The single quotation mark prevents the shell from looking inside the quotes. Now the shell interprets this line as the echo command with a single argument, which happens to be a string of characters including whitespace. All shell versions have words that have special meaning. Each shell version has different reserved word pertaining to its specific features. All shell versions have meta-characters, which allow the user to specify filenames.
Wild cards can be useful when processing a number of specific files.
This lists all files starting with t . This lists all files starting with test, any single character and ends with 5.dat . This lists all files starting with e , m , or t . The shell looks through this set of directories to find the program file for the command. The shell then passes the true filename to the kernel.