🔬 Lab1: Linux

🔬 Lab1: Linux#

📌 bjectives#

Students should be able to use commands like ls, pwd, and cd to explore directories, navigate using relative and absolute paths, and understand the structure of the Linux filesystem.
Students should be able to create files and directories using touch and mkdir, modify file timestamps, and delete items using rm and rmdir, including handling hidden and non-empty directories.
Students should be able to use advanced ls options such as -lah to display detailed file and directory information, including permissions, ownership, and sizes, and recognize hidden files.
Students should be able to use features like tab autocompletion, output redirection (>), and pipes (|) to streamline their workflow and combine commands for advanced functionality.
Students should be able to view system information and processes using commands like ps, top, and cat /proc/cpuinfo, and perform system actions such as shutting down or restarting from the command line.

Linux Commands 🐧#

During class, we covered several basic Linux commands. In this lab, we will get hands-on practice with those commands.

To log in the Master computer, use m3 if you’re in the M3 section and t5 if you’re in the T5 section.
Click the Terminal icon on the Dock to open a terminal and practice commands.
You can select an open terminal and use Ctrl + Shift + n to open a new terminal window or Ctrl + Shift + t to add a new tab to the current terminal.
When observing the terminal (or Shell) you will notice the following
- Syntax: username@hostname:(e.g., on the master: m3@masterX:, on the robot: pi@robotX)
- Current working directory: Represented as ~, which refers to the user’s home directory.
- Prompt: The $ character followed by a blinking cursor, which indicates the terminal is active and ready for commands.

Using Filesystem#

To master Linux, it’s crucial to understand the filesystem and become comfortable with the Terminal (or shell). While the command line may feel intimidating at first, regular practice will make it second nature.

View Directory Contents: Enter ls to see the contents of the current directory. You’ll see directories like Desktop, Documents, and Downloads. Color coding helps identify types: directories are blue, files are white, executable files are bright green, and archived files are red. For now, just note that directories are blue and files are white.
Display Current Directory Path: Enter pwd to display the path of the current directory. This will show your home directory path, such as /home/stanbaek.
Change Directory: To move from your home directory to the Downloads directory, enter cd Downloads. The cd command stands for “change directory.” Enter ls again to view the files inside the Downloads directory.
Navigate to Parent Directory: Use cd .. to navigate back to the home directory. In Linux, two dots .. refer to the directory above (the parent directory). A single dot . refers to the current directory. While you don’t use cd . to switch to the current directory, knowing this can be useful for some commands.
Root Directory: The root directory is always /. Enter ls / to list the contents of the root directory, and cd / to switch to the root directory. It’s important to understand the difference between ls Downloads and ls /Downloads. The first command lists the contents of the Downloads directory within the current directory, while the second tries to list a Downloads directory directly under the root, which usually doesn’t exist.
View Root Directory Contents: Enter cd / followed by ls to view the files inside the root directory.
Return to Home Directory: Enter cd to move back to your home directory. Note that cd is equivalent to cd ~.
Complex Paths: The ls and cd commands can also be used with more complex paths. For example, enter ls /opt/ros/humble to view the contents of the “humble” directory inside “ros,” which is inside “opt.” Switch to this directory using cd /opt/ros/humble. To move back up three parent directories, use cd ../../...
Autocomplete: Type cd /o followed by the tab key. It will autocomplete to cd /opt/. Press the tab key again to see options like google and ros. Type r and press the tab key to choose ros, then press the tab key again to choose humble as it is the only directory inside ros.

Listing Files#

Using the desktop GUI to list and move files is easier than using the Terminal and keyboard, but it’s an important skill that you’ll appreciate as you advance with ROS and Linux.

Basic Listing: We’ve already looked at ls, which lists the files in the working directory. You’re more likely to use a command like ls -lah. The bit after the command (the -lah) is known as the argument. This option modifies the behavior of the command. The -l argument lists files and directories in “long” format. Each file and directory is now on a single line, and before each file is a lot of text. First, you’ll see letters and dashes, like drwxr-xr-x. These are permissions, and we breifly discussed in class.
File Count: After the permission letters, there’s a single number. This is the number of files in the item. If it’s a file, it will be 1, but if it’s a directory, it will be at least 2. Each directory contains two hidden files: one with a single dot (.) and one with two dots (..). Directories containing files or other directories will have a higher number.
Owner and Group: Next, you’ll see your username twice on each line. The first is the owner of the file, and the second is the group. Typically, these will be the same, and you’ll see either root or your username. You can enter ls -l / to view the files and directories in the root directory that belongs to the root account.
File Size: The next number relates to the size of the files, in bytes. The h argument in ls -lah humanizes the number, making it easier to read.
Hidden Files: Be aware that many hidden files in Linux are listed using the -a argument. Hidden files and directories begin with a dot (.), so you should never start a file or directory with a dot, unless you want to hide it. Typically, you can combine all three arguments into the command ls -lah.

Creating and Deleting Files#

Creating and deleting files is a fundamental computing skill. When using the Linux Terminal, remember that deleted files are not sent to the system recycle bin, so extra care is needed.

Create a File: Enter cd to move to the home directory. Type touch testfile and ls -l to view the files. You’ll see a new file called testfile with a size of 0 because it’s empty.
Case Sensitivity: Linux is case sensitive. If you enter touch Testfile (with a capital T), it creates a second file called Testfile. Enter ls -l to see both files. To avoid confusion, most people use lowercase letters consistently.
Update Timestamp: Enter ls -l and note the timestamp of testfile. Then enter touch testfile followed by ls -l and notice the updated time. The touch command updates files or directories with the current system time.
No Spaces in File Names: Avoid using spaces in file names. Entering touch test file creates two files: test and file. Instead, use an underscore (_), like touch test_file.
Delete Files: If you’ve followed the steps, you should have five files: testfile, Testfile, test, file, and test_file. To delete files, use the rm command. Enter rm Testfile to delete the file named Testfile. Enter ls -l to confirm its deletion.
Use Wildcards: Enter ls test* to view files that match the word test followed by any characters. The * wildcard means “any characters here.” Enter rm test* to delete test, testfile, and test_file. Finally, enter rm file to delete it.

