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01-introduction.md

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+---
+title: Introducing the Shell
+teaching: 20
+exercises: 10
+---
+
+::::::::::::::::::::::::::::::::::::::: objectives
+
+- Describe key reasons for learning shell.
+- Navigate your file system using the command line.
+- Access and read help files for `bash` programs and use help files to identify useful command options.
+- Demonstrate the use of tab completion, and explain its advantages.
+
+::::::::::::::::::::::::::::::::::::::::::::::::::
+
+:::::::::::::::::::::::::::::::::::::::: questions
+
+- What is a command shell and why would I use one?
+- How can I move around on my computer?
+- How can I see what files and directories I have?
+- How can I specify the location of a file or directory on my computer?
+
+::::::::::::::::::::::::::::::::::::::::::::::::::
+
+## What is a shell and why should I care?
+
+A *shell* is a computer program that presents a command line interface
+which allows you to control your computer using commands entered
+with a keyboard instead of controlling graphical user interfaces
+(GUIs) with a mouse/keyboard/touchscreen combination.
+
+There are many reasons to learn about the shell:
+
+- Many bioinformatics tools can only be used through a command line interface. Many more
+  have features and parameter options which are not available in the GUI.
+  BLAST is an example. Many of the advanced functions are only accessible
+  to users who know how to use a shell.
+- The shell makes your work less boring. In bioinformatics you often need to repeat tasks with a large number of files. With the shell, you can automate those repetitive tasks and leave you free to do more exciting things.
+- The shell makes your work less error-prone. When humans do the same thing a hundred different times
+  (or even ten times), they're likely to make a mistake. Your computer can do the same thing a thousand times
+  with no mistakes.
+- The shell makes your work more reproducible. When you carry out your work in the command-line
+  (rather than a GUI), your computer keeps a record of every step that you've carried out, which you can use
+  to re-do your work when you need to. It also gives you a way to communicate unambiguously what you've done,
+  so that others can inspect or apply your process to new data.
+- Many bioinformatic tasks require large amounts of computing power and can't realistically be run on your
+  own machine. These tasks are best performed using remote computers or cloud computing, which can only be accessed
+  through a shell.
+
+In this lesson you will learn how to use the command line interface to move around in your file system.
+
+## How to access the shell
+
+On a Mac or Linux machine, you can access a shell through a program called "Terminal", which is already available
+on your computer. The Terminal is a window into which we will type commands. If you're using Windows,
+you'll need to download a separate program to access the shell.
+
+To save time, we are going to be working on a remote server where all the necessary data and software available.
+When we say a 'remote server', we are talking about a computer that is not the one you are working on right now.
+You will access the Carpentries remote server where everything is prepared for the lesson.
+We will learn the basics of the shell by manipulating some data files. Some of these files are very large
+, and would take time to download to your computer.
+We will also be using several bioinformatic packages in later lessons and installing all of the software
+would take up time even more time. A 'ready-to-go' server lets us focus on learning.
+
+## How to access the remote server
+
+You can log-in to the remote server using the [instructions from the Introduction to Cloud Computing for Genomics lesson](https://datacarpentry.org/cloud-genomics/02-logging-onto-cloud#logging-onto-a-cloud-instance).
+Your instructor will supply to you the `ip_address` and password that you need to login.
+
+Each of you will have a different `ip_address`. This will
+prevent us from accidentally changing each other's files as we work through the
+exercises. The password will be the same for everyone.
+
+After logging in, you will see a screen showing something like this:
+
+```output
+Welcome to Ubuntu 20.04.5 LTS (GNU/Linux 5.4.0-137-generic x86_64)
+
+ * Documentation:  https://help.ubuntu.com
+ * Management:     https://landscape.canonical.com
+ * Support:        https://ubuntu.com/advantage
+
+  System information as of Mon 13 Mar 2023 03:57:46 AM UTC
+
+  System load:  0.0                Processes:             192
+  Usage of /:   20.3% of 98.27GB   Users logged in:       0
+  Memory usage: 25%                IPv4 address for eth0: 172.31.12.214
+  Swap usage:   0%
+
+  Get cloud support with Ubuntu Advantage Cloud Guest:
+    http://www.ubuntu.com/business/services/cloud
+
+178 updates can be applied immediately.
+108 of these updates are standard security updates.
