Intro

Bash is a text-based shell for controlling your computer (or operating system). Bash is actually an acronym which stands for Bourne-Again SHell. It replaced the Bourne shell and "bashed together" the unix programs sh, csh and ksh. From it you can navigate the files on your computer and execute programs.

You can also connect to other computers and basically do everything you can do in your GUI Operating System (like OS X or Windows).

When you open a terminal, you're basically within your file system, or in a directory, just like you are when you open a Finder window or an Explorer window.

Navigation

Open up command prompt or terminal. Type in: pwd and hit return.

You should see some output describing the directory you are currently within.

/Users/avi

Note: avi is the author's username; your own username will appear here.

That output is describing a location on your computer. You have a file system and within that file system are directories and files.

The pwd command stands for "print working directory".

/User/avi means that I am currently working within a directory /Users on the root of my machine, and then within that directory, a directory named avi.

That's my home directory. It belongs to the user I am currently logged in as. The placeholder for a user's home directory is the ~ ("tilde") symbol.

Try this:

Note: Any time you see the $ character, you shouldn't type it in. This is just a standard way to represent a bash prompt. Yours may or may not be a $.

$ cd ..
$ pwd

You should now see that you are one directory above where you were, in my case

/Users

The cd command stands for "change directory". The .. is a placeholder meaning the directory above the working directory.

Try this:

$ cd .
$ pwd

You can see you are still in the same directory.

The . is a placeholder meaning the current directory.

So here are three default placeholders for your file system:

• ~ Your home directory
• . The current directory
• .. The directory in which your current directory is contained—referred to as the "parent" directory.

You can supply any path to the cd command to navigate to that location.

Try this:

$ ls

You should see a list of all the files within your working directory.

The command ls stands for "list".

Try this:

$ cd /Users/avi
$ pwd

The working directory is back to /Users/avi.

Paths

The path supplied to the cd command, for example /Users/avi, is known as an absolute path.

Systems can use either absolute or relative paths.

An absolute path is a path that points to the same location on the file system regardless of the working directory. They start with / ("forward slash") because that is the root of your file system.

This is an absolute path: /Users/avi.

A relative path is a path relative to the working directory of the user or application, so the full absolute path will not have to be given. They start with the name of a directory or a file.

This is a relative path: avi/Documents.

Paths use / to denote levels.

How many levels are within the following path?

/Users/avi/Development/code/flatiron-school/mixtape-app

• The One True Path
• More on paths - Wikipedia

Knowing where you are in your terminal - what directory you are working in - is very important.

(If you said 6 levels to the question above you are right!!)

Commands

Another cool command you can you use is touch, which simply creates a new file. Try:

$ touch hello_world.rb

Now try:

$ ls

You should see the file you just created, hello_world.rb, in the working directory. Note that this is an empty file and has nothing inside of it, because you just created it.

From within a shell you can also execute programs. Navigate to where you saved your hello_world.rb file and try:

$ ruby hello_world.rb

This command is no different than the cd command. We're executing the ruby program by supplying a path to a file to execute. Because the hello_world.rb file you just created is completely empty and has no contents inside of it, there is no program to run and your terminal won't actually produce any output when you tried running it via ruby hello_world.rb.

Most programs also accept flags, or options for execution.

A flag is denotated by a - ("dash"). Note: In some programs, options are passed directly to the command and not via flags.

A common flag that nearly all programs and commands accept is a standalone h, for "help" or "human".

$ ruby -h

Single-character options can typically be combined with each other. For example, in the ls command, h is a suffix on the l flag meaning "human readable formats." They can be combined with a meaning "all information including permissions". Try these three together:

$ ls -lah

And also:

$ ls -l -a -h

Both are valid input options.

Note: Combining flags is only valid for single-letter options. A "long option" such as --force is defined with more than one character and must be entered with its own flag.

You have a lot of programs and commands available to you. Useful ones include open, cat, and ps.

The open command is interesting because it will trigger the default action associated with the file type. So $ open . will popup a finder window with the current directory in finder (because remember that . is an alias to the current directory). Entering $ open hello_world.rb will open that file in your default editor.

Entering $ cat [file-name] (from "concatenate") reads a file and prints the content to your command line.

Entering $ ps lists the current processes being run by your terminal.

Tab Completion

As you type in commands you can use tab completion. Tab completion allows the shell to be smart and to try and guess what command you want to run when you hit tab. If there's only one logical way to complete your command it will auto populate, or will show you the possibilities and you can keep typing more letters until you can tab complete your command.

