Disk Space is Low' and this would give the appropriate output. While both single and double quotes solve the above problems, they are different. The difference comes in handling variables, which will be covered shortly. The single quote will not substitute a variable into an expression whereas the double quote will. Command substitution is a topic that ties up these last few sections.
Command substitution allows the output of a command to be substituted into another command.
The user now has the tools and flexibility to handle even the most specific or complicated command in the Bourne shell.