grep using input file as pattern to search other file

Sometimes we need to use another file which contains a multiple line that we want to use as input pattern
example:
You have a.txt
a
b
c
d
e
And you want to search a, c, e.
This case you can create input.txt
a
c
e
and use this command
grep -f -F input.txt a.txt
If the input txt is a list of regex you can remove the -F option.
-F, --fixed-strings
Interpret PATTERNS as fixed strings, not regular expressions.