Web programmers have to live on the command line. It gives us fast, reliable, and automatable control over computers. Web servers usually don't have graphical interfaces, so we need to interact with them through command line and programmatic interfaces. Once you become comfortable using the command line, staying on the keyboard will also help you keep an uninterrupted flow of work going without the disruption of shifting to the mouse.
For any command we discuss here, the command man, short for manual, will give a (hopefully) detailed explanation of that command. Sometimes that explanation will be too detailed for you. When you get lost in a man page and you want to understand it, start again from the beginning of of the man page and keep repeating. Hopefully you will get further into the page each time you read it.
The command line is my home. I literally think of using the command line as walking around a building.
/ is the entrance - the root directory - but I usually start out in /Users/raphael, which is my room, my home directory. My home directory can also be called ~.
The room I'm currently in is called the working directory. When I open a new terminal, my home directory is the working directory. Wherever we are, pwd, short for print working directory, will show us what directory we are in.
I look around the room I'm in - the working directory - with ls. ls stands for list directory contents. Try typing open . to open the current directory in Finder, then type ls. You should see something like the result below:
Applications Library Pictures downloads.txt output.pdf workspace
Desktop Movies Public dump.rdb sizereport
I can see everything, including hidden files and folders, with ls -a for List All:
-
In every room (folder) you'll see
., which I think of as being the floor. -
You'll also see
.., the door you came in through, which is the folder above the current folder. -
Any other files which begin with
.are considered hidden files.. .netrc .. .pip .ghc Applications .gitconfig Desktop .gnupg Documents .gnuplot_history Downloads
I can look at everything in a more detailed list with ls -l. List in a long format. I've put in a header line so you can see what some of the columns are.
Permissions Owner Group Size Last Modified Filename
drwxr-xr-x 45 raphael staff 1530 Nov 21 23:30 Applications
drwx------+ 11 raphael staff 374 Dec 4 09:09 Desktop
-rw-r--r-- 1 raphael staff 8872 Nov 13 2012 downloads.txt
-rw-r--r-- 1 raphael staff 191858 Nov 16 14:45 dump.rdb
lrwxr-xr-x 1 root staff 4 May 10 2011 usr -> /usr
Here's a tutorial on file permissions, if you're interested in unpacking that column: http://en.flossmanuals.net/command-line/permissions/
These can be combined into ls -la. List all in a long format.
Some of the entries I see are other rooms, which we call directories or folders. List the contents of another room with ls [dirname]. Try ls Downloads. We can go further down into the house, into a sub-directory.
I can change the room I'm in with cd. cd stands for change directory.
We can move back to the root directory: cd /. A path which starts from the entrance, the root directory, is called an absolute path. Now look around. What do you see?
We can move back home: cd ~. What do you see?
I can move into my workspace directory: cd workspace. A path which is relative to the current directory is called a relative path.
Hitting <TAB> autocompletes. Hit <TAB> constantly.
It's important to remember that being in one room is much like being in another room. The difference is in the contents and the relative position of other folders.
Now that we know how to move around, it's time to make some changes. We can make directories with the mkdir command. Look at man mkdir. What's the format of the command for making a directory?
MKDIR(1) BSD General Commands Manual MKDIR(1)
NAME
mkdir -- make directories
SYNOPSIS
mkdir [-pv] [-m mode] directory_name ...
DESCRIPTION
The mkdir utility creates the directories named as operands, in the order specified, using
mode rwxrwxrwx (0777) as modified by the current umask(2).
Operands are what comes after a command, so we write mkdir living_room to make a new room, where we will keep our couches. Keep your directory names lowercase in almost every case. Separating words with underscores is called snake_case.
Let's cd into our new living_room Look around with ls, and ls -a. What do you see?
I want my living room to have a bookshelf full of books. Let's make a file that lists all of our books. Type subl books to open Sublime Text editing a new file called books. Add a few books, copy and paste the section below so we all have some books in common, and save the file. Make sure the books you add are in the same format: <author_given_name>, <author_last_name>:<title>.
