Merge Standard Error with Standard Output when Using a Pipe

When piping the output of one terminal command to another on a Unix-based system, by default only the Standard Output (stdout) of the first command is piped to the Standard Input (stdin) of the second command.
We can merge streams to make sure stderr is piped to the second command.
The general syntax is:

$ cmd1 2>&1 | cmd2

For a specific example: the “No such file” error below is sent to Standard Error.
The word count command receives an empty input.

$ cat invalid-file | wc
cat: invalid-file: No such file or directory
    0     0     0

Now, if we merge streams, Standard Error is piped to word-count and the number of characters in the error message is counted and printed:

$ cat invalid-file 2>&1 | wc
    1     7     45

This shows that en error output was sent through the pipe to the second command.