which <command>...: Show the path of a command.

  which whatis # Output: /usr/bin/whatis

• whatis <command>...: Briefly describe a command.

  whatis pwd # Output: pwd - print name of current/working directory

• wc -<option>... <file>...: Count lines, words, and characters.

  wc file.txt # line, word, and character count of file.txt

• grep -<option>... <pattern> <file>...: Search for patterns in files and return matching lines.

  grep "hello" file.txt # Search for "hello"
  grep -i "HeLLo" file.txt # Case-insensitive search
  grep -r "hello" ./dir1 # Recursive search in dir1

• less: Open output in a controllable pager.

  • q: Quit.
  • space: Scroll down (next page).
  • b: Scroll up (previous page).
  • g: Go to the beginning.
  • G: Go to the end.
  • /<pattern>: Search forward (down).
  • ?<pattern>: Search backward (up).
  • n: Repeat search in the same direction (next result).
  • N: Repeat search in the opposite direction (previous result).
  • h: Help.

  less file.txt # View a file in a pager
  find ./dir1 -name "*.txt" | less # View the output of find in a pager

• cat -<option>... <file>...: Print file contents to the standard output (terminal).

  cat ./file.txt # Print a file
  cat ./file1.txt ./file2.txt # Print multiple files

  -n # Number all lines
  -b # Number non-empty lines
  -s # Squeeze blank lines into one

File & Directory

• OUTPUT > <file>...: Redirect output to files (>: overwrites, >>: appends).

  > file.txt # Create/empty a file and start writing in it
  pwd > file.txt # Save the command (pwd) output to file
  cat file1.txt > file2.txt # Copy contents of file1.txt to file2.txt
  cat file1.txt file2.txt >> file3.txt # Append contents of file1.txt and file2.txt to file3.txt

• mkdir -<option>... <directory>...: Create directories.

  mkdir new_directory # Create a directory called new_directory
  mkdir -p ./new_directory/sub_directory # Create a nested directory structure
  mkdir -p ./new_dir/{dir1,dir2,dir3} # Create a directory with multiple subdirectories in it

• rm -<option>... <file/directory>...: Delete files/directories.

  rm file.txt # Remove a file
  rm -r ./dir1 # Recursively remove a directory and its contents
  rm -ri ./dir1 # Prompt before each removal
  rm -rf ./dir1 # Force delete without prompt

• cp -<option>... <source>... <destination>: Copy files and directories (mv to move).

  cp file.txt ./dir1 # Copy a file to dir (overwrite if exists)
  cp -i file.txt ./dir1 # Prompt before overwrite
  cp file.txt ./dir1/file2.txt # Copy and rename in destination
  cp -r ./dir1 ./dir2 # Copy entire directory with its contents to another directory

• touch <file>...: Create empty files.

  touch file.txt # Create a file called file.txt

• find <directory>... -name <pattern> -<option>: Search for files and directories.

  find ./dir1 -name file.txt # Find specific file in dir1 and its subdirectories
  find ./dir1 -name "*.txt" # Find all .txt files in dir1 and its subdirectories
  find ./dir1 -name file.txt -delete # Find and delete a specific file in dir1 and its subdirectories

Process & Monitoring

• du -<option>... <file/directory>...: Show disk usage.

  du -a # All files and dirs in the current directory
  du -h # Human-readable format
  du -s # Summary total for each argument

• pidof <program_name>...: Show the PID(s) of a running process.

  pidof bash # PID of bash
  pidof docker # PID of Docker

• ps -<option>...: View running processes.

  ps -u <username> # User-specific processes
  ps -ef # More detailed list

• lsof -<option>... <pattern>...: List open files and their processes.

  lsof -p <PID> # Files opened by a process with PID
  lsof -i # All network connections
  lsof -i tcp # TCP connections
  lsof -i tcp:3000 # TCP on port 3000

• kill -<signal> <PID>: Send a signal to a process.

  Signal Name	Number	Description
  SIGHUP	1	Reload config. Tells a program to restart or reload its settings. Often used after editing config files.
  SIGINT	2	Graceful stop. Sent when you press Ctrl + C to stop a program.
  SIGQUIT	3	Like SIGINT (2), but also creates a debug log (core dump).
  SIGABRT	6	Tells a program to abort/cancel (used when something goes wrong).
  SIGKILL	9	Force-stops a program immediately.
  SIGTERM	15	Asks a program to stop nicely. (Default signal for kill)

  kill -9 <PID> # Force kill a process with given PID
  kill -9 $(pidof nginx) # Force kill all processes with the name nginx

• watch -<option>... <command -option>...: Re-run a command periodically.

  watch -n 5 free -h # Show memory usage with a 5 seconds of refresh rate

Command Chaining

• cmd1 ; cmd2 ; cmd3: Run commands in order, regardless of success.

• cmd1 && cmd2 && cmd3: Run next only if the previous succeeds.

• cmd1 || cmd2 || cmd3: Run next only if the previous fails.

• cmd1 | cmd2 | cmd3: Pipe output of one command to the next.

  find . *.* | less # View the command result in a pager