Introduction to Bash

In this tutorial, we cover the basics of interacting with a computer using Bash (the Bourne Again Shell). You may have heard the terms “command line”, “terminal”, “console”, “shell”, “interactive prompt”, “git-bash”, etc. To be imprecise, they all refer to using the keyboard to control your computer by typing commands.

Preparation for this lesson: To follow along with this lesson, you will need to have bash installed. For Linux and OSX users, bash is included as part of the operating system. In OSX, it can be accessed with terminal.

For Windows users, there are many options (e.g. Cygwin, Windoes Subsystem for Linux). However, we recommend downloading git bash, which is bundles with with git for windows

Why use a command line?

The command line is a very precise and powerful way to interact with a computer. Because it is a scripting language, you can automate and perform complex tasks without much overhead.

Shells are particularly well suited to interacting with files and running programs – possibly your own! Not having a graphical interface turns out to be a strength, because it makes remote computing simple and reliable.

At first, typing in commands will feel a bit cumbersome, but like learning any language the only way to get better is with practice. Lucky for us, bash is based on English with a very simplified grammar.

Starting the lesson

Let’s go ahead and open up our shells. On Macs the program is called Terminal and it’s installed by default in /Applications/Utilities/. You can also use Spotlight and search for Terminal. On Windows, search for git-bash

With your bash shell open, you can now start issuing commands.

Bash command syntax

You can execute commands using the following syntax: commmand name followed (optionally) by arguments or flags.

Let’s issue a command to figure out our current working directory, which is where we currently are in the computer’s directory structure. We can have bash remind us what (and where) this is using pwd, or print working directory:

pwd

Of course, we’re probably more interested in the contents of a directory over just its name. The ls command lists the contents of our working directory.

ls

We can modify how we want the output of ls to look by adding what’s called a flag. For instance, the -l flag will output the contents of a directory in a more detailed fashion.

ls -l

It’s worth keeping in mind that nearly every shell command has its own set of flags/modifiers. We’ll be going over the more generally useful ones as they come. If you want to see how to use a particular command along with its available flags, you can use the manual command.

man ls

Working with directories

Back to our folder here. While peeking at the contents of a directory is certainly useful, we’ll often find ourselves wanting to switch our current directory. This is done using the cd (change directory) command. You should have downloaded a folder call intro-bash-demo and saved it somewhere practical (e.g. your Desktop or home folder) - let’s use cd to navigate to it. I’ve a included a snippet of some useful cd commands below:

Tip: You can press TAB to autocomplete directory and filenames. If a file in the working directory matches the start of the file or directory you’ve started typing, the shell with autocomplete its name. You can also double tap tab to list directory name options if you can’t autocomplete it based on what you’ve typed in.

Once you’ve navigated to the intro-bash-demo folder, list the files in the folder to view its contents.

You notice some text files (e.g. animals.txt) and directories. We’ll return to these shortly.

We can create directories with the mkdir command. Let’s create a directory called text-files.

mkdir text-files

Similarly, you can remove directories with the rmdir command. Let’s remove the folder-to-delete folder.

rmdir folder-to-delete

Hmmm…that didn’t work. It turns out rmdir only works on empty directories. Let’s make sure we don’t actually need the files in that directory before trying again to delete it. You can pass directory names to ls as an argument to see its contents.

ls folder-to-delete

Now that we’re sure we don’t need that folder anymore, let’s forcefully and recursively delete it using the rm (for remove) command. Note, this command will not warn you and the content will be gone forever. Unlike Microsoft Word, there is no “undo” button here once you’ve done this!

rm -rf folder-to-delete

rm can also be used on files without the -rf flags. Try remove to-delete.txt

Working with files

Beyond navigating directories, bash offers a suite of useful tools to do all sorts of things with files themselves.

Let’s examine hello_world.py in greater detail. We can do this with cat, which simply spits out the contents of a file to the terminal’s output. hello_world.py is a Python script that we’ll run from bash later in the tutorial. At the end of the day, scripts are just plain text files that the computer can execute to perform tasks.

cat hello_world.py

When it comes to looking at individual files, however, cat is generally not very practical for anything but tiny files like this one. Generally, we would want to use less, a program known as a pager that is designed to let users scroll through files (no matter how large) by only generating a single ‘page worth’ at a time. As such, it has very little memory overhead. You can also use your arrow keys to navigate through the pages. Use q to quit. It can be a little confusing at first, and it will seem like you’ve lost your bash interface and can’t find it again, but that is normal, so don’t panic.

Let’s use less to view the contents of the larger file, animals.txt.

less animals.txt

When you want to get out of the “pager” view, you can press ‘q’ to exit.

Finally, bash features two useful tools that output the first ten lines (head) and last ten lines (tail) of a file. These are especially handy for large files that we don’t want to open in their entirety.

head animals.txt
tail animals.txt

Copying and moving files

cp is used to copy files. When using cp, you have to type out both the file you want to copy as well as the name of the file you want to make.

cp animals.txt animals_copy.txt

cp also allows you to copy into a folder by providing a folder name instead of a new file name.

cp animals.txt text-files/animals_copy.txt
ls text-files # Confirm that file is there

mv (move) is somewhat similar, but requires you to type out the file you want to move as well as the directory you want to move it to. Let’s move our copy of animals.txt into text_files.

mv animals.txt text-files/
ls text-files

To move animals_copy.txt intotext-files, we should make sure to rename it, which can also be done with the mv command. Here we are going to both rename animals_copy.txt to animals_copy_2.txt and place it in text-files. Otherwise, the version of animals_copy.txt already in text-files` would be overwritten. Note: Bash will not warn you before overwritting a file if a file with the same name already exists in the directory.

mv animals_copy.txt text-files/animals_copy_2.txt
ls text-files

Note on file naming: Notice that none of the file or directory names have spaces in them. This is because spaces are used to separate commands and arguments/flags for command-line programs. In other words, if your directories or files have spaces in their names, they may be interepreted as commands or arguments. It’s best to avoid spaces in files and directories.

