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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.
    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

    A shell script have syntax just like any other programming language. If you have any prior experience with any programming language like Python, C/C++ etc. it would be very easy to get started with it.
    A shell script comprises following elements –

    • Shell Keywords – if, else, break etc.
    • Shell commands – cd, ls, echo, pwd, touch etc.
    • Functions
    • Control flow – if..then..else, case and shell loops etc.

    Why do we need shell scripts

    There are many reasons to write shell scripts –

    • To avoid repetitive work and automation
    • System admins use shell scripting for routine backups
    • System monitoring
    • Adding new functionality to the shell etc.

    Advantages of shell scripts

    • The command and syntax are exactly the same as those directly entered in command line, so programmer do not need to switch to entirely different syntax
    • Writing shell scripts are much quicker
    • Quick start
    • Interactive debugging etc.
Program 1:
Write program to find sum of two numbers
echo "Enter two numbers"
read num1 num2
sum = 'expr $num1 + $num2'
echo "The sum is = $sum"
Program 2:
shell program to multiply two numbers using expr
num1=10
num2=20

ans=`expr $num1 \* $num2`

echo $ans

Output
200

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