Carroll, Lewis:Through the Looking-Glass
Shakespeare, William:Hamlet
Bartlett, John:Familiar Quotations
Mill, John Stuart:On Nature
London, Jack:John Barleycorn
Bunyan, John:Pilgrim's Progress, The
Defoe, Daniel:Robinson Crusoe
Mill, John Stuart:System of Logic, A
Milton, John:Paradise Lost
Johnson, Samuel:Lives of the Poets
Shakespeare, William:Julius Caesar
Mill, John Stuart:On Liberty
Bunyan, John:Saved by Grace
Now try ls again. Do you see the books file? Look at the contents with cat. Let's make another, smaller file, which will be our bookshelf. Describe the bookshelf, and just say the description to ourselves.
echo "The particle board of this bookshelf flexes under the weight of the books it holds, but it serves its purpose and does it cheaply."
echo is a command that just echoes (outputs) what we give to it as arguments (same as operands). Now we want to put that line in a file called bookshelf.
echo "The particle board of this bookshelf flexes under the weight of the books it holds, but it serves its purpose and does it cheaply." > bookshelf
Using the closing angle bracket > in this way is called redirection. Every command that we run in the shell has an input, an output, an error output, and arguments/operands. We are saying: "Run echo with this string as an operand, and take the output and put it in a new file called bookshelf." Try running ls again, and cat our new file.
Adapted from http://en.flossmanuals.net/command-line/piping/
Let's look back at our books. Read out the file. Notice that the list of books is unsorted! We need to organize this using the sort command.
Try cat books, and cat books | sort. The character | is called the pipe. We take the output from cat books and send it through a pipe to sort. The output of cat books becomes the input of sort. Now send the output of sort to a file:
cat books | sort > sorted_books
Look around again to see how the room has changed.
There are dozens of powerful tools we can leverage using pipes. One of the ones you'll be using the most is grep.
cat books | grep Mil
See how we filtered out just the lines that contain Mil? Try grepping for something else.
Now that we have our books sorted, we really don't need our unsorted list of books. mv stands for move, and that's how we move files and folders from place to place.
mv sorted_books books
Look around and see how the room has changed.
To copy files, we use the cp command. Extrapolate from the way we used mv to copy the file bookshelf to add a file second_bookshelf.
rm is short for remove. Use rm to remove the second_bookshelf file we just created with cp.
In interacting with the terminal like we did today, we've been using a programming language called bash, which stands for Bourne-again shell. There are many different shell languages, but the commands we went over today will work in almost any. Another common shell is zsh.
The fact that we interact with the computer by programming empowers us. Instead of struggling to talk about the difference between two menus in a graphical program, we can communicate precisely about shell commands that have a predictable effect.
Sometimes it's important to know how the shell finds the commands that you run. The command which [command-name] will tell you the location of the file which will be run when you execute that command. For example, which rm on my computer gives /bin/rm.
which ruby gives /Users/raphael/.rvm/rubies/ruby-2.0.0-p247/bin/ruby
The shell finds these commands by looking at the PATH variable in the shell. echo $PATH will show you the contents of PATH. It should be a list of directories separated by :. When you run any command, the shell looks in the directories for files that match the name of the command you're trying to run, and executes the first one it finds. Most of you should have a line changing PATH in your ~/.bash_profile file, which is run every time you open a new terminal session (tab or window).
Linux is almost the same as OSX, and this is a great manual about the Linux command line:
The GNU Guide to the Command Line
Here's an interesting anecdote about why Standard Output and Standard Error are separate:
Programs and code are basically all text. We can edit them with any text editor. We want an editor that's easy to use but has a programmer's features. Some document editors are not text editors. For example, Microsoft Word saves files by default in the Microsoft Word Document format which is not text, and we can't view it using cat and grep, and can't manipulate it using a text editor.
We use Sublime Text because it's easy, has been extended to be good for editing a lot of programming languages, and it's made for programmers.
You also might see Vim or other terminal editors, which are good because they don't require a graphical (non-command line) interface to use, so we can use them over the internet to edit files on remote servers, and to edit files in the context of terminal commands.
Files are files, names are just names, file extensions are just a convenience so programs like Sublime Text can be smart about showing the files to you, it doesn't make a difference when it comes to running them.
Programmer's editors have many features that editors like Word do not have, like:
- Auto-reindentation. Edit -> Line -> Reindent in Sublime Text will reindent code to make it more readable.
- Syntax highlighting. Sublime Text will show code in helpful colors to make it more readable.
- Parenthesis matching. Sublime text will show you the matching close parenthesis when you highlight an open parenthesis, and vice versa.
Sublime Text's settings can be adjusted. Be proactive in making your environment comfortable and you will become more productive.