When working with files you can also use special characters called wildcards that represent one or many unspecified characters. The use of wild cards to pattern match strings is sometimes referred to as globbing. Here are some common patterns for globbing:

Let’s move all of the Python scripts into the scripts folder.

mv *.py scripts/

Let’s also remove the two copies of animals.txt in text-files.

rm text-files/*copy*
ls text-files

Here is a summary of some of the commands we’ve used until now.

Bash pipes and redirects

Let’s change directories into text-files and start looking at some more powerful uses of bash.

cd text-files

Pipes (stdin/out)

Commands can be strung together with pipes |. Those of you using R may have seen the concept in the dplyr package. Commands typically have an input (stdin) and an output (stdout). The pipe attaches the first commands command into the input of the next.

Say we were interested in knowing how many fish are in animals.txt. Here we’ll first use a powerful command called grep (global regular expression print) to search animals.txt for all of the occurences of fish.

grep 'fish' animals.txt

Notice that by default (and like most commands), grep prints its output to the shell window. Of course, this is not always what we want. In this case, we just want to count the occurences of fish rather that list them. In that case, we can pipe (|) the output of grep to the wc command (for word count) with the -l flag to count the number of lines from the grep output.

grep 'fish' animals.txt | wc -l

Redirect

What if we wanted to create a separate file with the list of fish, called fish.txt. Here, we can use > to redirect the output of grep to a file called fish.txt

grep 'fish' animals.txt > fish.txt

Notice nothing is printed to the shell but a new file with all of the fish has appeared. Cool!

Importantly, using > and passing the output to a file that already exists will overwrite that file. If you just want to add to the end of the file, you can append using the >> operator.

Running programs from the command-line

It can be quite useful to write entire scripts and run them in their entirety. This is particularly helpful if running scripts (e.g. in R, Python, etc) on a server or computer cluster where Graphical User Interfaces (GUI’s) are uncommon.

Before starting, let’s change into our scripts folder. Note, we could run the scripts from any folder by specifying the full path to the file, but for now we’ll change over.

cd ../scripts

We’ll be executing the python script hello_world.py. First, let’s make sure that we have Python and that the computer knows where to find it (more on this in a bit). which is a helpful command for this as it tells us which executable file is being run when we type a command in the shell.

which python

If successful, the shell will print the full path to the executable used in performing the command. Incidentally, these are the files that are executed when your using your computer for everyday use; their execution is simply hidden because you’re often using a GUI.

Let’s go ahead and run our script

python hello_world.py

We can similarly execute bash scripts in the same way. First, let’s look at our script.

less print-numbers.sh

See! The script is just a plain text file~

Let’s go ahead and execute this script.

bash print-numbers.sh

R scripts can be run using the Rscript <script-name> command.

Environment variables

There are special variables called environment variables which are used by bash and other programs to affect behaviour. Here, we’ll only cover two of them. The first is one of the most important: $PATH:

echo $PATH

That’s a little tough to read. Let’s put the directories in our $PATH on separate lines by piping the output of $PATH to the translate characters command, which allows us to quickly substitute the : in $PATH with newline (\n) characters.

echo "$PATH" | tr ':' '\n'

Notice the $ symbol. This is how variables are accessed in bash. Environment variables are always capitalized. The $PATH variable stores the directories you computer searches for programs, commands, and utilities that have been installed. If you installed something and can’t get it to work on the command-line, it’s likely because the executable file for the program is not in the computers $PATH. When we used the which command earlier, these are the directories that were searched. If the command is not in $PATH, nothing will be printed.

which made-up-command

Another useful environment variable is $PYTHONPATH. This is the directories that Python will search for installed Python modules.

echo $PYTHONPATH | tr ':' '\n'

Bash profile and aliases

It turns out we can more finely tune our shell environment and add shortcuts and customizations to make our lives easier. This is done using .bash_profile or .bashrc configuration files. You can read about the differences between these here. These are shell configuration files that are executed every time you start shell and are executed just prior to the bash prompt appearing in the shell. Importantly, these files live in your computer’s home directory; configuration files located anywhere else will not be read by the shell. Let’s first see whether you have a configuration file.

cat ~/.bashrc
cat ~/.bash_profile

If the shell told you the files don’t exist, you can make one (.bash_profile) using vim, a command-line editor. Run the following command

vi ~/.bash_profile

You will now be in a vim editor window. We’ll only add a couple of Aliases to our configuration file. Aliases allow us to create short hands and custom commands to facilitate performing repetitive tasks or for configuring certain commands to always include certain flags (for example).

Press i to enter insert mode in vim and enter the following into the window.

alias ll="ls -alh"
alias ..="cd .."

Press ESCAPE and save and quit the file by typing :wq (don’t ask).

Since we’re still in a terminal, we need to re-execute the .bash_profile (remember it’s executed before the terminal prompt). We can do this with the source command.

source ~/.bash_profile

Now you can use the aliases that we defined in place of the commands that we assigned to them. For example, .. will go back one directory (no need to type cd .. everytime).

The .bash_profile file allows all manner of customizations (e.g. terminal prompt colouring, custom functions, permanently adding directories to you $PATH). Be careful, it’s a slippery slope…