The Terminal, Shell, and Text Editors

If you have never used a terminal before, this page will get you oriented. If you have, use it as a quick reference. This is not a comprehensive guide; it covers enough to be productive in this course.

Tip

For a deeper treatment of these topics, see MIT’s The Missing Semester of Your CS Education.

Terminal vs. Shell

A terminal (or terminal emulator) is the window you type commands into. A shell is the program running inside that window that interprets your commands and executes them.

Common shells:

• bash (Bourne Again Shell): the default on most Linux servers and older macOS versions.
• zsh (Z Shell): the default on modern macOS. Nearly identical to bash for basic usage.

When you open Terminal on macOS or a WSL window on Windows, you are running a shell inside a terminal. The prompt (something like ubuntu@ip-172-31-0-5:~$) tells you who you are, where you are, and that the shell is ready for input.

Navigating the Filesystem

Everything in Linux is a file or a directory. Paths use forward slashes (/). The root of the filesystem is /.

Command	What it does	Example
pwd	Print the current directory	pwd
ls	List files in the current directory	ls -la (all files, detailed)
cd	Change directory	cd /var/log
cd ..	Go up one directory	cd ..
cd ~	Go to your home directory	cd ~
cd -	Go to the previous directory	cd -

Absolute paths start with / (e.g., /etc/apache2/apache2.conf). Relative paths start from your current directory (e.g., ./config.txt or ../other-dir/).

Working with Files

Command	What it does	Example
cat	Print a file’s contents	cat /etc/hostname
less	View a file with scrolling (press q to quit)	less /var/log/syslog
head / tail	Show the first/last lines	tail -20 /var/log/syslog
cp	Copy a file or directory	cp file.txt backup.txt
mv	Move or rename a file	mv old.txt new.txt
rm	Remove a file (no undo)	rm temp.txt
rm -r	Remove a directory and its contents	rm -r old-dir/
mkdir	Create a directory	mkdir -p logs/2026
touch	Create an empty file or update a timestamp	touch notes.txt

Caution

rm deletes permanently. There is no recycle bin on the command line. Double-check before pressing Enter.

Searching

Command	What it does	Example
grep	Search for a pattern in files	grep "error" /var/log/syslog
grep -r	Search recursively in a directory	grep -r "DB_HOST" /var/www/
grep -i	Case-insensitive search	grep -i "warning" log.txt
find	Locate files by name or attributes	find /etc -name "*.conf"
which	Show the path of a command	which python3

Pipes and Redirection

The shell’s real power is combining small commands. The pipe (|) sends one command’s output as input to another.

# Count how many lines contain "error" in the syslog
grep "error" /var/log/syslog | wc -l

# Show the 10 largest files in /var
du -ah /var | sort -rh | head -10

# Find which process is listening on port 80
ss -tlnp | grep :80

Redirection sends output to a file instead of the screen:

echo "hello" > file.txt     # Write (overwrite) to file
echo "world" >> file.txt    # Append to file
command 2> errors.log        # Redirect errors only
command > out.log 2>&1       # Redirect both stdout and stderr

File Permissions

Every file has an owner, a group, and permissions for read (r), write (w), and execute (x).

ls -l script.sh
# -rwxr-xr-- 1 ubuntu www-data 512 Mar 15 script.sh
#  ^^^         owner permissions (rwx)
#     ^^^      group permissions (r-x)
#        ^^^   others permissions (r--)

Command	What it does	Example
chmod	Change file permissions	chmod +x script.sh
chmod 755	Set specific permissions (owner rwx, group r-x, others r-x)	chmod 755 script.sh
chown	Change file owner	sudo chown www-data:www-data index.html

The numeric form (like 755) encodes permissions as three digits: owner, group, others. Each digit is the sum of read (4), write (2), and execute (1). So 755 means rwxr-xr-x.

sudo

sudo runs a command as the superuser (root). Most system administration tasks require it.

sudo apt update              # Update package lists
sudo systemctl restart nginx # Restart a service
sudo vim /etc/hosts          # Edit a system file

Use sudo only when necessary. Running everything as root is a security risk.

Text Editors

You will need to edit files on remote servers where there is no graphical interface. Two terminal editors are available on virtually every Linux system.

nano (beginner-friendly)

nano /path/to/file.txt

nano shows keyboard shortcuts at the bottom of the screen. The ^ symbol means Ctrl.

Shortcut	Action
Ctrl+O	Save (write out)
Ctrl+X	Exit
Ctrl+K	Cut the current line
Ctrl+U	Paste
Ctrl+W	Search
Ctrl+G	Help

vim (powerful but has a learning curve)

vim /path/to/file.txt

vim has modes. You start in Normal mode (for navigation and commands). Press i to enter Insert mode (for typing text). Press Esc to return to Normal mode.

Key/Command	Mode	Action
i	Normal	Enter Insert mode (start typing)
Esc	Insert	Return to Normal mode
:w	Normal	Save
:q	Normal	Quit
:wq	Normal	Save and quit
:q!	Normal	Quit without saving
dd	Normal	Delete the current line
u	Normal	Undo
/text	Normal	Search for “text”
n	Normal	Jump to next search match

Tip

If you get stuck in vim, press Esc a few times, then type :q! and press Enter. This will exit without saving. You can always start over.

Package Managers

Package managers install, update, and remove software. The package manager depends on your distribution.

Distribution	Manager	Install example
Ubuntu/Debian	apt	sudo apt install nginx
Amazon Linux/RHEL	dnf / yum	sudo dnf install nginx
macOS	brew	brew install wget

Always run sudo apt update (or equivalent) before installing packages to refresh the package list.

Environment Variables

Environment variables store configuration that programs can read. They are written in ALL_CAPS by convention.

echo $HOME         # Print your home directory
echo $PATH         # Print the list of directories the shell searches for commands
export MY_VAR=42   # Set a variable for the current session

Variables set with export last only until you close the terminal. To make them permanent, add the export line to your shell configuration file (~/.bashrc for bash, ~/.zshrc for zsh).

SSH

SSH (Secure Shell) connects you to a remote machine over an encrypted channel. You will use it constantly in this course.

# Connect to a remote server
ssh -i ~/Downloads/cs312-key.pem ubuntu@54.123.45.67

# Copy a file to a remote server
scp file.txt ubuntu@54.123.45.67:/home/ubuntu/

# Copy a file from a remote server
scp ubuntu@54.123.45.67:/var/log/syslog ./syslog-copy.txt

If you get a “permission denied” error on your key file, fix its permissions:

chmod 400 ~/Downloads/cs312-key.pem

Getting Help

When you encounter an unfamiliar command, these are the fastest ways to learn what it does:

man ls              # Open the manual page for "ls" (press q to quit)
ls --help           # Print a brief help summary
tldr tar            # Community-maintained cheat sheets (install with "brew install tldr" or "sudo apt install tldr")

Reading manual pages (man) is a core skill. The format is dense at first but becomes natural with practice.