let's see an example: in my main.sh, I'd like to source a.sh and b.sh. a.sh, however, might have already sourced b.sh. Thus it will cause the codes in b.sh executed twice. Is there any mechanism alike "include guard" in C++?
If you're sourcing scripts, you are usually using them to define functions and/or variables.
That means you can test whether the script has been sourced before by testing for (one of) the functions or variables it defines.
For example (in b.sh):
if [ -z "$B_SH_INCLUDED" ]
then
B_SH_INCLUDED=yes
...rest of original contents of b.sh
fi
There is no other way to do it that I know of. In particular, you can't do early exits or returns because that will affect the shell sourcing the file. You don't have to use a name that is solely for the file; you could use a name that the file always has defined.