User-defined variables in the Bourne shell

The Bourne shell recognizes alphanumeric variables to which string values can be assigned.

To assign a string value to a name, type the following:

Name=String

A name is a sequence of letters, digits, and underscores that begins with an underscore or a letter. To use the value that you have assigned to a variable, add a dollar sign ($) to the beginning of its name. Thus, the $Name variable yields the value specified by the String variable. Note that no spaces are on either side of the equal sign (=) in an assignment statement. (Positional parameters cannot appear in an assignment statement. You can put more than one assignment on a command line, but remember that the shell performs the assignments from right to left.

If you enclose the String variable with double or single quotation marks (" or '), the shell does not treat blanks, tabs, semicolons, and newline characters within the string as word delimiters, but it imbeds them literally in the string.

If you enclose the String variable with double quotation marks ("), the shell still recognizes variable names in the string and performs variable substitution; that is, it replaces references to positional parameters and other variable names that are prefaced by dollar sign ($) with their corresponding values, if any. The shell also performs command substitution within strings that are enclosed in double quotation marks.

If you enclose the String variable with single quotation marks ('), the shell does not substitute variables or commands within the string. The following sequence illustrates this difference:

You:            num=875
                number1="Add $num"
                number2='Add $num'
                echo $number1
System:         Add 875
You:            echo $number2
System:         Add $num

The shell does not reinterpret blanks in assignments after variable substitution. Thus, the following assignments result in $first and $second having the same value:

first='a string with embedded blanks'
second=$first

When you reference a variable, you can enclose the variable name (or the digit designating a positional parameter) in braces { } to delimit the variable name from any string following. In particular, if the character immediately following the name is a letter, digit, or underscore, and the variable is not a positional parameter, then the braces are required:

You:            a='This is a'
                echo "${a}n example"
System:         This is an example
You:            echo "$a test"
System:         This is a test