The shell scans the results of parameter expansion, command substitution, and arithmetic expansion that did not occur within double quotes for word splitting.
The shell treats each character of $IFS as a delimiter, and splits
the results of the other expansions into words on these characters.
If IFS is unset, or its value is exactly
the default, then sequences of
at the beginning and end of the results of the previous
expansions are ignored, and any sequence of IFS
characters not at the beginning or end serves to delimit words.
If IFS has a value other than the default, then sequences of
the whitespace characters
are ignored at the beginning and end of the
word, as long as the whitespace character is in the
value of IFS (an IFS whitespace character).
Any character in IFS that is not IFS
whitespace, along with any adjacent IFS
whitespace characters, delimits a field. A sequence of IFS
whitespace characters is also treated as a delimiter.
If the value of IFS is null, no word splitting occurs.
Explicit null arguments (
'') are retained.
Unquoted implicit null arguments, resulting from the expansion of
parameters that have no values, are removed.
If a parameter with no value is expanded within double quotes, a
null argument results and is retained.
Note that if no expansion occurs, no splitting is performed.