1. Shell programming.

Shell Programming:

Usually shells are interactive that mean, they accept command as input from users and execute them. However some time we want to execute a bunch of commands routinely, so we have type in all commands each time in terminal.

A shell program, sometimes referred to as a shell script, is simply a program constructed of shell commands.

Shell programs are interpreted each time they are run. This means each command is processed (i.e. executed) by the shell a single line at a time.

This is different from languages such as C or C++, which are translated in their entirety by a compiler program into a binary image.

A shell program may be simple and consist of just a few shell commands, or it may be very complex and consist of thousands of shell commands.

As shell can also take commands as input from file we can write these commands in a file and can execute them in shell to avoid this repetitive work. These files are called Shell Scripts or Shell Programs. Shell scripts are similar to the batch file in MS-DOS. Each shell script is saved with .sh file extension eg. myscript.sh

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 # 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.

It would be a simple text file in which we would put all our commands and several other required constructs that tell the shell environment what to do and when to do it.

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

Example Script

Assume we create a test.sh script.

Note all the scripts would have the .sh extension. Before you add anything else to your script, you need to alert the system that a shell script is being started.

• A shell script contains a list of commands which have to be executed regularly.
• Shell script is also known as shell program.
• The user can execute the shell script itself to execute commands in it.
• A shell script runs in interpretive mode. i.e. the entire script is compiled internally in memory and then executed.
• Hence, shell scripts run slower than the high-level language programs.
• ".sh" is used as an extension for shell scripts.

Example: A shell script (program1.sh) to execute few commands.

#! /bin/sh

echo “Welcome to Shell Programming”         # print message

echo “Today’s date : `date`”                # print date

The hash symbol # indicates the comments in the script.

The first line "#! /bin/sh" indicates the path where the shell script is available.

The shell ignores all the characters that follow the # symbol. However, this does not apply to the first line.