Command Line Basics

Command Line Basics

• The user interface program (shell or GUI) is the lowest level of user-mode software.
• This level allows users to start other programs like web browsers or music players.

Command Interpreter (Shell)

• A process called the command interpreter or shell reads commands from a terminal.
• Once a user types a command (e.g., requesting program compilation), the shell creates a new process to run the compiler.
• After the compilation finishes, the process executes a system call to terminate itself.

Shell Input and Output

• The shell uses the terminal as standard input/output.
• It displays a prompt (e.g., a dollar sign) to indicate it's ready for commands.
• For instance, when a user types date, the shell creates a child process to run the date program.
• The shell waits for the child process to terminate.
• After the child process finishes, the shell displays the prompt again to read the next input line.

Recommended Resources

• Free command line course: https://www.codecademy.com/learn/learn-the-command-line
• Command line videos: https://www.youtube.com/playlist?list=PLW5y1tjAOzI2ZYTlMdGzCV8AJuoqW5lKB
• Jason Cannon's "Linux Succinctly" (2014).
• Jason Cannon's "Command Line Kung Fu: Bash Scripting Tricks, Linux Shell Programming Tips, and Bash One-liners" (2014).
• Online command line tutorials at tutorialspoint.com: https://www.tutorialspoint.com/unix_terminal_online.php

Basic Commands

• ls: Lists files in the current directory.
• ls --help: Displays help information on the use of the ls command.
• ls /home: Lists files in the /home directory.
• ls /: Lists files in the root directory.
• ls Documents/ Downloads/: Lists the contents of the Documents and Downloads directories.

ls Command Options

• By default, ls does not show files or directories starting with a period, as they are considered hidden in Linux.
• ls -a: Displays all files, including hidden ones.
• ls -t: Sorts the output of the ls command by time, showing the most recently modified items first.
• ls -rt: Sorts the output of the ls command by time in reverse order.

Filenames with Spaces

• ls -l 'my to do list.txt' or ls -l my\ to\ do\ list.txt: Correctly lists files with spaces in their names.

File Content Commands

• cat: Concatenates or displays files.
• more: Allows browsing through a text file.
  • Press Spacebar to advance to the next page.
  • Press Enter to advance to the next line.
  • Type q to quit viewing the file.
• less: Similar to more, but allows backward movement and pattern searches.
• head: Displays the beginning portion of a file.
• tail: Displays the ending portion of a file.

Sorting and Comparing Files

• diff: Displays the differences between files.
• sdiff: Displays two files side-by-side, highlighting the differences.

Standard Input/Output Redirection

• date > date.txt: Redirects standard output to a file named date.txt.
• >>: Used to append output to a file.
• sort < fruits.txt: Standard input is redirected.
• sort < fruits.txt > outfile.txt
• ls info.txt nofile.txt 2> errors.txt:
• ls info.txt nofile.txt 1> out.txt 2> errors.txt:
• ls info.txt nofile.txt > out.txt 2>&1: Redirects one file descriptor to another.
• ls info.txt nofile.txt 2> /dev/null: Redirects output to the null device, preventing it from displaying or saving.

Pipes

• A pipe is a pseudofile that connects two processes.
• Processes communicate through a pipe.
• Process A writes data to process B.
• Process B reads data in UNIX, like ordinary file reads/writes.
• cat fruits.txt vegetables.txt | sort: Connects cat and sort with a pipe.
• cat fruits.txt vegetables.txt | sort > out.txt: Connects cat and sort with a pipe that redirects to out.txt

Screen, Disk Space, and Memory Commands

• clear: Clears the contents of the screen.
• cat fruits.txt \ vegetables.txt | sort > out.txt:
• df: Shows available disk space.
• free: Shows the amount of free memory.

File Permissions

• Before reading or writing a file, it must be opened, and permissions are checked.
• UNIX protects files with a 9-bit binary protection code.
• The code contains three 3-bit fields.
  • Owner.
  • Owner's group.
  • Everyone else.
• Each has read, write, and execute access bits.
• The three bits are known as rwx bits
• Example: rwxr-x--x
  • The owner can read/write/execute the file.
  • Group members can read/execute.
  • Everyone else can execute.
• For a directory, x gives search permission.
• A dash means that the permission is absent.
• ls -l: Shows permission settings for a file.

File Type

• First character is the type of the file.

Changing Permissions

• Permissions are also known as modes.
• chmod (change mode) changes permissions.
• chmod g+w somefile.txt: Adds write permission for the group.
• chmod g-w somefile.txt: Removes write permission for the group.
• chmod g+wx somefile.txt: Adds write and execute permissions for the group.
• chmod ug+wx somefile.txt: Adds write and execute permissions for the user and group.
• chmod u=rwx,g+x somefile.txt: Sets read, write, and execute permissions for the file owner and adds execute permission for the group.
• chmod a=r somefile.txt: Sets the file to be readable by everyone.
• The equal sign (=) replaces current permissions.
• chmod a=r,a=w somefile.txt produces different results to chmod a=w,a=r somefile.txt
• chmod a= somefile.txt: Removes permissions following the equal sign.

Wildcards

• ls m*: Lists files starting with m.
• ls *.txt: Lists files with .txt extension.
• The asterisk matches anything, while the question mark matches a single character.
• ls ??????.txt

Symbolic Links (Symlinks)

• A symlink points to an actual file or directory.
• The symlink acts as a pointer.
• Symbolic links create shortcuts to long names or paths.
• Another common use for symlinks helps point to an application's current version.

Working Directory

• Each process at every instance has a current working directory.
• Path names not beginning with a slash are looked for.
• pwd: Print working directory
• System call changes working directory using the: cd /usr/share/doc
• Change directory (absolute path): cd doc
• Go to parent directory: cd ..
• Go home: cd ~
• The tilde represents the current user's home directory

Directory and File Manipulation

• mkdir test: Creates a directory called test.
• cd test: Navigates to the test directory.
• touch newfile.txt: Creates a new empty file.
• mv newfile.txt ..: Moves or renames file or directories to the parent directory.
• mv newfile.txt test: Moves file back to the directory test.
• ls test: Checks that the file is back in the directory test.
• mv newfile.txt mynewfile.txt: Renames the file.
• mv test mydir: Renames the directory.
• cp mynewfile.txt myoldfile.txt: Copies files.
• cp info.txt Documents: Copies info.txt into the Documents directory.

Directory Operations

• cp -a mydir Documents: Copies a whole directory.
• rmdir mydir: Removes an empty directory.
• rm -rf mydir: Removes a non-empty directory.

Finding Files

• find /etc -name l*: example of finding files

Find Command

• Locates files or directories on a Linux system.

Man Pages

• The man command: displays the online manual for a given command.

Environment Variables

• An environment variable is a name-value pair.
• Programs determine their actions.
• env: To display all the environment variables

Processes

• ps: To Lists the currently running processes.
• ps -e | less: To see every process running on the system.
• kill 2994: To kill the process, using it's PID.
• kill -9 2994: Kills the process if it's still running.

Superuser

• su: Become the superuser.
• su -c pwd: Execute a command as a superuser.
• su -c 'ls -l /': Quoted command to execute if the command is more than one word in length.
• whoami: To show what user you are currently working as

Sudo Command

• Allows you to run a command with the security privileges of another user.
• Sudo uses the superuser in no username is specified.
• sudo ls: example to "run ls as root user".
• sudo -l: List commands to execute with sudo
• sudo -u someuser command: Allows you to Run command as another user
• sudo su: Switch to the superuser account