WHAT IS SHELL SCRIPTING AND WHEN TO USE SHELL SCRIPTS

 1.SHELL:

SHELL IS COMMAND LINE INTERPRETER.IT TAKES COMMANDS FROM USER AND EXECUTES THEM.IT IS AN INTERFACE BETWEEN USER AND KERNEL.

THE THREE MOST WIDELY USED UNIX SHELL ARE BOURNE SHELL .KORN SHELL AND C SHELL.

SHELL:

BOURNE SHELL

DEVELOPED BY:

STEVE BOURNE

SHELL PROMPT:

$

EXECUTION COMMAND:

SH

SHELL:

KORN SHELL

DEVELOPED BY:

DAVID KORN

SHELL PROMPT:

$

EXECUTION COMMAND:

KSH

SHELL:

C SHELL

DEVELOPED BY:

BILL JOY,CALIFORNIA UNIVERSITY STUDENT

SHELL PROMPT:

%

EXECUTION COMMAND:

CSH

EACH SHELL HAS MERITS AND DEMERITS OF ITS OWN,MOREOVER THE SHELL SCRIPTS WRITTEN FOR ONE SHELL MAY NOT WORK WITH THE OTHER SHELL.THIS IS BECAUSE DIFFERENT SHELL USE DIFFERENT MECHANISMS TO EXECUTE THE COMMANDS IN THE SHELL SCRIPT.BOURNE SHELL SINCE IT IS ONE OF THE MOST WIDELY USED UNIX SHELL IN EXISTENCE TODAY.

ALMOST ALL UNIX IMPLEMENTATIONS OFFER THE BOURNE SHELL AS PART OF THEIR STANDARD CONFIGURATION.IT IS SMALLER THAN THE OTHER TWO SHELLS AND THEREFORE MORE EFFICIENT FOR MOST SHELL PROCESSING.HOWEVER ,IT LACKS FEATURES OFFERED BY THE C AND THE KORN SHELL.

ALL SHELL PROGRAMS WRITTEN FOR THE BOURNE SHELL ARE LIKELY TO WORK WITH THE KORN SHELL.THE REVERSE HOWEVER MAY NOT BE TRUE,THIS IS SO SINCE THE FACILITIES LIKE ARRAYS,COMMAND ALIASING AND HISTORY MECHANISM AVAILABLE IN THE KORN SHELL ARE NOT SUPPORTED BY THE BOURNE SHELL.

THE C SHELL PROGRAMMING LANGUAGE RESEMBLES THE C LANGUAGE AND IS QUITE DIFFERENT FROM THE LANGUAGE OF THE BOURNE SHELL.ONLY THE VERY BASIC SHELL SCRIPTS WILL RUN UNDER BOTH THE C AND BOURNE SHELL; A VAST MAJORITY WILL NOT SHELL KEEPS TRACK OF COMMANDS AS YOU ENTER THEM AND ALLOWS YOU TO GO BACK AND EXECUTE THEM AGAIN WITHOUT TYPING THE COMMANDS OR IF YOU WANT TO,YOU CAN RECALL THEM,MAKE MODIFICATIONS,AND THEN EXECUTE THE NEW COMMAND.

A SHELL PROGRAM IS NOTHING BUT A SERIES OF SUCH COMMANDS.INSTEAD OF SPECIFYING ONE JOB AT ATIME,WE GIVE THE SHELL A TO-DO LIST A PROGRAM THAT CARRIES OUT AN ENTIRE PROCEDURE.SUCH PROGRAMS ARE KNOWN AS 'SHELL SCRIPTS'.

WHEN TO USE SHELL SCRIPTS:

1.CUSTOMIZING YOUR WORK ENVIRONMENTS.FOR EXAMPLE,EVERY TIME YOU LOG IN IF YOU WANT TO SEE THE CURRENT DATE, A WELCOME MESSAGE AND THE LIST OF USERS WHO HAVE LOGGED IN YOU CAN WRITE A SHELL SCRIPT FOR THE SAME.

2.AUTOMATING YOUR DAILY TASKS.FOR EXAMPLE,YOU A MAY WANT TO BACK UP ALL YOUR PROGRAMS AT THE END OF DAY.THIS CAN BE DONE USING A SHELL  SCRIPT.

3.AUTOMATING REPETITIVE TASKS.FOR EXAMPLE,THE REPETITIVE TASK OF COMPILING A C PROGRAM,LINKING IT WITH SOME LIBEARIES AND EXECUTING THE EXECUTABLE CODE CAN BE ASSIGNED TO A SHELL SCRIPT.

4.EXECUTING IMPORTANT SYSTEM PROCEDURES LIKE SHUTTING DOWN THE SYSTEM,FORMATTING A DISK,CREATING A FILE SYSTEM,MOUNTING THE FILE SYSTEM,LETTING THE USER THE FLOPPY AND FINALLY UNMOUNTING THE DISK.

5.PERFORMING SAME OPERATION ON MANY FILES,OR EXAMPLE ,YOU MAY WANT TO REPLACE A STRING PRINTF WITH A STRING MY PRINT IN ALL THE C PROGRAMS PRESENT IN A DIRECTORY.