line 1 (should be empty for markdown, but for testing we only care numbers)
[make README.md]: # (include test_content.txt)

[source link/to/file](link/to/file) # old markdown link

```
old input
line 8
line 9
```

line 12

[make README.md]: # (echo "new outputed content")

```
previous command output here
line 18
line 19
```

some text after
line 23
