Assume that I have a file named "letters" in my dictionary contains multiple letters, each line per one letter such as:

 Assume that I have a file named "letters" in my dictionary contains multiple letters, each line per one letter such as:

a

b

c

d

e

etc

In the second files named "words", I have this line:

we want to replace by these possible letter which in the letters file with sed instruction

such as can be , OR OR .....

How can we do it?

So basically I want to replace a content of 1 file by the contents of another file

Solution: 

We can use this command

[ sed -i ' s/old-text/new-text/g ' filename ]

sed -i 's/Letters/WORDS/g ' filename

even we can use
cp file1 file2

and also cat file1 >> file2