Word expansion means the process of splitting a string into
words and substituting for variables, commands, and wildcards
just as the shell does.
For example, when you write `ls -l foo.c', this string is split
into three separate words—`ls', `-l' and `foo.c'.
This is the most basic function of word expansion.
When you write `ls *.c', this can become many words, because
the word `*.c' can be replaced with any number of file names.
This is called wildcard expansion, and it is also a part of
word expansion.
When you use `echo $PATH' to print your path, you are taking
advantage of variable substitution, which is also part of word
expansion.
Ordinary programs can perform word expansion just like the shell by
calling the library function wordexp.
