Episode 1 echo "Hello!"

Welcome to Command Line TV.

This is a video podcast designed to help you learn the Unix Shell.

I’m Chris League. I am a professor of computer science at LIU Brooklyn.

And with me is my co-host Christian Lopes. Tell us a little about yourself.

Great! Let’s begin by discussing the scope of what we’re going to do here.

We’re going to learn about shell commands available on Linux and Mac OS X.

Not so much the Windows command prompt. A little bit about my technological biases.

and where I’m coming from. I have been a full time Unix user since the early 90’s.

I have used Linux quite a bit since then, and also part time on Mac OS X.

What’s your background with tech?

Great! That’s what we’ll be doing.

Yeah, it’s kind of unusual, these days to stick to a command terminal.

It’s not the easiest thing to learn, but it gives you a lot of flexibility, power, and precision.

So, any commands that you type on a command line can be almost as easily scripted and automated,

So that you can take your work flow and automate a lot of things.

Which I think is a really great way to work.

Also, when you control things from the command line it can be configured endlessly

And adapted to the way that you like to work. Desktop environments and graphical applications

Are often called “WYSIWYG”. Which is, uhh, What You See Is What You Get,

And that’s a positive to some extent because, you know, when you’re dealing with a graphical program,

You want to see what you’re producing. But another way of thinking of that is

What You See Is All You’ve Got, and so the power of those types of programs can be limited

by having to manually select and click on things and work with menus.

So those types of apps have their place. I probably wouldn’t create graphics for a website

using only the command line. I certainly wouldn’t edit a video using only a command line,

but there are command line tools for doing image processing and video transcoding, and

I use them all the time because we can automate them. So we’re going to look at some of

those commands later on as we get a little more advanced. Also, Unix is often used in server situations.

So the Unix machine is just in a data center somewhere. It’s not something

Where there’s even a monitor or a keyboard attached to it, and so a graphical desktop

environment would be a waste of resources. You can log in to that machine remotely

and just get the command prompt and it is very simple to take care of things that way.

Yeah, free software it’s contributed by lots of people around the world. So the goal

of this podcast is to help our viewers become adept at the command line and to join in this

community of people that are users and power users, developers – although you don’t have to

be a software developer to be interested in this – and that’s what we’ll do.

So, in the Finder application, which is your basic file explorer, on the ‘Go’ menu,

it refers to a folder called ‘Utilities’. Within the Utilities folder you can find an app called

‘Terminal’, and when you open that, it gives you this little window with your prompt in it.

There are other terminal programs available for the Mac. Some people might actually prefer

them over the one that is built in. One that I’ve used is called iTerm. They just have

different minor capabilities and we can discuss those differences at a later time.

So, I’m going to actually log in as a different user so I can have an environment set up

strictly for this podcast, and let’s not worry about what that command is at the moment.

You can make your terminal font bigger or smaller using command and + or -, and then we’re

also going to go full-screen so that we aren’t distracted by other pictures and things.

We’re starting out here with this prompt. What does all of this mean?

Right!

Then the “dollar sign” just means that the terminal is here waiting for me to type a command.

That “dollar sign” is kind of interesting because when you do things as an administrator, as the

‘super user’ of the system, than that often will change to a pound sign, and that’s your

indication that, when there is a “pound sign” there it means that the commands can have a

greater effect on the system. I can do a lot more things in that case because I am the administrator.

It’s very similar, but there’s not a whole lot of uniformity in the Linux world.

There are a ton of different Linux distributions and we’ll talk later about the

differences between some of them. Even within a distribution there can be different desktop

environments and window managers and terminal programs. So, for now all you want to do is go

through menus to try and find something called terminal. It might be “Gnome” terminal or

Konsole, or xterm is one of the most basic terminals that’s available. So if you find

a program like that and you get to your command line prompt, that’s all we need for now.

Yeah, it’s not quite the same. There is a command line for Microsoft Windows and it’s

– the application that is called “Command Prompt”, but it’s a completely different language,

and in my judgement not nearly as flexible and useful. However, there are some ways to run a

Unix like shell on Windows. One of them if you’re a software developer is, you may have

heard of the version control system called Git, and when you install Git on Windows it comes

with an app called Git Bash. Bash is one of the Unix shells, and what it is doing there is

emulating the Unix environment on Windows so that you can operate Git more efficiently, or

more effectively. So that has a limited form of the Unix shell you can use. There’s another

system you can get for Windows called Cygwin, and Cygwin is more of a complete command line

environment. You can install different applications in it and it’s not perfect as far as an emulation

of Linux, but it allows you to do a lot of the same things we are going to cover on Windows.

So we are going to start just by exploring the filesystem. The files and folders on your

system are structured as a tree. Here’s a diagram that we’ve got of a directory tree.

This exact set of files and folders can be downloaded if you want to follow along with what

we’re doing exactly. Check out the link in the show notes for directions for doing that.

So what were the first couple of commands we learned for navigating around the file system?

Sure, and what do they do? What’s the difference between them?

Yeah, and just as a note, we’re going to use the terms ‘folder’ and ‘directory’ interchangeably.

They mean the same thing. Directory is just an older-school term for it, but a lot of people

more familiar with the desktop environments would use folder, and that means the same thing.

So at the top of this directory tree we have the tilde character, which represents our home directory.

two of its sub-directories are cs101 and Downloads.

So let’s say I want to look into that Downloads folder. How would I get there?

Right! So I just type cd Downloads, and you see that part of the prompt has changed to

show me that I am in the Downloads folder, and then I can do ls to show the files within there.

ls

So say that we want to get down to this js folder within django. How would I get there?