+To see these additional updates run: apt list --upgradable
+
+
+Last login: Fri Mar 10 03:14:44 2023 from 72.83.168.14
+```
+
+This provides a lot of information about the remote server that you're logging into. We're not going to use most of this information for
+our workshop, so you can clear your screen using the `clear` command.
+
+Type the word `clear` into the terminal and press the `Enter` key.
+
+```bash
+$ clear
+```
+
+This will scroll your screen down to give you a fresh screen and will make it easier to read.
+You haven't lost any of the information on your screen. If you scroll up, you can see everything that has been output to your screen
+up until this point.
+
+:::::::::::::::::::::::::::::::::::::::::  callout
+
+## Tip
+
+Hot-key combinations are shortcuts for performing common commands.
+The hot-key combination for clearing the console is `Ctrl+L`. Feel free to try it and see for yourself.
+
+::::::::::::::::::::::::::::::::::::::::::::::::::
+
+## Navigating your file system
+
+The part of the operating system that manages files and directories
+is called the **file system**.
+It organizes our data into files,
+which hold information,
+and directories (also called "folders"),
+which hold files or other directories.
+
+Several commands are frequently used to create, inspect, rename, and delete files and directories.
+
+:::::::::::::::::::::::::::::::::::::::::  callout
+
+## Preparation Magic
+
+You may have a prompt (the characters to the left of the cursor) that looks different from the `$` sign character used here.
+If you would like to change your prompt to match the example prompt, first type the command:
+`echo $PS1`
+into your shell, followed by pressing the <kbd>Enter</kbd> key.
+
+This will print the bash special characters that are currently defining your prompt.
+To change the prompt to a `$` (followed by a space), enter the command:
+`PS1='$ '`
+Your window should look like our example in this lesson.
+
+To change back to your original prompt, type in the output of the previous command `echo $PS1` (this will be different depending on the
+original configuration) between the quotes in the following command:
+`PS1=""`
+
+For example, if the output of `echo $PS1` was `\u@\h:\w $ `,
+then type those characters between the quotes in the above command: `PS1="\u@\h:\w $ "`.
+Alternatively, you can reset your original prompt by exiting the shell and opening a new session.
+
+This isn't necessary to follow along (in fact, your prompt may have other helpful information you want to know about).  This is up to you!
+
+::::::::::::::::::::::::::::::::::::::::::::::::::
+
+```bash
+$
+```
+
+The dollar sign is a **prompt**, which shows us that the shell is waiting for input;
+your shell may use a different character as a prompt and may add information before
+the prompt. When typing commands, either from these lessons or from other sources,
+do not type the prompt, only the commands that follow it.
+
+Let's find out where we are by running a command called `pwd`
+(which stands for "print working directory").
+At any moment, our **current working directory**
+is our current default directory,
+i.e.,
+the directory that the computer assumes we want to run commands in,
+unless we explicitly specify something else.
+Here,
+the computer's response is `/home/dcuser`,
+which is the top level directory within our cloud system:
+
+```bash
+$ pwd
+```
+
+```output
+/home/dcuser
+```
+
+Let's look at how our file system is organized. We can see what files and subdirectories are in this directory by running `ls`,
+which stands for "listing":
+
+```bash
+$ ls
+```
+
+```output
+R  r_data  shell_data
+```
+
+`ls` prints the names of the files and directories in the current directory in
+alphabetical order,
+arranged neatly into columns.
+We'll be working within the `shell_data` subdirectory, and creating new subdirectories, throughout this workshop.
+
+The command to change locations in our file system is `cd`, followed by a
+directory name to change our working directory.
+`cd` stands for "change directory".
+
+Let's say we want to navigate to the `shell_data` directory we saw above.  We can
+use the following command to get there:
+
+```bash
+$ cd shell_data
+```
+
+Let's look at what is in this directory:
+
+```bash
+$ ls
+```
+
+```output
+sra_metadata  untrimmed_fastq
+```
+
+We can make the `ls` output more comprehensible by using the **flag** `-F`,
+which tells `ls` to add a trailing `/` to the names of directories:
+
+```bash
+$ ls -F
+```
+
+```output
+sra_metadata/  untrimmed_fastq/
+```
+
+Anything with a "/" after it is a directory. Things with a "\*" after them are programs. If
+there are no decorations, it's a file.
