reader-01-the-command-line.md

Reader 01 - The Command Line

The command line is a text-based interface for interacting with your computer. From the command line you can launch programs, view files, and manipulate your file system by making, moving, and copying files and directories. You can think of it as the Finder in Mac, without the graphic interface, but much more powerful.

Setup

On a Mac you can access the command line by opening up the Terminal application, located in /Applications/Utilities/Terminal

To get started on Windows you will need to set up the Windows Subsystem for Linux, which allows you to run Ubuntu (a Linux distribution) from within your current Windows 10 installation. Follow this guide to do so.

The Prompt

When you open up your terminal application you'll see something like this:

SamsComputer:~ sam$

This is called the "prompt". By default (on a Mac) it shows the name of the computer, the directory that you are currently in, your username, and then a $ sign.

The basic use of the command line is: 1) you type a command, 2) you hit return, and 3) some output of the command is printed to the screen.

Basic Navigation & File Operations

Please note I use the word "directory" and "folder" interchangeably.

When you open a new terminal window, you are placed inside your home folder. On a Mac this is /Users/myusername and on Linux, /home/myusername.

To see the folder you are currently in, type: pwd and hit return. pwd stands for "print working directory", or in other words, "show me the directory I am currently working from".

Here are some basic commands for getting around, making, deleting and copying files and folders.

pwd stands for "print working directory". It prints out where you are:

pwd

ls stands for "list". It lists the contents of current directory.

ls

cd stands for "change directory". Type cd and then the directory you want to go to. For example, change to the Desktop from your home folder:

cd Desktop

To go into the parent folder, up one level in the file structure, type .. or ../ instead of a folder name, like so:

cd ..

If you type cd without a folder name after, it takes you back to your home folder.

mkdir stands for "make directory". Type mkdir and then a name to make a folder. For example, make a folder called "cool_project":

mkdir cool_project

mv stands for "move". It lets you move files and folders and also rename them. To rename a file:

mv oldname.txt newname.txt

cp stands for "copy". It lets duplicate files:

cp draft.txt draft_copy.txt

rm stands for "remove". It lets you delete files:

rm bad_selfie.jpg

Please note, rm will not ask for confirmation, and it will not move files to the trash. It'll just delete them immediately, so be careful.

cat stands for "concatenate" and it shows you the contents of a file and also allows you to join two files together. For example, to print out the entirety of Moby Dick:

cat mobydick.txt

more is like cat but will paginate the output if it is larger than the size of your terminal window:

more mobydick.txt

(now use the up and down arrows to go up or down by a line, the space to go down by a page and q to exit if needed)

file provides basic info about a file:

file mysterfile.what

sort sorts a file alphabetically by line and prints the output to the screen

sort names.txt

grep searches each line of a file for some input, and prints those lines to the screen. For example, the following searches for all lines in Moby Dick containing the word "whale".

grep whale mobydick.txt

Command Line Options and Getting Help

Most commands have extra options that you can input when you run the command. They are usually preceded by either one or two dashes (- or --).

The structure of a typical command looks like this:

command_name [options] arguments

("arguments" refers to the file or files your are running the command with)

For example, the sort command outputs in ascending order by default, but you can have it use reverse order with the -r option, like so:

sort -r mobydick.txt

You can also tell sort to only output unique lines (ie, to remove any duplicate lines) with the -u option:

sort -u mobydick.txt

Finally you can combine options:

sort -u -r mobydick.txt

Sometimes, options have parameters. For example, the cut command cuts out portions of each line of a file. To use it you must specify a delimiter character with the -d option and field number to extract with the -f option. To get the first word of every line in Moby Dick I might enter:

cut -d " " -f 1 mobydick.txt

To see all the options and view a manual for any command, use the man tool (short for "manual")

man cut

Use the arrow keys to navigate, and q to exit.

Piping and Directing Output

Most commands will produce output on the screen. However we can also automatically save that output to the filesystem using the > character followed by a filename.

Sort a file called "names.txt", and save the output to a new file called "sorted_names.txt":

sort names.txt > sorted_names.txt

> will create a file if it does not already exist, or overwrite one if it does. You can use >> instead to append to a file.

Unix also has a very powerful concept called "pipes" which allow us to chain commands together, effectively feeding the output of one command into the input of another. To do so, we use the | symbol.

Extract all lines of Moby Dick containing "whale", then sort them.

grep whale mobydick.txt | sort -u

The | here means "take the output of the grep command and send it to sort -u". You can use as many pipes as you desire, and combine this technique with the output redirection.

Extract all lines of Moby Dick containing "whale", then sort them, then save to a new file called "sorted_whales.txt"

grep whale mobydick.txt | sort -u > sorted_whales.txt

The Structure of the Filesytem

Everything on your computer is either a file or a folder, and these files and folders are organized hierarchically, like a tree. At the very bottom of the tree is the "root folder", indicated by a single forward slash, like so /. Here's a basic example of directory structure:

/
	Users/
  		sam/
   			Desktop/
	  			trotsky.jpg
	  			the_man_without_qualities.txt
	 		Documents/
	 		Downloads/			
		Guest/
	Applications/
	Volumes/

Each file and folder has a unique location on the filesystem. This location is called a "path". You can reference files and folders either by their relative path, or by their absolute or full path. In the previous examples I have been using the relative path - that is, I have been referencing files relative to where I currently am. A path is absolute if it begins with a /

For example the absolute path to the_man_without_qualities.txt in the above filesystem is /Users/sam/Desktop/the_man_without_qualities.txt. I can look inside the contents of this file, from any working directory, with this command:

more /Users/sam/Desktop/the_man_without_qualities.txt

There are a few shortcuts for dealing paths as well.

. (single dot) or './' (single dot with slash) means the current folder that I am in.

.. (two dots) or ../ (two dots with slash) means the parent folder. For example, if am in my Desktop folder and I want to list the contents of my Downloads folder I could type:

ls ../Downloads/

Wildcards

It's also possible to reference multiple files using the * character in combination with other characters. This can be really useful in a lot of situations.

For example, can list all files that begin with the word "the" like so:

ls the*

List all jpg images:

ls *.jpg

Make a folder called images and move all jpeg images into it:

mkdir images
mv *.jpg images/

Tips

It can take a while to get used to the command line, but there are a few tips and trick that make it much easier to use.

• Use the up and down arrows to view a history of the commands you have entered.
• Hit the tab key to autocomplete commands and file paths
• Type open and then a filename to open the file in its default program
• Drag a folder or file onto the terminal to fill in its absolute path
• Type ctrl-a to move your cursor to the beginning of the line, and ctrl-e to the end