This rule is triggered when there are code blocks showing shell commands to be
typed, and *all* of the shell commands are preceded by dollar signs ($):

<!-- markdownlint-disable commands-show-output -->

```markdown
$ ls
$ cat foo
$ less bar
```

<!-- markdownlint-restore -->

The dollar signs are unnecessary in this situation, and should not be
included:

```markdown
ls
cat foo
less bar
```

Showing output for commands preceded by dollar signs does not trigger this rule:

```markdown
$ ls
foo bar
$ cat foo
Hello world
$ cat bar
baz
```

Because some commands do not produce output, it is not a violation if *some*
commands do not have output:

```markdown
$ mkdir test
mkdir: created directory 'test'
$ ls test
```

Rationale: It is easier to copy/paste and less noisy if the dollar signs
are omitted when they are not needed. See
<https://cirosantilli.com/markdown-style-guide#dollar-signs-in-shell-code>
for more information.