1. Introduction to Unix

This lectures covers an introduction to Unix system, commands and the file system.

Attribution The content of this lecture are modified from three excellent sources: Introduction to Bash (Shell) from Earth Lab CU Boulder; Intro to Unix from Columbia University; and The Unix Shell from Software Carpentry.

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1.1. Background

We interact with computers in many different ways, such as through a keyboard and mouse, touch screen interfaces, or using speech recognition systems. The most widely used way to interact with personal computers is called a graphical user interface (GUI). With a GUI, we give instructions by clicking a mouse and using menu-driven interactions.

While the visual aid of a GUI makes it intuitive to learn, this way of delivering instructions to a computer scales very poorly and it is not reproducible. Imagine the following task: for a literature search, you have to copy the third line of one thousand text files in one thousand different directories and paste it into a single file. Using a GUI, you would not only be clicking at your desk for several hours, but you could potentially also commit an error in the process of completing this repetitive task. This is where we take advantage of the Unix shell. The Unix shell is both a command-line interface (CLI) and a scripting language, allowing such repetitive tasks to be done automatically and fast. With the proper commands, the shell can repeat tasks with or without some modification as many times as we want. Using the shell, the task in the literature example can be accomplished in seconds.

1.2. The Shell

Shell is the primary program that computers use to receive code (i.e. commands) and return information produced by executing these commands (i.e. output). These commands can be entered via a Terminal, which you will work with in this course.

Using a Shell helps you:

• Navigate your computer to access and manage files and folders (i.e. directories).

• Efficiently work with many files and directories at once.

• Run programs that provide more functionality at the command line such as git for version control.

• Launch programs from specific directories on your computer such as Jupyter Notebook for interactive programming.

• Use repeatable commands for these tasks across many different operating systems (Windows, Mac, Linux).

Shell is also important if you need to work on remote machines such as a high performance computing cluster (HPC) or the cloud.

The most popular Unix shell is Bash (the Bourne Again SHell — so-called because it’s derived from a shell written by Stephen Bourne). Bash is the default shell on most modern implementations of Unix and in most packages that provide Unix-like tools for Windows. Note that ‘Git Bash’ is a piece of software that enables Windows users to use a Bash like interface when interacting with Git.

1.3. Terminal Sessions On Your Computer

The terminal program that you use to run Bash commands will vary depending upon your computer’s operating system.

Mac (OS X)

You can use the program called Terminal, which uses the Bash implementation of Shell and is installed natively on the Mac OS.

You can open Terminal by finding and launching it from Spotlight (or from /Applications/Utilities).

Linux

Many Linux computers use the Bash implementation of Shell, which you will learn to test for in the section below.

You can open the program called Terminal (or Terminal Emulator) by finding and launching it from your list of programs.

Windows

There are many options for running Bash on Windows. In this course, we recommend using Windows Subsystem for Linux (WSL). WSL is a feature of the Windows operating system that enables you to run a Linux file system, along with Linux command-line tools and GUI apps, directly on Windows, alongside your traditional Windows desktop and apps. You can read more about it here.

To enable WSL on windows, follow the steps outlined here. The default Ubuntu distribution works well with the content of this course, and you do not need to change it.

1.4. Home Directory

To understand what a “home directory” is, let’s have a look at how the file system as a whole is organized. For the sake of this example, check the following figure that illustrates the file system on Hamed’s computer. After this illustration, you’ll be learning commands to explore your own filesystem, which will be constructed in a similar way, but not be exactly identical.

On a Unix computer, the filesystem looks like something this:

Fig. 1.1 Hamed’s file system tree

The filesystem looks like an upside down tree. The topmost directory is the root directory that holds everything else. We refer to it using a slash character, /, on its own; this character is the leading slash in /Users/nelle.

Inside that directory are several other directories: bin (which is where some built-in programs are stored), lib (for the software “libraries” used by different programs), users (where users’ personal directories are located), tmp (for temporary files that don’t need to be stored long-term), and so on.

By default, when you open your terminal it lands in your home directory, which in our example is /Users/hamed. We know that this directory is stored inside /Users because /Users is the first part of its name. Similarly, we know that /Users is stored inside the root directory / because its name begins with /.

Tip

There are two meanings for the / character. When it appears at the front of a file or directory name, it refers to the root directory. When it appears inside a path, it’s just a separator.

1.5. Bash Commands to Navigate Files and Directories

Now, let’s review some of the useful bash commands to navigate and manipulate files and directories in your filesystem.

