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What is the the difference between "Shell" and "Bash" and what do these terms mean?

As far as I know, there is no difference. But I've seen many books about "Shell" and others about "Bash"!

So in case I want to work with the Terminal on Mac OS X and write some bash scripts, I'm wondering which kind of books I should go for.

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It is a class versus instance distinction. –  Kaz Oct 15 '12 at 0:44
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Interesting reading material: unix.stackexchange.com/questions/4126/… –  Bernhard Oct 15 '12 at 6:32
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the first shell was written by a guy called Bourne. BASH is an acronym for 'Boune Again Shell'. It is important to have fun! –  Kinjal Dixit Oct 15 '12 at 8:10

6 Answers 6

A "shell" is any software that provides an interface to an operating system. For instance, explorer.exe is the default shell in Windows (though alternatives exist), and on OS X Finder provides much of the same functionality. On Linux/*nix, the shell could be part of the desktop environment (like Gnome or KDE), or can be a separate software component sitting on top of it (like Unity or Cinnamon).

The above examples are all graphical shells that use a combination of windows, menus, icons and other such elements to provide a graphical user interface (GUI) that can be interacted with using the mouse cursor. However, in the context of software like Bash, or writing scripts, "shell" is usually taken to mean a command-line interpreter, which performs largely the same duties as a graphical shell, except is entirely text-based.

Bash is a specific example of a command-line shell, and is probably one of the most well-known ones, being the default in many Linux distributions as well as OS X. It was designed as a replacement for the Bourne shell (Bash stands for "Bourne again shell"), one of the first Unix shells.

Examples of command-line shells on Windows include cmd.exe (aka Command Prompt) and PowerShell.

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Finder isn't usually called a shell on OS X. Most of the GUI and window management features are handled by other processes. –  Lri Oct 15 '12 at 2:42
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@LauriRanta Wikipedia seems to disagree. Regardless of whether or not it has help from other processes, its job is to enable the user to interact with the underlying OS through a GUI, and thus it fits the description of a graphical shell. –  Indrek Oct 15 '12 at 2:54
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I guess the term graphical shell can also apply to file management applications. But it's not generally used on OS X, and Finder is more like Nautilus than the Windows shell or Unity. –  Lri Oct 15 '12 at 3:29
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@LauriRanta: quick quiz, what's the icon of Nautilus? –  Lie Ryan Oct 15 '12 at 5:56
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@LauriRanta Wouldn't Dock be a better candidate for a shell? Launching programs and opening documents, Expose/Spaces/Mission Control, AFAIK Launchpad and the task switcher as well are all Dock features. –  Daniel Beck Oct 15 '12 at 6:37

Bash is one of several shells.

A shell on a Unix or Unix-like system like OSX or Linux is a application program that provides a command-line interface to the operating system, allowing you to type commands and run them. There are a number of different shells to choose from but they all provide filename wildcarding, piping, here documents, command substitution, variables and control structures for condition-testing and iteration.

The original Unix shell was the Bourne shell, sh, written by Stephen Bourne at Bell Labs. Then came the C shell, written by Bill Joy at Berkeley, since updated as tcsh. Other shells include the Korn shell, ksh, written by David Korn, also at Bell Labs, and bash, the "Bourne again shell", written by Brian Fox for the GNU project as a free replacement for sh.

Today, bash is probably the most popular Unix shell but a lot of people (me included) still prefer the C shell based on (what to some of us seems like) its nicer syntax. Basically, it's a matter of taste, so I recommend reading the Wikipedia articles I've linked to help you get started.

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Shells are not necessarily command-line based, there are many graphical shells as well, e.g. Nautilus, Windows Explorer, Finder, etc. The essential point for shell is it is a wrapper/user interface/shell around core OS functionalities/system calls, i.e. shells provides management of resources (e.g. file management) and management of processes. –  Lie Ryan Oct 15 '12 at 14:13
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@LieRyan those are not shells, they are file managers. Shells allow integrated command/script interpreters to run within them. See my answer. Just being able to run a program from a file manager doesn't make it a shell. –  BillR Oct 17 '12 at 22:08
    
@BillR: You ought to inform the Gnome Shell guys of your definition. –  paradroid Oct 19 '12 at 22:31
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I'm inclined to agree with Lie Ryan and paradroid, that graphical shells are now considered shells. I'm as invested in the command line paradigm as anyone and resisted accepting graphical shells as shells through the 90s. But with all the control panel widgets and whatnot in a modern graphical shell, I now agree the common usage is to call them shells and that that usage is correct. They match the definition of a shell, a relatively thin user interface layer around the underlying OS. Graphical shells are weaker on how anyone would script anything but lots of users don't care about that. –  Nicole Hamilton Oct 19 '12 at 22:53

The term 'shell' is well named. It is literally a shell around the O/S allowing the user to interact with the computer. When it was originally conceived there were very little if any graphical user interfaces (no windows :( ). Everything was done on the command line. But even the command line needed a place to live. It lived, and still does, in a shell.

