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Linux Command-Line Basics

If this is your first time using a terminal, don't worry — you only need a small set of commands to survive this course. This page covers what you'll actually use in Labs 1–6 and the ROS2 lectures.

Info

You'll mostly be working inside Ubuntu (either in a VM or via WSL). Everything below applies there.

What actually happens when you type a command

When you type something into the terminal and hit enter, you're running a program with that name. ls, for example, is just a small program whose job is to list the contents of your current folder. Try it now:

ls

Most programs accept parameters (also called flags or options) that change how they behave — think of them as instructions you're giving the program about what you want. There are two styles:

  • a single - followed by one-character parameters, e.g. -l
  • a double -- followed by full-word parameters, e.g. --all

For example:

ls -l

The l parameter tells ls to print its output in long format (permissions, size, modified date, etc.) instead of just names. Try it and compare with plain ls.

You can also stack several single-character parameters together after one -. The a parameter tells ls to show all files, including hidden ones (any file or folder whose name starts with a .):

ls -la

Now you should see extra entries like .bashrc or .config that weren't visible before — these are hidden by default because a leading . tells Linux "don't show me unless asked."

Tip

This -flag / --flag pattern isn't unique to ls — almost every command-line tool in this course, including ros2 and colcon, works the same way (e.g. colcon build --symlink-install).

Finding out what a command does: man

Every built-in command comes with a manual page. If you forget what a command or parameter does, don't guess — look it up:

man ls

This opens a scrollable manual (use the arrow keys, press q to quit). It lists every available parameter for that command with an explanation. This is often faster than searching the web, and it always matches the exact version installed on your machine.

Tip

Not every command has a man page (e.g. some ros2 subcommands don't). In those cases, try adding -h or --help instead, e.g. ros2 pkg -h.

Why command line at all?

Linux and ROS2 tools (ros2 run, colcon build, rosdep, etc.) don't always have a full graphical interface. The terminal is the interface. Once you get comfortable with a dozen commands, it becomes much faster than clicking through folders.

Opening a terminal

  • Ubuntu Desktop / VM: look for a terminal icon, or right-click the desktop → "Open Terminal Here"
  • WSL on Windows: open PowerShell and type wsl, or launch the "Ubuntu" app from the Start menu

A few symbols you'll see everywhere

Before the commands, it helps to know what these shorthand symbols mean when they appear in a path:

Symbol Meaning
/ The root of the entire file system — everything on the machine lives somewhere under /. Also used as a separator between folder names, e.g. home/user/Documents.
~ Your home folder (shorthand for something like /home/yourusername). Typing cd ~ always takes you home, no matter where you are.
. The current folder you're in. Mostly used when running something in the current directory, e.g. ./my_script.sh.
.. The parent folder, one level up from where you are. cd .. moves you up one level.

Tip

You can chain .. to go up multiple levels: cd ../.. moves up two folders. And ~/Documents means "the Documents folder inside my home folder," regardless of your current location.

Commands

Command What it does
pwd Print working directory — "where am I?"
ls List files in the current folder
ls -la List files, including hidden ones, with details
cd foldername Move into a folder
cd .. Move up one folder
cd ~ or cd Go to your home folder
cd - Go back to the previous folder

Tip

Press Tab while typing a path to auto-complete it. This saves you from typos and is the single most useful habit to build early.

Working with files and folders

mkdir my_folder          # create a folder
touch my_file.txt        # create an empty file
cp file.txt copy.txt     # copy a file
mv file.txt newname.txt  # rename or move a file
rm file.txt              # delete a file
rm -r my_folder          # delete a folder and its contents

Danger

rm does not move files to a trash bin, they are deleted immediately. Double-check the filename before pressing enter, especially with rm -r.

Viewing file contents

cat file.txt      # print the whole file
less file.txt     # scroll through a long file (press q to quit)
nano file.txt     # simple text editor inside the terminal

Permissions and sudo

Some commands (usually installing software) require admin rights:

sudo apt update
sudo apt install ros-humble-turtlesim

sudo will ask for your password. Nothing appears on screen as you type it — that's normal, just type and press enter.

If you see Permission denied, it usually means the command needs sudo, or the file isn't marked executable (see below).

chmod +x my_script.sh   # make a file executable

Running programs and stopping them

Shortcut Effect
Ctrl + C Stop the currently running program
Ctrl + Z Pause the current program
command & Run a command in the background
↑ / ↓ arrows Scroll through your command history

Multiple terminals

Many labs (e.g. Lecture 3's turtle_teleop_key example) ask you to open several terminals at once — one per node. This is completely normal in ROS2: each terminal runs one program and they talk to each other in the background. Keep track of what's running in each one.

The one ROS2-specific habit to build

Every new terminal needs to "source" ROS2 before any ros2 command will work:

source /opt/ros/humble/setup.bash

and, inside your workspace, also:

. install/setup.bash
Why does ros2 say 'command not found' sometimes?

Almost always because this terminal hasn't been sourced yet. Run the two commands above and try again.

Quick reference cheat sheet

Click to expand: most-used commands at a glance 
pwd                      # where am I
ls -la                   # list files
cd foldername            # move into folder
cd ..                    # move up
mkdir name               # new folder
touch file.txt           # new file
cp a.txt b.txt           # copy
mv a.txt b.txt           # rename/move
rm file.txt              # delete file
rm -r folder             # delete folder
cat file.txt             # print file
sudo apt install pkgname # install software
chmod +x script.sh       # make executable
Ctrl+C                   # stop program

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