Unix - Getting Started

What is Unix ?

The Unix operating system is a set of programs that act as a link between the computer and the user.
The computer programs that allocate the system resources and coordinate all the details of the computer's internals is called the operating system or the kernel.
Users communicate with the kernel through a program known as the shell. The shell is a command line interpreter; it translates commands entered by the user and converts them into a language that is understood by the kernel.

Unix - File Management

In this chapter, we will discuss in detail about file management in Unix. All data in Unix is organized into files. All files are organized into directories. These directories are organized into a tree-like structure called the filesystem.
When you work with Unix, one way or another, you spend most of your time working with files. This tutorial will help you understand how to create and remove files, copy and rename them, create links to them, etc.
In Unix, there are three basic types of files −

Unix - File Permission / Access Modes

In this chapter, we will discuss in detail about file permission and access modes in Unix. File ownership is an important component of Unix that provides a secure method for storing files. Every file in Unix has the following attributes −

• Owner permissions − The owner's permissions determine what actions the owner of the file can perform on the file.
  

Unix - Environment

In this chapter, we will discuss in detail about the Unix environment. An important Unix concept is the environment, which is defined by environment variables. Some are set by the system, others by you, yet others by the shell, or any program that loads another program.
A variable is a character string to which we assign a value. The value assigned could be a number, text, filename, device, or any other type of data.

Unix Basic Utilities - Printing, Email

In this chapter, we will discuss in detail about Printing and Email as the basic utilities of Unix. So far, we have tried to understand the Unix OS and the nature of its basic commands. In this chapter, we will learn some important Unix utilities that can be used in our day-to-day life.

Unix - Pipes and Filters

In this chapter, we will discuss in detail about pipes and filters in Unix. You can connect two commands together so that the output from one program becomes the input of the next program. Two or more commands connected in this way form a pipe.
To make a pipe, put a vertical bar (|) on the command line between two commands.
When a program takes its input from another program, it performs some operation on that input, and writes the result to the standard output. It is referred to as a filter.

Unix - Processes Management

In this chapter, we will discuss in detail about process management in Unix. When you execute a program on your Unix system, the system creates a special environment for that program. This environment contains everything needed for the system to run the program as if no other program were running on the system.

Unix - Network Communication Utilities

In this chapter, we will discuss in detail about network communication utilities in Unix. When you work in a distributed environment, you need to communicate with remote users and you also need to access remote Unix machines.
There are several Unix utilities that help users compute in a networked, distributed environment. This chapter lists a few of them.

Unix - The vi Editor Tutorial

In this chapter, we will understand how the vi Editor works in Unix. There are many ways to edit files in Unix. Editing files using the screen-oriented text editor vi is one of the best ways. This editor enables you to edit lines in context with other lines in the file.
An improved version of the vi editor which is called the VIM has also been made available now. Here, VIM stands for Vi IMproved.

Unix - What is Shells?

A Shell provides you with an interface to the Unix system. It gathers input from you and executes programs based on that input. When a program finishes executing, it displays that program's output.
Shell is an environment in which we can run our commands, programs, and shell scripts. There are different flavors of a shell, just as there are different flavors of operating systems. Each flavor of shell has its own set of recognized commands and functions.

Unix - Using Shell Variables

In this chapter, we will learn how to use Shell variables in Unix. A variable is a character string to which we assign a value. The value assigned could be a number, text, filename, device, or any other type of data.
A variable is nothing more than a pointer to the actual data. The shell enables you to create, assign, and delete variables.

Unix - Special Variables

In this chapter, we will discuss in detail about special variable in Unix. In one of our previous chapters, we understood how to be careful when we use certain nonalphanumeric characters in variable names. This is because those characters are used in the names of special Unix variables. These variables are reserved for specific functions.

Unix - Using Shell Arrays

In this chapter, we will discuss how to use shell arrays in Unix. A shell variable is capable enough to hold a single value. These variables are called scalar variables.
Shell supports a different type of variable called an array variable. This can hold multiple values at the same time. Arrays provide a method of grouping a set of variables. Instead of creating a new name for each variable that is required, you can use a single array variable that stores all the other variables.
All the naming rules discussed for Shell Variables would be applicable while naming arrays.

Unix - Shell Basic Operators

There are various operators supported by each shell. We will discuss in detail about Bourne shell (default shell) in this chapter.
We will now discuss the following operators −

• Arithmetic Operators
• Relational Operators
• Boolean Operators
• String Operators
• File Test Operators

Unix - Shell Decision Making

In this chapter, we will understand shell decision-making in Unix. While writing a shell script, there may be a situation when you need to adopt one path out of the given two paths. So you need to make use of conditional statements that allow your program to make correct decisions and perform the right actions.
Unix Shell supports conditional statements which are used to perform different actions based on different conditions. We will now understand two decision-making statements here −

Unix - Shell Loop Types

In this chapter, we will discuss shell loops in Unix. A loop is a powerful programming tool that enables you to execute a set of commands repeatedly. In this chapter, we will examine the following types of loops available to shell programmers −

• The while loop
• The for loop
• The until loop
• The select loop

Unix - Shell Loop Control

In this chapter, we will discuss shell loop control in Unix. So far you have looked at creating loops and working with loops to accomplish different tasks. Sometimes you need to stop a loop or skip iterations of the loop.
In this chapter, we will learn following two statements that are used to control shell loops−

• The break statement
• The continue statement
  

Unix - Shell Substitution

What is Substitution?

The shell performs substitution when it encounters an expression that contains one or more special characters.

Example

Here, the printing value of the variable is substituted by its value. Same time, "\n" is substituted by a new line −

#!/bin/sh

Unix - Shell Quoting Mechanisms

In this chapter, we will discuss in detail about the Shell quoting mechanisms. We will start by discussing the metacharacters.

The Metacharacters

Unix Shell provides various metacharacters which have special meaning while using them in any Shell Script and causes termination of a word unless quoted.

Unix - Shell Input/Output Redirections

In this chapter, we will discuss in detail about the Shell input/output redirections. Most Unix system commands take input from your terminal and send the resulting output back to your terminal. A command normally reads its input from the standard input, which happens to be your terminal by default. Similarly, a command normally writes its output to standard output, which is again your terminal by default.