I am aware that giving the path of file or directory in the .gitignore file does not add those files to my git repository, but I wanted to ignore all "build" directories created in my repository, I don't want to specify the complete path of my individual build directory every time. Please suggest the solution.
If all your target directories are called "build", then you can write this in gitingnore:
[Bb]uild/
This should solve the problem. From the documentation:
If the pattern ends with a slash, it is removed for the purpose of the following description, but it would only find a match with a directory. In other words, foo/ will match a directory foo and paths underneath it, but will not match a regular file or a symbolic link foo (this is consistent with the way how pathspec works in general in Git).