To combine stderr and stdout into the stdout stream, we append this to a command:

 2>&1

e.g. to see the first few errors from compiling g++ main.cpp:

 g++ main.cpp 2>&1 | head

What does 2>&1 mean, in detail?

File descriptor 1 is the standard output (stdout).
File descriptor 2 is the standard error (stderr).

At first, 2>1 may look like a good way to redirect stderr to stdout. However, it will actually be interpreted as "redirect stderr to a file named 1".

& indicates that what follows and precedes is a file descriptor, and not a filename. Thus, we use 2>&1. Consider >& to be a redirect merger operator.