Shell Parameter Expansion

Being a developer it is unlikely to have not played around with environment variables at one point or the other.
Considering that we will be spending more time with them in the foreseeable future, Shell parameter expansion is one great arrow to have in your quiver.

Let’s quickly learn the basics of it.

• To expand an env variable simply use:

echo $ENV_VARIABLE

• To check if the variable is present in the current shell or not use

echo ${ENV_VARIABLE-Not Present} //prints Not Present, as variable 
                                   not present
export ENV_VARIABLE=             //variable present but not set
echo ${ENV_VARIABLE-Not Present} //prints nothing as variable is  
                                   present

• To check if the variable is unset, null or not present

echo ${ENV_VARIABLE:-Not Present} //prints Not Present, as 
                                    variable is not present
export ENV_VARIABLE=              //variable present but not set
echo ${ENV_VARIABLE-Not Set}      //prints Not Set as variable is 
                                    null

• Assign default value if the variable is unset, null or not present

echo ${ENV_VARIABLE:=DEFAULT_VALUE} //assigns and prints 
                                      DEFAULT_VALUE to variable as 
                                      variable not present
export ENV_VARIABLE=…