Getting Started with the Shell

Getting Started with the Shell

If you have never used the shell before, then this is where we'll start. We're not going to go into lots of detail, there's plenty of that later on in book. Instead we'll do a quick crash course on the basics. If you have not used the shell before this'll give you a chance to see how it works.

Start by opening your shell. This is covered in Opening the Shell. Your shell should be Bash - if this doesn't sound familiar, then make sure you have followed the instructions in Configuring the Shell.

You should see your terminal program running your shell. You can see what the version is of your shell by running:

bash --version

Screenshot: Bash Version

Let's quickly dissect this. We have run the bash command. A command can be a program on your computer, or it can be something built into the shell. We'll look at this in a lot more detail later, but for now it's important to understand that a lot of what you will be doing is running commands.

The --version text is a parameter. Parameters affect how commands work. This is actually easier to see with an example.

Let's move to the home folder. On most computers your home folder is your personal space where things like documents, photos, music, downloads and so on are kept.

Let's switch to the home folder by running the following command:

cd ~

Once you've done that, run the pwd command:

pwd

Screenshot: Moving to the home directory

So what has happened here? The first command:

cd ~

Is used to change directory - that's what cd stands for. The parameter we passed to cd was just the ‘tilde’ character (~). This character has a special meaning in the shell - it means “the current user's home directory”.

Finally, we ran the pwd command. This command is short for print working directory. It writes out to the screen where you currently are. On my Mac, my home directory is located at /Users/dwmkerr, which is what the command has shown me.

Let's take another look at a command. Run the following in your shell:

ls

The ls command is short for list directory contents - it shows you everything that is in the current directory. On my computer you can see things like the ‘Downloads’, ‘Music’ and ‘Pictures’ folders, which are set up by default on a Mac, as well as some of my own folders.

Screenshot: List directory contents

We can pass different parameters to ls. The main parameter is the location of the folder we'd like to list the contents of. So if we wanted to see what was in the Music folder, we'd just run:

ls Music

Not much to see here:

Screenshot: List the contents of the Music directory

Many commands actually allow us to pass multiple parameters. For example, we could list the contents of my Movies and my personal applications:

ls Movies Applications

Screenshot: List the contents of the Movies and Applications directory

There's not much in either. You might wonder why Applications is so empty - that's because we're looking at the applications only installed for the current user, because we are in the user's home directory. To see the applications for everyone we'd need to use the folder where applications are kept for all users.

We can do this by running ls /Appliciations:

Screenshot: List global applications

The trick here is that we start with a leading forward slash - this means the Applications folder in the root of the computer, not the one in my current folder.

On Windows, applications are kept in different places, but we can see some of the installed applications by running ls "c:\program files\":

Screenshot: List applications on Windows

Why do we have the extra quotation marks here? If we ran the command without the quotation marks, the shell would think we were giving it two parameters. It would think we wanted to see the contents of the c:\program and files folders - and they don't exist!

Screenshot: List applications on Windows incorrectly

The error above shows what happens when we miss the quotation marks.

Now we can take a look at how a flag would work. A flag is a parameter which changes how a command works. Flags normally start with a hyphen. Let's say we wanted to know the size of the files in the folder. We do this by using the -lh pass the parameter, which is short for long list, human readable:

ls -lh Downloads/*.jpg

Screenshot: List downloaded photos

Now I can see all of the jpg files (jpg files are images) in my Downloads folder. I can see it looks like I've got two pictures of “Mardi Himal” (a mountain in the Himalayas) which are both 384 Kilobytes in size, as well as some other images. Blow by blow, this is what we've got:

  • ls - List the contents of a folder
  • -lh - This is the long list in human-readable sizes parameter, which means we see how big the files are in a friendly format (like 911K for Kilobytes, rather than showing something like 911012 which would be the number of bytes - and harder to read!)
  • Downloads/*.jpg - Show the contents of the Downloads folder, including any files which end with .jpg - the * is a wildcard which means that we don't mind what the filename is

The -lh parameter is shorthand. Many commands offer longhand parameters (such as --version) as well as shorthand (such as -v as an alternative for --version). Longhand is easier to read, shorthand is faster to type.

Don't worry - in the next section we'll see how to look up the available parameters for a command. You don't need to remember all of these details, only understand which part is the command and which parts are the parameters. This is just an introduction for now!

Now let's look at one more command.

The Echo Command

The ‘echo’ command is used to write out a message in the shell. Here's an example of how it works:

echo "Hello Shell!"

This command writes out the text Hello Shell!:

Screenshot: Echo command

Why would we do this? One of the most common reasons would be to see what the shell thinks a certain value is. For example, try this command:

echo "My home directory is at: $HOME"

You'll see something like this:

Screenshot: Echo the home directory

The $HOME part of the text is called a variable. We can recognise variables because they start with a dollar symbol. $HOME is a built-in variable which holds the location of the current user's home directory.

We're going to see all sorts of cool things we can do with echo as we continue in the book!

Move Around

One common thing we can do in a visual file explorer is move around. We can open folders, and go ‘up’ from the current folder. We often also see visually where we are in the folder structure with an ‘address bar’.

A useful reference might be the picture below:

Screenshot: Shell quick reference

Here we map the shell commands to the visual interface's equivalents:

  • pwd shows the current working directory - where you currently are in the file system
  • ls lists the files in the current directory (or any directory you tell it)
  • cd .. changes the directory to another location - if you use the special .. directory, you are telling it to change to the parent directory, i.e. ‘go up’ in the file system

As a final trick, lets see how we open a file or folder. Let's say I want to open one of the photos in my Downloads folder. Here's how I can do it:

cd ~/Downloads
open himalayas.jpg

We can see the result here:

Screenshot: Open a photo

Running open himalayas.jpg has opened the photo in the application which is used for photos by default in the operating system.

Be aware - this command is different on different operating systems (but we're going to see later on how to fix that and make it consistent everywhere!). The open command will open a file on MacOS. On Windows you can use start, and on Linux you can generally use xdg-open.

As a nifty trick, trying running open .1:

Screenshot: Open the current directory

. This will open the current folder. Every folder contains two ‘special’ folders. The first is .., which we've seen means ‘my parent folder’ and the second is ., which means ‘myself’. Having this . folder is convenient, as it means we can do things like this - run a command to open the current folder.

We're going to go into a lot more detail on how to work with files and folders, move around, but hopefully this has provided a crash course for the basics. They key concepts to remember, which are much more important than the individual commands we've see are:

  • In the shell we run commands
  • We can change how commands work by using parameters
  • Some parameters just go at the end of the command - like ls Downloads
  • Some parameters start with a hyphen, and change how the command behaves - these are often called ‘flags’. An example is ls -lh, which lists the files in the current folder with a human-readable file size

We've also learned:

  • cd changes the current directory
  • pwd prints the current directory
  • ls lists the files in a directory
  • echo can be used to write out text to the screen
  • open, start and xdg-open can be used to open a file or folder on MacOS, Windows and Linux respectively

Now we can start to get into more detail!


Footnotes


  1. On Windows you might need to run start . and on Linux, xdg-open .. ↩︎

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Copyright © , Dave Kerr