Grep for "ab?c12345678" From a file
 

Hello,

How to grep a string like "ab?c12345678" where 3rd character is unknown, while other characters are known.

Stick a dot in there instead, unless by using the question mark you are saying that it may or may not be there?
These are different. I will leave you to find out what the difference is :)

Type "man grep" at a terminal and then "/REGULAR EXPRESSIONS":

"The period . matches any single character."

So: grep 'ab.c12345678' file

will search file for lines containing the above string with any character in the third position.

And: echo $string | grep 'ab.c12345678'

will search $string for the same pattern.