1.5.1. Print a List of Files and Subdirectories (ls)

ls prints the names of the files and directories in the current directory in alphabetical order, arranged neatly into columns. Here is the output of running ls inside the directory of this book on a local computer:

$ ls

CONDUCT.md		_config.yml		lectures
CONTRIBUTING.md		_toc.yml		logo.png
LICENSE			assignments		references.bib
README.md		docs			requirements.txt
_build			intro.md

We can make its output more comprehensible by using the flag -F, which tells ls to add a trailing / to the names of directories:

$ ls -F

CONDUCT.md		_config.yml		lectures/
CONTRIBUTING.md		_toc.yml		logo.png
LICENSE*		assignments/		references.bib
README.md		docs/			requirements.txt
_build/			intro.md

To print all files and directories, including hidden ones, you can add the -a flag:

$ ls -aF

./			LICENSE			intro.md
../			README.md		lectures/
.DS_Store		_build/			logo.png
.git/			_config.yml		references.bib
.gitignore		_toc.yml		requirements.txt
CONDUCT.md		assignments/
CONTRIBUTING.md		docs/

The ../ directory in the output of ls -aF is a special directory name meaning “the directory containing this one” or simply the parent of the current directory. You will learn in the following how to navigate to the parent directory.

1.5.2. Print Current Working Directory (pwd)

To print the name of the current working directory, use the command pwd. This commands prints the full path to the directory (meaning that you can see the parent directory).

$ pwd

/Users/hamed

1.5.3. Change Current Working Directory (cd)

The command to change locations is cd followed by a directory name to change our working directory. cd stands for “change directory”.

Let’s say we are inside the directory of this book, and we want to move to the lectures directory we saw above. We can use the following command:

$ cd lectures

There is no output from this command. But if you are pwd you can confirm that you are in lectures directory now.

You can use the same cd command to also go to the parent directory of the current directory:

$ cd ..

Tip

If you run cd without any arguments it will return you to your home directory. This is equivalent to running cd ~

So far, when specifying directory names, or even a directory path (as above), we have been using relative paths. When you use a relative path with a command like ls or cd, it tries to find that location from where we are, rather than from the root of the file system.

However, it is possible to specify the absolute path to a directory by including its entire path from the root directory, which is indicated by a leading slash. The leading / tells the computer to follow the path from the root of the file system, so it always refers to exactly one directory, no matter where we are when we run the command.

A useful shortcut

You can use the - (dash) character with cd to move into the previous directory you were in. This is very helpful instead of having to remember the full path.

1.5.4. Create a New Directory (mkdir)

To create a new directory you can use mkdir followed by the name you would like to give the new directory. If you only give a name, the new directory will be created in the current directory. You can also give the absolute path to mkdir to create a new directory anywhere on your file system. The following command will create a new directory in Hamed’s home directory named new-directory:

$ mkdir /Users/hamed/new-directory/

1.5.5. Create a New File Using a Single Command (touch)

You can create a new empty file using the single command touch. This command was originally created to manage the timestamps of files. However, if a file does not already exist, then the command will make the file.

This is an incredibly useful way to quickly and programmatically create a new empty file that can be populated at a later time. Here is an example:

$ touch samples.txt

1.5.6. Copy a File (cp)

You can copy a specific file to a new directory using the command cp followed by the name of the file you want to copy and the name of the directory to where you want to copy the file. The names can be relative path or absolute path.

For example, to copy the file samples.txt from the current directory to /Users/hamed/documents/

$ cp samples.txt /Users/hamed/documents/

1.5.7. Copy a Directory and Its Contents (cp -r)

To copy a directory and all its content to a new directory, you need to use the flag -r (meaning recursive) with cp.

For example, to copy the directory documents (and all its content) from Hamed’s home directory to /Users/hamed/projects/ you can run:

$ cp -r /Users/hamed/documents/ /Users/hamed/projects/

1.5.8. Delete a File (rm)

To delete a specific file, you can use the command rm (abbreviated for remove) followed by the name of the file you want to delete.

For example, you can delete the samples.txt file under the current directory:

$ rm samples.txt

1.5.9. Delete a Directory (rm -r)

To delete a directory and all its content (be careful when using this command as Unix doesn’t have a trash bin), you can use the -r flag with rm.

For example, the following command will delete the projects directory and all its content from the current directory:

$ rm -r projects/

1.6. Getting help

Every command in bash has multiple options that you can pass to change the output. There are two ways to find out what options are available:

1. Pass --help to any command:

$ ls --help

2. Read the manual using man:

$ man ls