+
+`ls` has lots of other options. To find out what they are, we can type:
+
+```bash
+$ man ls
+```
+
+`man` (short for manual) displays detailed documentation (also referred as man page or man file)
+for `bash` commands. It is a powerful resource to explore `bash` commands, understand
+their usage and flags. Some manual files are very long. You can scroll through the
+file using your keyboard's down arrow or use the <kbd>Space</kbd> key to go forward one page
+and the <kbd>b</kbd> key to go backwards one page. When you are done reading, hit <kbd>q</kbd>
+to quit.
+
+:::::::::::::::::::::::::::::::::::::::  challenge
+
+## Challenge
+
+Use the `-l` option for the `ls` command to display more information for each item
+in the directory. What is one piece of additional information this long format
+gives you that you don't see with the bare `ls` command?
+
+:::::::::::::::  solution
+
+## Solution
+
+```bash
+$ ls -l
+```
+
+```output
+total 8
+drwxr-x--- 2 dcuser dcuser 4096 Jul 30  2015 sra_metadata
+drwxr-xr-x 2 dcuser dcuser 4096 Nov 15  2017 untrimmed_fastq
+```
+
+The additional information given includes the name of the owner of the file,
+when the file was last modified, and whether the current user has permission
+to read and write to the file.
+
+:::::::::::::::::::::::::
+
+::::::::::::::::::::::::::::::::::::::::::::::::::
+
+No one can possibly learn all of these arguments, that's what the manual page
+is for. You can (and should) refer to the manual page or other help files
+as needed.
+
+Let's go into the `untrimmed_fastq` directory and see what is in there.
+
+```bash
+$ cd untrimmed_fastq
+$ ls -F
+```
+
+```output
+SRR097977.fastq  SRR098026.fastq
+```
+
+This directory contains two files with `.fastq` extensions. FASTQ is a format
+for storing information about sequencing reads and their quality.
+We will be learning more about FASTQ files in a later lesson.
+
+### Shortcut: Tab Completion
+
+Typing out file or directory names can waste a
+lot of time and it's easy to make typing mistakes. Instead we can use tab complete
+as a shortcut. When you start typing out the name of a directory or file, then
+hit the <kbd>Tab</kbd> key, the shell will try to fill in the rest of the
+directory or file name.
+
+Return to your home directory:
+
+```bash
+$ cd
+```
+
+then enter:
+
+```bash
+$ cd she<tab>
+```
+
+The shell will fill in the rest of the directory name for
+`shell_data`.
+
+Now change directories to `untrimmed_fastq` in `shell_data`
+
+```bash
+$ cd shell_data
+$ cd untrimmed_fastq
+```
+
+Using tab complete can be very helpful. However, it will only autocomplete
+a file or directory name if you've typed enough characters to provide
+a unique identifier for the file or directory you are trying to access.
+
+For example, if we now try to list the files which names start with `SR`
+by using tab complete:
+
+```bash
+$ ls SR<tab>
+```
+
+The shell auto-completes your command to `SRR09`, because all file names in
+the directory begin with this prefix. When you hit
+<kbd>Tab</kbd> again, the shell will list the possible choices.
+
+```bash
+$ ls SRR09<tab><tab>
+```
+
+```output
+SRR097977.fastq  SRR098026.fastq
+```
+
+Tab completion can also fill in the names of programs, which can be useful if you
+remember the beginning of a program name.
+
+```bash
+$ pw<tab><tab>
+```
+
+```output
+pwck      pwconv    pwd       pwdx      pwunconv
+```
+
+Displays the name of every program that starts with `pw`.
+
+## Summary
+
+We now know how to move around our file system using the command line.
+This gives us an advantage over interacting with the file system through
+a GUI as it allows us to work on a remote server, carry out the same set of operations
+on a large number of files quickly, and opens up many opportunities for using
+bioinformatic software that is only available in command line versions.
+
+In the next few episodes, we'll be expanding on these skills and seeing how
+using the command line shell enables us to make our workflow more efficient and reproducible.
+
+:::::::::::::::::::::::::::::::::::::::: keypoints
+
+- The shell gives you the ability to work more efficiently by using keyboard commands rather than a GUI.
+- Useful commands for navigating your file system include: `ls`, `pwd`, and `cd`.
+- Most commands take options (flags) which begin with a `-`.
+- Tab completion can reduce errors from mistyping and make work more efficient in the shell.
+
+::::::::::::::::::::::::::::::::::::::::::::::::::
+
+