This provides access to the value. How-To: Bash Parameter Expansion and Default Values 2 minute read Bash is a sh-compatible command language interpreter that executes commands read from the standard input or from a file. You can use it for manipulating and expanding variables on demands without using external commands such as perl, python, sed or awk. The following expressions are the complement to the match in expr string operations (see Example 16-9 ). The ‘ $ ’ character introduces parameter expansion, command substitution, or arithmetic expansion. The parameter name or symbol to be expanded may be enclosed in braces, which are optional but serve to protect the variable to be expanded from characters immediately following it which could be interpreted as part of the name. We're only going to touch briefly on parameter expansion in this lesson, but we'll be covering it more later. Substitution is introduced with the / or // operators, followed by two arguments separated by another / representing the pattern and the string to substitute. In addition, parameter expansion allows you to wrap curly braces ({and }) around your expansion. The bash shell is available on many Linux® and UNIX® systems today, and is a common default shell on Linux. Variable Expansion. How-To: Bash Parameter Expansion and Default Values 2 minute read Bash is a sh-compatible command language interpreter that executes commands read from the standard input or from a file. Positional Parameters. Using these forms of parameter expansion in your shell scripts can simplify and shorten your scripts. These are not the only forms of parameter expansion that bash supports but they're the ones that I've found most useful over time. These braces are used to tell bash what the beginning and end of your parameter name is. It's a feature that is more useful in shell scripts than directly on the command line. T he $ character is used for parameter expansion, arithmetic expansion and command substitution. These braces are used to tell bash what the beginning and end of your parameter name is. Parameter Expansion. Substitution in parameter expansion. These things are described here. This guide shows you how to use parameter expansion modifiers to transform Bash shell variables for your scripting needs. Using these forms of parameter expansion in your shell scripts can simplify and shorten your scripts. Variable Expansion. Variable expansion is the term used for the ability to access and manipulate values of variables and parameters. Basic expansion is done by preceding the variable or parameter name with the $ character.