If you want to assign the value of var to the variable lcount, you can do that as follows:

lcount=$var                   pdksh and bash







set lcount = $var             tcsh

Positional Parameters

It is possible to write a shell script that takes a number of parameters at the time you invoke it from the command line or from another shell script. These options are supplied to the shell program by Linux as positional parameters. The positional parameters have special names provided by the system. The first parameter is stored in a variable called 1 (number 1) and can be accessed by using $1 within the program. The second parameter is stored in a variable called 2 and can be accessed by using $2 within the program, and so on. It is possible to omit one or more of the higher numbered positional parameters while invoking a shell program. For example, if a shell program mypgm expects two parameters—such as a first name and a last name—then you can invoke the shell program with only one parameter, the first name. However, you cannot invoke it with only the second parameter, the last name.

Here's a shell program called mypgm1, which takes only one parameter (a name) and displays it on the screen:

#Name display program

if [ $# == 0]

then

   echo "Name not provided"

else

   echo "Your name is "$1

In pdksh and bash, if you execute mypgm1 as follows:

. mypgm1

you will get the output:

Name not provided

However, if you execute mypgm1 as follows:

. mypgm1 Sanjiv

then you will get the following output:

Your name is Sanjiv

The shell program mypgm1 also illustrates another aspect of shell programming, the built-in variables. In mypgm1, the variable $# is a built-in variable and provides the number of positional parameters passed to the shell program.

Built-in Variables

Built-in variables are a special type of variable that Linux provides to you. These variables can be used to make decisions within a program. You cannot modify the values of these variables within the shell program.

Some of these variables are

To show these built-in variables in use, here is a sample program called mypgm2:

#my test program

echo "Number of parameters is "$#

echo "Program name is "$0

echo "Parameters as a single string is "$*

In pdksh and bash, if you execute mypgm2 from the command line, as follows:

. mypgm2 Sanjiv Guha

you will get the following result:

Number of parameters is 2

Program name is mypgm2

Parameters as a single string is Sanjiv Guha

Special Characters

Some characters have special meaning to Linux shells, so using them as part of variable names or strings will cause your program to behave incorrectly. If a string contains such special characters, then you also have to use escape characters to indicate that the special characters should not be treated as special characters.

Some of these special characters are shown in the following table.

There are a few special characters that deserve special note. They are the double quotes ("), the single quote (`), the backslash (\), and the backtick (`). They are discussed in the following sections.

Double Quotes

If a string contains embedded spaces, you can enclose the string in double quotes (") so that the shell interprets the whole string as one entity instead of more than one. For example, if you assigned the value of abc def (abc followed by one space followed by def) to a variable called x in a shell program as follows:

x=abc def                pdksh and bash







set x = abc def          tcsh

you would get an error as the shell would try to execute def as a separate command. What you need to do is surround the string in double quotes as follows:

x="abc def"             pdksh and bash







set x  = "abc def"       tcsh

The double quotes resolve all the variables within the string. Here is an example for pdksh and bash:

var ="test string"

newvar="Value of var is $var"

echo $newvar

Here is the same example for tcsh:

set var = "test string"

set newvar = "Value of var is $var"

echo $newvar

If you execute a shell program containing these three lines, you will get the following result:

Value of var is test string

Single Quote

You can use the single quote (`) to surround a string in order to stop the shell from resolving a variable. In the following example, the double quotes in the preceding example have been changed to single quotes.

pdksh and bash:

var ='test string'

newvar='Value of var is $var'

echo $newvar

tcsh:

set var = `test string'

set newvar = `Value of var is $var'

echo $newvar

If you execute a shell program containing these three lines, you will get the following result:

Value of var is $var

As you can see, the variable var did not get resolved.

Backslash

You can use a backslash (\) before a character to stop the shell from interpreting the following character as a special character. Say you want to assign a value of $test to a variable called var. If you use the following command:

var=$test              pdksh and bash







set var = $test         tcsh

then a null value will be stored in var. This is because the shell will interpret $test as the value of the variable test, and as test has not been assigned any value, var will contain null. You should use the following command to correctly store $test in var: