Hello,
I have a few files with simple data on them. All of the files may contain the same and different data. I need to create a new file that contains data that is on all files at the same time. Let say if word "ffffffff" is on all files it must be in the new file. If word "kkkkkkk" is not on all files then it must not be on the new file. Can anyone give me a solution at least for the case when there are only two files.

Thank you in advance.

For two files, you can use the comm command. Use the -12 switch to supress the output of lines present in only one of the two files. You will end up with the output that is common to the two files. Like so:

# cat test1
aaa
bbb
ccc
# cat test2
aaa
bbb
ddd
# comm -12 test1 test2
aaa
bbb