Yup! So that’s doing it with three separate commands, and I’m down in that js folder

which it indicates in my prompt. I’ll also point out this command pwd, which tells you the

pwd

exact path to where you currently are. So, what’s showing up in my prompt is just the last

piece of that, but this shows me the complete path to get there. So that also shows us that

we can write these directory names, like when one directory is nested in another we can separate

them with slashes. It can be more convenient to do it all at once. The way to do that is to

just join the directories together with a slash. So I would be able to say cd django/media/js,

cd django/media/js

And that’s the forward slash not the back slash, but this won’t work from the current place

because I’m already in js, and so there is no django folder to descend into. So what I’m going

to do first is go back up. Do you remember the command to go back up to a parent directory?

Yup! So .. goes up one. So I can go back up to media, or you can also combine

those together with slashes to get all the way back up to Downloads. Then I’ll be able to

cd ../..

do the cd django/media/js. I’m getting back previous commands I typed by

cd django/media/js

just using the up and down arrows, and now that will work just as well. So if you look

at our file system tree, js has a sibling directory called css.

How would we get to the css directory?

Right. So the .. goes up to the parent and then css goes down again to the sibling,

and now I’m in css.

So one way is to just go up to the top of the tree and down again. So you can see that it’s –

it looks like three levels up to your home directory. So if we go up three times from here.

Now we’re in home, but I can just continue from there and do cs101/hello/lib/sys.

cd ../../../cs101/hello/lib/sys

So you just trace the path up and down the tree, and I’m over in sys. Another way to

do that is, you can use that ~ (tilde) to shortcut directly to your home. So I’m going to use a

command, or another form of cd, which is cd - and that takes me back to the place I was

cd -

previously. So now I’m back in templates, and instead of using all of those dots, we’ll

just do cd ~/cs101/hello/lib/sys and now I’m back there again.

cd ~/cs101/hello/lib/sys

So ls lists files and directories in the current directory. It’s a little unclear what’s

ls

what if you just have a basic ls output like this. So one thing I like to do is, there’s an

option to ls which is dash… capital F. You can’t have any space in there. It has to be

-F. When I do that, it adds a character to certain types of entries here. Which indicates,

ls -F

in this case, that slash indicates that those are sub-directories and everything else that doesn’t

have the slash is a regular file. So that helps me orient myself a little more in

the tree structure. A more detailed version of that is ls -l. This is a long, or a

detailed listing, and it gives lots of information. The file names are here on the

right side, and then we’ve got some permissions. Later, we’ll go into exactly what those mean,

but for now you see that the d here corresponds to the directories. Ignore that

number. We’ve got a user and a group name, so that’s the owner of this file. The file size, and then

a time stamp, which is when the file was last modified.

Yeah, there are a few ways to change how it gets sorted. So, one that is very useful is to

Sort by modification times. So that’s called ls -t, and you might also want to do

ls -t

that with l. So that when I combine multiple options together, I can either have them as

ls -l -t

separate switches like this with a space, or most commands support having them merge together

ls -lt

So that it’s just -lt. And now what it’s doing here is it’s showing these entries

sorted by the modification time. So the most recently modified is at the top, and I often want

to reverse that. So, if you want to reverse however it’s being sorted, you can add an r

or -r to it, and that will put those in reverse order. So now the most recent is at

ls -ltr

the bottom. I find that really useful because a lot of times you’ll have more files than fit

on the screen, but then the bottom most file that you see is the most recent.

Right. Yeah a lot of the Unix system is case sensitive. So it matters which way you use

uppercase and lowercase, and that includes for commands. So you can’t type capital LS to get

The ls command, and switches to commands like you noticed here. But also the file and folder

names. So if I go up here again and I try type downloads, then – actually that works on

a Mac because the file system on a Mac is not case sensitive. Even though the Unix shell is

case sensitive. So you get some weird artifacts with that. Let me try that again, but this time

I am going to use the Tab key. Tab is a way to complete a file name, so you only have to

type part of it and then you hit Tab. So if I try to type downloads and hit Tab with

a lowercase d it doesn’t complete because the shell, which is doing that Tab completion, knows

that nothing starts with a lowercase down like that. But if I do Down with a capital D and

I hit Tab, then it works again. So, the strange thing is that although the Mac

cd Downloads/

filesystem is not case sensitive, the shell that’s interpreting your command still is. And

when we switch to Linux next time, Linux has a case sensitive filesystem, so it’s important

to be aware of that distinction.

Yeah, spaces are a real issue in file names because spaces are used to separate the

different parameters on the command line, and so a lot of people that use the command line

have a hard and fast rule I just never put spaces in file names. I’ll use a dash or maybe

an underscore, but the real world is messy and sometimes you’ve got to deal with spaces in file

names and directory names. So for example here I want to cd into “Command Line TV”, and one

way to do that is to put quotes. It can be a double quote or a single quote, as long as it

is consistent, and then you type it out with the spaces in it and close the quote.

cd 'Command Line TV'

That allows me to descend into that directory. Another way is with a back slash. So instead of

putting the quote you just start to type it. Again I’ll use Tab to do the completion

because the Tab completion helps me out. What it’s going to do is insert these

back slashes before each space, and that tells the shell that this isn’t really a space that

separates the next parameter of the command, but instead it’s a space that’s a part of that

same directory name. So that works fine too.

cd Command\ Line\ TV

So next time we’re going to look at how to view different file types from the command line,

and also how to construct some very simple pipelines in order to view the content of

different things. So see you next time!