Shell script - 1

Shell script - 1:

          This tutorial will give an over view and a good idea how to write shell script, I think speaking of history like other tutorials is mere waste so I directly starting with details.

If you need any reference or if you are not aware of UNIX basic commands please go through it by clicking here it will be so useful.

#! The she bang

            All shell scripts start with this #! Line this is called she bang it tell the what shell script we are going to use and its location(interpreter location)

            Example 1:

                        #!/bash/sh

            For further reference refer http://en.wikipedia.org/wiki/Shebang_(Unix)

# Comment

            The # is used for commenting the a line

            Example 2:

                        #Adding two numbers

Variables:

            Variable name can contain only, (a to z, A to Z, 0 to 9 and _)

Defining and Assigning values:

            An variable can be defined and assigned as follows

                        Var1=10

                        Var2=”jai”

Variable Types:

When a shell is running, three main types of variables are present:

Local Variables: A local variable is a variable that is present within the current instance of the shell. It is not available to programs that are started by the shell. They are set at command prompt.

Environment Variables: An environment variable is a variable that is available to any child process of the shell. Some programs need environment variables in order to function correctly. Usually a shell script defines only those environment variables that are needed by the programs that it runs.

Shell Variables: A shell variable is a special variable that is set by the shell and is required by the shell in order to function correctly. Some of these variables are environment variables whereas others are local variables.

Command line arguments and some special symbols with $

Example program 3:

#!/bin/sh

echo "file name: $0"

echo "first argument: $1"

echo "Second argument: $2"

echo "Number of arguments: $#"

echo "All the arguments individually double quoted: $@"

echo "Process number of previous command:$!"

echo "Process number of CURRENT SHELL: $$"

echo "All the arguments double quoted: $*"

Out put:

Testhome/l685360# ./test jaya chandran

file name: ./test

first argument: jaya

Second argument: chandran

Number of arguments: 2

Given argument is: jaya chandran

Process number of previous command:

Process number of CURRENT SHELL: 20775420

All the arguments: jaya chandran

Testhome/l685360#

Arrays:

Example 4:

#!/bin/sh

name[0]="jayachandran"

name[1]="Mohan"

name[2]="Devi"

name[3]="Shiva"

name[4]="Ranjith"

echo ${name[@]}

echo ${name[*]}

Output

Testhome/l685360# ./test

Mohan Devi Shiva Ranjith

Mohan Devi Shiva Ranjith

Testhome/l685360#

Arithmetic and Relation operators:

The bourne shell does not do any arithmetic it uses external programs expr and awk to the arithmetic

expr

Example 5:

#!/bin/sh

val1=2

val1=`expr $val1 + 10`

echo "value of val1 aft adding 10 is : $val1"

            Out put:

                        Testhome/l685360# ./test

value of val1 aft adding 10 is : 12

Testhome/l685360#

In the above program we can see usage of expr

1.                          Any arithmetic expression with symbols should be separated by space.( $val1 + 10 )

2.                          Any expr should be given in between `` only.

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