91 lines
4.1 KiB
Markdown
91 lines
4.1 KiB
Markdown
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---
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tags:
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- shell
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---
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# Introduction to the Unix shell
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## Unix based systems
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Many operating systems are based on the UNIX software architecture. macOS/OSX
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(Darwin) and GNU/Linux are two very popular examples. Most web servers run a
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version of Linux as their native OS thus a knowledge of command line for UNIX
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systems is invaluable for web developers. Windows systems are not UNIX based so
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everything that is written here is not applicable to the Windows command line
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program Command Prompt although there are obvious conceptual overlaps.
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UNIX was initially developed by AT&T and is still owned by them. GNU (the basis
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for Linux) is a recursive acronym for ’GNU is not UNIX’. Functionally it is the
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same as Unix however, it just doesn’t contain the proprietary code.
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## Key terms
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### Kernal
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The kernel is the central part of an operating system. It manages the operations
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of the computer and the hardware; most notably memory and CPU time. There are
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two types of kernels: a **micro-kernel**, which only contains basic
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functionality; and a **monolithic kernel**, which contains many device drivers.
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### Shell
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A shell is a user interface for access to an operating system’s services. In
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general, operating system shells use either a command-line interface or GUI,
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depending on a computer’s role and particular operation. It is named a shell
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because it is the outermost layer around the operating system kernel
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### Bash
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Bash is a Unix shell and command language written by Brian Fox for the GNU
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Project as a free software replacement for the Bourne shell. First released in
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1989, it has been distributed widely as the default login shell for most Linux
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distributions and Apple’s macOS.
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## What is the command line?
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Command line is both a general computing concept and a specific utility. In its
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general sense, engaging with a computer via command line means inputting
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abbreviated written commands into a console or terminal window on a computer.
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The computer then runs these commands on the basis of the instructions you have
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given.
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You can get the computer to ‘do things’ (make files, connect to networks etc)
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via command line and you can use it to check the status of systems within your
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computer.
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This mode of engagement contrasts with the engagement facilitated by modern
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graphical user interfaces (GUIs) where the user navigates via a combination of
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keyboard and mouse inputs and interacts with windows whose functionality is
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usually self-explanatory and therefore more user-friendly. Command line predates
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GUIs but it is considered a more efficient and expansive means of interacting
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with the whole of your computer especially if you are a developer or programmer.
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In the more specific sense, the ‘command line’ refers to the location in the
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console window where you input your commands. The command line consists in a
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user name and the current directory you are in followed by a dollar sign and a
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cursor waiting for input.
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## Basic orientation
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When you are using command line you are always situated somewhere within a file
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system. Typically this will be your own user files. directory structure By
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default you are always in your home directory, this is represented by the tilda
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symbol: `~`. This is why you see the tilda symbol as part of the command line.
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In command line, we do not use the term ‘folders’, instead they are called
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directories. (I will follow this convention from hereon.) You do not have to
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worry too much about remembering the names of specific folders and files you can
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always ask the computer to display them to you by using `ls` argument or `pwd`.
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## Command line syntax
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The syntax of the language used to input commands is analogous to the grammar of
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natural languages. We have a verb that is operative on an object/noun and which
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can be modified through adverbs. The syntax of a command sequence is as follows:
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1. Command (verb): what we want to do
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1. Option (adverb): modifying the command - always starts with a hyphen
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1. Argument (noun/object): what we want our command to operate on
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We will see that not all commands require arguments, but this is the general
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structure
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