In simple terms, in order for the command line to be useful it needed instructions that it could call. So programs were made to run inside the shell for the command line to use. The programs were grouped tightly in their own packages, and intended to work together. They include programs like "ls" and "grep", "ps", "sed", etc. They also include file redirection commands like ">" and "<" and pipes ("|"). More importantly they also include programming constructs like conditional operations (if, then, else, for loops, while loops, ways to check the status returned when you run a statement (e.g. if you run "ls" did it find anything?), thing like that). These are the foundations of more complex command line (shell) scripts, and in fact is what the command line interpreter is.

When someone uses the term 'Bash Shell' they are talking about a command line interpreter called 'Bash' that runs in the O/S shell. You could think of it as being short for 'Bash Shell Interpreter'. There are other interpreters like Bourne (Bash is a 'new and improved Bourne Shell and it is short for Bourne Again Shell). There is also the C-Shell, the K-Shell (favoured by many who write complex shell scripts), and other GNU variants. Over the years it has become customary to refer to the particular command line interpreter you are using as the shell because one can't be used without the other. But the reality is that they are different.

As to why they are properly known as command line interpreters and not as the actual shell: it is because they live in the shell and interpret all commands as if they were running in a program. And the shell doesn't care what interpreter you run in it, as long as it meets the correct standards.

And as to why they are called interpreters, it is because they are really interpreters. Even if you are not explicitly running a script (and a script is really just a text file of commands you create so that you can execute the same commands over and over without having to type them in again). For example, take the humble 'ls' command. When you run it, it returns a list of files. But how it runs is more important to your question: It actually runs inside the context of the command line interpreter, even if you just run what appears to be a simple one off command. That is, it runs as if it were a statement in part of a bigger program. It runs as if it were in a shell script script file without actually being in a shell script file. An anonymous shell script file as it were.

Anything you run on the command line has this in common (whether it be a single command like 'ls' or a script file full of commands and iterators and conditional statements): it is all processed by the command line interpreter; be it Bash, C-Shell, K-Shell (default on AIX btw).

To see what I mean, make a directory 'test':

mkdir test

Enter it and run the folowing commands

grep hello * 

You will get some sort of response like 'no such file or directory'. Now enter the command

echo $?

($? says, tell me what you found in cryptic computer speak.) You should see it return a number (it should be) '2'. That is the return code from grep that means 'no such file or directory'. Now run the following:

echo hello > hello.txt
grep hello *
echo $?

You will see the file 'hello.txt' returned from the initial grep command and should now see the 'echo $?' return the number '0' meaning it actually found something.

Even if these seemingly one off commands are run, the command line interpreter acts like they are part of a larger program and keeps track of their return values. That is why if you forget the * at the end of the grep command it doesn't return. It is knows the statement is incomplete and expects more input. After all you could conceivably be going to ask it to grep the results of some loop which is perfectly legal to write and run on the command line.

Bottom line is the shell is the shell, and the interpreter (whatever the name of the one you use, 'Bash', k-shell, etc.) are different. But often they are used interchangeably, because at any given instant they are completely tied together.

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bash is one of the shell family, but there's plenty of other shells.

By example, on Minix3, there's the ash shell, it doesn't support associative arrays like bash4.

The POSIX standard is an attempt to make a portable API between differents shells and OSes.

See http://en.wikipedia.org/wiki/Unix_shell#Bourne_shell_compatible

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bash is one of the many shells that exist.

All shells have their similarities and differences. For example a script written in bash, might be fully or largely compatible with another shell (for example zsh).

Due to the fact that bash is very widespread, it is often implied that a script is compatible with it.

If you're looking to buy a book, buy one written specifically for the shell you intend to use. It would be a good idea to read up on their differences before spending money though.

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Shell is a text based user interface.

Bash is a type of shell.

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What is a "test based user interface"? Also it would be nice to expand your answer a little.. As you can see from the other answers, it always helps to give more context. –  slhck Oct 18 '12 at 15:19
    
Sorry. Text-based user interface. How would you like for me to expound upon that? A shell is your command line way of interacting with the operating systems kernel. The Shell interprets your commands and relays them to the machine which then, in turn, performs the necessary operations relevant to the command you've given? Without a 500 page manual it's difficult to effectively expound on this ambiguous question. The effort of the answer fit the effort of the question. This question is apples to oranges. –  HayekSplosives Oct 18 '12 at 15:20
    
If you think a question shows now effort that doesn't automatically mean you shouldn't put effort in an answer to it. Your answer, in comparison to the others, lacks a little detail. Of course it's up to you to add that. –  slhck Oct 18 '12 at 16:13
    
I appreciate the correction and lesson in personal responsibility. Thank you for protecting those who aren't willing to put in the effort from those who do. I am totally out of line forgetting how much I owed the poster. –  HayekSplosives Oct 19 '12 at 17:53

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