So to review: we showed how you can use arrow keys to browse your history, and use Ctrl+r to search your history. So it shouldn’t a surprise to you that the commands you type in are saved. To view your complete history, you…. history.
2075 ll
2076 cd courseNotes/
2077 ll
2078 rm friendly*
2079 head -3 Acceptable-Use-Policy
2080 tail -3 Acceptable-Use-Policy
2081 ll *.png
2082 ll *.jpg
2083 file lecture2a.md
2084 file lecture2a-nautilus.png
2085 cp lecture2a-nautilus.png test.txtyou can call any of these commands using the ! and the number from history:
!2079
head -3 Acceptable-Use-Policy
--------------------------------------------------------------------------------
Policy : Information Technology Acceptable UseAnother incredibly useful shortcut is !!. This will repeat the last command you typed in. This doesn’t seem too useful until you start knowing something about permissions.
cd cambridge, cd ../gen_ed.cd ~/Documents, cd ~uli101/a1.cd /etc/shared.Let’s assume that your filesystem looks like this:
/
├── etc
│ ├── network
│ │ └── interfaces
│ └── selinux
│ └── config
├── example.html
├── example.md
├── home
│ └── student
│ ├── Documents
│ │ └── resume.docx
│ ├── Music
│ └── Projects
│ └── Seneca
│ └── final-project.c
└── usr
└── local
├── bin
│ └── program.py
└── lib
├── node
└── python…and you are given a question such as this:
You are logged in as Student. Your current location is
/usr/local/lib. Copyprogram.pyinto the directory calledSeneca.
You will be required to use the shortest possible filepath for both the target and the destination.
Our target is program.py. Our current location (.) is /usr/local/lib. Our home (~) is /home/student. We have three possible filepaths.
.. gets us to /usr/local. bin gets us into the proper subdirectory. program.py identifies the target. So the relative path is: ../bin/program.py. (3 steps).~. Unfortunately our target isn’t in a subdirectory of home, so this is going to get complicated! The relative-to-home path is: ~/../../usr/local/bin/program.py (7 steps)./. usr is a subdirectory of /. Then bin is a subdirectory of usr. So the absolute path is: /usr/local/bin/python.py (4 steps).We can see that the relative path is the shortest, most efficient path to program.py.
Again, for the destination we have three options.
../../.. gets us to root, so already we can see this isn’t a very efficient route. home/student/Projects/Seneca will get us to the destination. The complete relative path is ../../../home/student/Projects/Seneca (7 steps).~. From here, we type Projects to get into a subdirectory, and Seneca is our destination. The relative-to-home filepath is ~/Projects/Seneca (3 steps)./). The absolute filepath is /home/student/Projects/Seneca (4 steps).So for the destination, we should use a relative-to-home filepath.
Our answer for this question is: cp ../bin/program.py ~/Projects/Seneca
So as a normal user, you don’t have permission to install applications or change much of the system configuration. This is because you don’t have root privileges. Oftentimes in the real world, you will type in a command, and have the shell tell you that permission denied. sudo is a command that invokes your root privileges for a command. So you have situations like this:
This is a command to check for new software updates in Ubuntu:
apt update
sudo apt update
sudo apt update
[sudo] password for eric:
Hit:2 http://ca.archive.ubuntu.com/ubuntu bionic InRelease Get:3 http://security.ubuntu.com/ubuntu bionic-security InRelease [83.2 kB] Hit:4 http://archive.canonical.com/ubuntu wily InRelease Reading package lists... Done
Building dependency tree
Reading state information... Done
All packages are up to date.Instead of typing in ‘apt update’ twice, the same thing can be accomplished with:
sudo !!
This isn’t too relevant for you yet, since in this course you won’t be given root privileges. But it’s a common issue with Linux users. And it will become relevant in the following situation:
echo "Hello User !!"
What just happened?
Well, the shell saw this: !! and performed the substitution that it thought was appropriate. Namely, it substituted !! with the last command I ran, which in this case was sudo apt update. This is an important concept to keep in mind. Putting a string in double quotes (") did not prevent the substitution from happening.
Here’s another example of the same thing. We’ve talked a bit about Bash being a programming language, and programming languages make use of variables. We have variables in Bash, and here’s how they work.
Finally, the echo command is becoming useful. To assign value to a variable, we use an equal sign (=). But to call that variable, we start with a dollar sign ($).
This throws us an error, because the shell is interpreting the second word ‘College’ as a command. Let’s try again by putting the string in double quotes.
Double quotes indicate to the shell the boundaries of our string. You can think of them as a weak fence, but also notice how the dollar sign still gets interpreted not as a dollar sign, but something that identifies a variable.
Now let’s use single quotes (’).
Single quotes are a strong barrier. No variable or command substitution is allowed. Everything is very literal: ! are exclamation points, and $ are dollars signs.
Note: If you only wish to escape one character such as a dollar sign, use the escape character (\).
cost=100
echo $cost
100
echo "Wow, I can't believe this textbook costs \$$cost."
Wow, I can't believe this textbook cost $100.If there is only one golden rule for Bash scripting, it is this: use quotes, and use the quotes appropriate for what you want to do.
The concept of open source software (or free software) is a very important part of Linux. But what exactly does it mean? I’ll pass along the definition of ‘free software’ from Richard Stallman. Free software follows four freedoms:
One important distinction you hear made is that free software is “free” as in “free speech”, but not necessarily “free” as in “free beer.” You also get into distinction between ‘free software’ and ‘open source software’, but for the most part we use the two terms interchangeably. Like a lot of things in the Linux world, there’s a lot of politics and backstory if you feel like reading up on it!
What this means is that a lot of the software provided with your version of Linux is free: you can use it without cost, you can read its source code and choose to contribute to the project if you wish. And open source has had a huge impact on how computing has developed. The Internet would look very different (or would still be in its infancy!) if there had never been the LAMP stack, which is all open source.
Let’s say you work on a project that you think might benefit other people, and decide to open source your project. There are several licenses available that you might consider. One of these is the GPL. In a nutshell, under a GPL license (version 3), anyone who modifies your code must also release their modified code under the GPL license. This would prevent, say, a company from taking your code, modifying it a bit, and then making it proprietary.
Software Licensing and ownership is a huge but interesting topic. I encourage you to read about it further.
Summary
Shortcuts
- !!: Repeat last command
- &&: Combine two commands; if the first is successful, run the second.
- !####: Repeat a command from history
- \: Escape character in Bash
- "": Weak Quotes. Allows Bash to expand $ and !
- ’’: Strong Quotes. $ and ! will be taken literally
Introduction to Variables and Quoting
- To assign a value to a variable:
var=value- To print the value of a variable:
echo $value