BASH - Output - Understand the difference between standard output and standard error

A sane command writes the output that you request to standard output (stdout) and only writes errors to standard error (stderr).

For example:

dig +short A wiki.sharewiz.net
5.42.134.35
 
ip=$(dig +short A wiki.sharewiz.net)
echo "{$ip}"
{5.42.134.35}
 
ls no-such-file
ls: cannot access 'no-such-file': No such file or directory
 
output=$(ls no-such-file)
ls: cannot access 'no-such-file': No such file or directory
 
echo "{$output}"
{}

NOTE: In this example:

dig wrote output to stdout, which was also captured in the ip variable.
This was all successful.

ls encountered an error, so it did not write anything to stdout.
It wrote to stderr, which was not captured (because we 2>&1 was not used).
The error message appeared directly on the terminal instead.