Unix - What is Shells?

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The 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.

A shell is an environment in which we can run our commands, programs, and shell scripts. There are different flavors of shells, just as there are different flavors of operating systems. Each flavor of shell has its own set of recognized commands and functions.

Shell Prompt:

The prompt, $, which is called command prompt, is issued by the shell. While the prompt is displayed, you can type a command.

The shell reads your input after you press Enter. It determines the command you want executed by looking at the first word of your input. A word is an unbroken set of characters. Spaces and tabs separate words.

Following is a simple example of date command which displays current date and time:

$date
Thu Jun 25 08:30:19 MST 2009

You can customize your command prompt using environment variable PS1 explained in Environment tutorial.

Shell Types:

In UNIX there are two major types of shells:

  1. The Bourne shell. If you are using a Bourne-type shell, the default prompt is the $ character.

  2. The C shell. If you are using a C-type shell, the default prompt is the % character.

There are again various subcategories for Bourne Shell which are listed as follows:

  • Bourne shell ( sh)

  • Korn shell ( ksh)

  • Bourne Again shell ( bash)

  • POSIX shell ( sh)

The different C-type shells follow:

  • C shell ( csh)

  • TENEX/TOPS C shell ( tcsh)

The original UNIX shell was written in the mid-1970s by Stephen R. Bourne while he was at AT&T Bell Labs in New Jersey.

The Bourne shell was the first shell to appear on UNIX systems, thus it is referred to as "the shell".

The Bourne shell is usually installed as /bin/sh on most versions of UNIX. For this reason, it is the shell of choice for writing scripts to use on several different versions of UNIX.

In this tutorial, we are going to cover most of the Shell concepts based on Borne Shell.

Shell Scripts:

The basic concept of a shell script is a list of commands, which are listed in the order of execution. A good shell script will have comments, preceded by a pound sign, #, describing the steps.

There are conditional tests, such as value A is greater than value B, loops allowing us to go through massive amounts of data, files to read and store data, and variables to read and store data, and the script may include functions.

Shell scripts and functions are both interpreted. This means they are not compiled.

We are going to write a many scripts in the next several tutorials. This would be a simple text file in which we would put our all the commands and several other required constructs that tell the shell environment what to do and when to do it.

Example Script:

Assume we create a test.sh script. Note all the scripts would have .sh extension. Before you add anything else to your script, you need to alert the system that a shell script is being started. This is done using the shebang construct. For example:

#!/bin/sh

This tells the system that the commands that follow are to be executed by the Bourne shell. It's called a shebang because the # symbol is called a hash, and the ! symbol is called a bang.

To create a script containing these commands, you put the shebang line first and then add the commands:

#!/bin/bash
pwd
ls

Shell Comments:

You can put your comments in your script as follows:

#!/bin/bash

# Author : Zara Ali
# Copyright (c) Tutorialspoint.com
# Script follows here:
pwd
ls

Now you save the above content and make this script executable as follows:

$chmod +x test.sh

Now you have your shell script ready to be executed as follows:

$./test.sh

This would produce following result:

/home/amrood
index.htm  unix-basic_utilities.htm  unix-directories.htm  
test.sh    unix-communication.htm    unix-environment.htm

Note: To execute your any program available in current directory you would execute using ./program_name

Extended Shell Scripts:

Shell scripts have several required constructs that tell the shell environment what to do and when to do it. Of course, most scripts are more complex than above one.

The shell is, after all, a real programming language, complete with variables, control structures, and so forth. No matter how complicated a script gets, however, it is still just a list of commands executed sequentially.

Following script use the read command which takes the input from the keyboard and assigns it as the value of the variable PERSON and finally prints it on STDOUT.

#!/bin/sh

# Author : Zara Ali
# Copyright (c) Tutorialspoint.com
# Script follows here:

echo "What is your name?"
read PERSON
echo "Hello, $PERSON"

Here is sample run of the script:

$./test.sh
What is your name?
Zara Ali
Hello, Zara Ali
$


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