Creating and Removing Directories#

After learning to create files, you’ll want to know how to make directories and move items around.

Create a Directory: Enter ls to view directories in the home directory. Use mkdir to create directories. Enter mkdir testdir and ls again to see it.
Timestamp Update: Unlike touch, mkdir does not update the timestamp if used on an existing directory.
Multiple Directories: You can create multiple directories at once with mkdir. Enter mkdir testdir2 testdir3 and ls to see several directories.
Nested Directories: You can create directories within directories using the directory path. Enter mkdir Documents/photos to create a photos directory inside Documents. The parent directory must exist, so mkdir articles/reports will fail if articles does not exist.
Create Directory Path: Use the -p option with mkdir to create a directory path. Enter mkdir -p articles/reports and ls to view the articles directory, and ls articles to see the reports directory inside.
Remove a Directory: Ensure you’re in the home directory by entering cd. Then enter ls to view contents. Use rmdir to delete directories. Enter rmdir testdir3 and ls to confirm deletion.
Non-Empty Directories: Try to delete the articles directory containing the reports directory. Enter rmdir articles. You will get an error. The rmdir command only removes empty directories. To delete non-empty directories, use rm with the -r (recursive) option. Enter rm -r articles to delete the articles directory containing the reports directory.
Interactive Deletion: Use the -i (interactive) option with rm to prompt before each deletion. Enter rm -ri test* and enter Y or y for each prompt. Using -i is a good practice with the rm command.

Copying, Moving, and Renaming Files#

In Linux, renaming a file is essentially moving it from one name to another, and copying a file involves moving it without deleting the original.

Create Test File and Directory: Enter touch testfile and mkdir testdir to create a test file and directory in your home directory. Enter ls to confirm their presence.
Move Files and Directories: Use the mv command with two arguments: source and destination. Enter mv testfile testdir to move testfile into testdir. Enter ls to see it’s no longer in the home directory, and ls testdir to see it inside testdir. Create a new directory with mkdir newparent.
Move Directories: Move directories with files using the same command. Enter mv testdir newparent to move testdir into newparent. Enter cd newparent/testdir and ls to see testfile inside testdir.
Move Up Levels: Use the double dot (..) to move up directories. Enter ls -la to view files, including single and double dot entries. Move testfile up one level with mv testfile .. and enter cd .. to move to the parent directory.

Hint

Use the tab key to autocomplete paths for the following step.
Longer Paths: Move files using longer paths. Enter cd ~ to return to the home directory, then mv newparent/testfile newparent/testdir/testfile to move testfile back to testdir. Enter ls newparent/testdir to confirm.
Rename Files: Rename and move a file at the same time. Enter mv newparent/testdir/testfile newparent/testfile2. Enter ls newparent to see testfile2 in the newparent directory.

Useful Commands#

Linux is a vast and versatile command line language with hundreds of commands you can learn. Here are a few that can help you get more from your Ubuntu.

View Processor Details: Enter cat /proc/cpuinfo to see details about the processors.
Using cat: The cat command lists the contents of a text file, such as cpuinfo. You can also open this text file using gedit, a GUI text editor. Enter gedit /proc/cpuinfo to view the file in gedit. Note that cpuinfo is read-only.
View Memory Information: Enter cat /proc/meminfo to get information about your memory.
Process Status: Type ps to see two items: bash and ps. To view processes used by other users (including those started by root), enter ps -a. This option shows processes for all users but does not include background processes. For that, enter ps -A or ps -e, which shows every process on the system, including background processes. You may need to pipe it through less using ps -e | less. The less command allows you to view the contents one screen at a time. Press q to exit less.
Real-Time Process Monitoring: While ps is useful, you may need to view processes in real-time, especially to check CPU and memory usage. Use the top command for this.
Shutdown and Restart: To shut down the computer from the command line, enter sudo shutdown -h now. The -h option stands for “halt.” To restart, enter sudo shutdown -r.

Input, Output, and Pipes#

Redirect Output: Change the standard output using the > character after your command. For example, ls -l /etc lists all items in the /etc directory. Using ls -l /etc > etc.txt outputs the list to a new text file called etc.txt.
View Output File: The etc.txt file now contains the output from the ls command. Check it using cat etc.txt or nano etc.txt. The output from ls -l was sent to this file instead of the screen. Press Ctrl+X to quit nano. The > character allows you to output to files, but you can also get input from a file.
Using Pipes: As you advance in Linux, you create more powerful commands using the pipe character (|). Enter cat ~/.bashrc to display the .bashrc file’s content. Now enter cat ~/.bashrc | wc. The output from cat is piped into the wc (word count) command, showing the number of lines, words, and characters in the document.
Chaining Commands: You can pipe commands multiple times. Enter cat ~/.bashrc | sort | grep source* to list lines starting with “source” in alphabetical order. The output from cat is passed to sort, and sort’s output is passed to grep, which filters out content starting with “source.”

🚚 Deliverables#

Visit Git Repo Setup to create a ROS workspace on the Master computer and configure your ECE387 classroom repository.
Submit Lab 1 on Gradescope.

References

“Raspberry Pi Tips, Tricks & Fixes Vol. 35”, Edited by James Cale, BDM Limited, 2019, ISBN: 2046-2743.