For example let's say we have the following directory structure:

/flatiron_school
/flatiron_building

If I'm in my root directory (typing pwd gives me /Users/avi) and I type $ cd f and then hit tab, it will fill in everything up until the conflict so I'll see $ cd flatiron_. If I then add the s and hit tab it will fill in $ cd flatiron_school and I can hit enter.

Login Routine

Every time you open your terminal or a new tab, you are relogging into your shell. Your system has a login routine of things to do when you login. One of the things it does is read a file called .bash_profile. Try this:

$ cd ~

(This command moves you to your home directory). Then type:

$ ls -lah

You may or may not see a .bash_profile file listed. If not, don't worry. We'll make one and add some cool stuff to it later.

Hidden Files

When you entered $ ls -lah above, you should have a received a list of files including some that you hadn't seen from entering just $ ls before:

drwxr-xr-x   6 avi  staff   204B Jun  2 11:21 .
drwxr-xr-x   5 avi  staff   170B May 28 15:52 ..
-rw-r--r--@  1 avi  staff   6.0K May 28 15:52 .DS_Store
drwxr-xr-x  13 avi  staff   442B Jun  2 11:02 .git
-rw-r--r--   1 avi  staff    66B May 28 15:49 .learn
-rw-r--r--   1 avi  staff    11K Jun  2 11:21 README.md

Notice that at the top of the file output there are a bunch of files that start with a ., like .DS_Store

But try $ open . and just $ ls, those files, like .DS_Store, are not listed. That's because files that start with a . are hidden files. Your .bash_profile is a hidden file in your home directory. If you want to see the hidden files you can add the a flag to ls by typing $ ls -a.

If you're interested in where this convention came from check out The history of hidden files .

PATH and Environment Variables

You may be wondering what the computer is actually doing when you type a command at the command line. It's running an executable program. But how does the computer know what to do when I type $ ls whatever? The PATH variable gives the computer an ordered list of directories to search in to find an executable with the name you typed. In our case, it's going to search for the ls executable. If you type $ ls /bin, you'll see that this is an actual program or "binary" which is why it's usually found in the bin directory (short for "binary"). If you're trying to run a ruby program and typing $ ruby myprogram.rb the computer goes through all the files in the path until it finds an executable called ruby and then runs that code with the provided argument (myprogram.rb). If you type $ echo $PATH you can see what your path is. If you're using RVM (if you don't know what this is at the moment don't worry), it will look something like this:

/Users/blake/.rvm/gems/ruby-1.9.3-p392/bin:/Users/blake/.rvm/gems/ruby-1.9.3-p392@global/bin:/Users/blake/.rvm/rubies/ruby-1.9.3-p392/bin:/Users/blake/.rvm/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/bin:/usr/local/sbin:/Users/blake/bin:/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin:/Applications/Xcode.app/Contents/Developer/usr/bin

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Each directory in the path is separated by the : ("colon") symbol.

If you ever get errors where you type something in the terminal and it says it can't be found, the executable you're trying to run needs to be added to the path. If the wrong executable is getting run, the order of directories in your path is wrong.

The PATH variable is an environmental variable. These are variables you can set specific to your computer's environment which can then be used in other programs. For example, in Ruby you can type ENV[name_of_variable] to access an environmental variable. These are typically set in your bash profile, in a bash script, or at the command line.

Tip: If you want to find out where the program being run is located when you type a command at the command line, use the which command; entering $ which ruby will tell you where the Ruby binary is located.

Piping "|"

Piping, a verb derived from the | symbol named "pipe", will send the output of one command into the input of another command. The most common command you'll probably use is piping the process list to grep to search for a running program.

$ ps aux | grep ruby

This would run the ps command with the a, u, and x options and send the output of that to grep (a search utility), which would then search for the term "ruby". You'll notice that ps is one of the commands that only accepts options without a flag. Try:

$ ps -aux

If you're on OS X, you should have gotten an error — something like ps: No user named 'x'. Just keep in mind which commands take options with a flag (-) and which take options without a flag.

Manual

If you're curious what the options on ps mean, enter:

$ man ps

Read about what the a and u options do. Notice that the x option is a suffix on the a option. The man ("manual") command reveals very useful reference documentation on the various bash commands. You'll notice that your command prompt has disappeared. Don't panic! You're just inside the documentation. Enter $ q ("quit") to return to your command prompt.

More Resources:

• Lifehacker on the Command Line
• Environment Variables
• Built-in Shell Commands Very useful
• 15 Useful Bash Commands

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