To combine stderr and stdout into the stdout stream, we append this to a command:
2>&1
For example, the following command shows the first few errors from compiling main.cpp:
g++ main.cpp 2>&1 | head
But what does 2>&1 mean?
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.