im trying to append to the end of the line using sed but I want to do it without creating a new line
the text to which I want to append is all in capital letters.

I want to do something like this:

LINE]Foo

/[A-Z]/a\
] Foo

LINE
]Foo

Try something like this:
I will add Eon at the end of the line

$ cat data
Knight Eon
Hello World
Hello Earth
How is everyone

$ sed -e '$s/\(.*\)/\1Eon/g' data
Knight Eon
Hello World
Hello Earth
How is everyone Eon

echo "LINE" | sed 's/$/]FOO/'

thanks for the replays but what I really want to do is append to only the lines that are in all capital letters.

sort of like if i Have:

TITLE
this is a line
this is a line

TITLE]Foo
this is a line
this is a line

try this

touch /tmp/test.$$
while read line
do
check=`echo $line | tr -dc [A-Z]`
if [ ! -z "$check" ];then
echo "$line" | sed 's/$/]Foo/' >> /tmp/test.$$
else
 echo "$line" >> /tmp/test.$$
fi
done < one
cat /tmp/test.$$

It can be written in sed one liner. Here is how:

$ cat data
Knight Eon
Hello WORLD
Hello Earth
LINE
How is everyone
$ sed -e 's/\(\L.*\)/\1]Foo/g' data
Knight Eon
Hello WORLD]Foo
Hello Earth
LINE]Foo
How is everyone

sed 's/^[A-Z][A-Z]*$/&\]FOO/' infile

sed 's/^[ \t]*[A-Z][A-Z]*[ \t]*$/&\]FOO/' infile