print code fences, optionally only those tagged with a language

print_code_fences.awk

#!/usr/bin/awk -f
#
# To print all fenced code blocks in `my_file.md`:
#     ./print_code_fences my_file.md
#
# To print only fenced code blocks tagged with `javascript`:
#     ./print_code_fences.awk -v lang=javascript my_file.md

BEGIN { in_code_block = 0 }
/^```/ {
    if (!in_code_block) {
        sub(/^`*/, "");
        this_lang = $0;
        if (!lang || lang == this_lang) {
            in_code_block = 1;
            next;
        }
    } else {
        in_code_block = 0;
        print "";
    }
}
in_code_block

sample.in

Here is some text

```javascript
// with some JavaScript code
```

and some more text

```c
/* and some C code */
```