┌───────────────────────────────────────────┐
                 │ fsub — substitute a file into other files │
                 └───────────────────────────────────────────┘

fsub is a command-line utility to replace substrings within files with other
files.  This has a variety of usecases, such as static-site generators.  Take
for example the following skeleton HTML file:

    <!-- base.html -->
    <table>
      <tbody>
        <!-- TABLE DATA -->
      </tbody>
    </table>

We want to be able to generate the table data externally, and then fill in the
above table.  With fsub this is made very easy.  The following commands generate
table data which is written to “data.html”, and then replace the comment in the
base file with the contents of “data.html” and write the result to “index.html”:

    $ ./generate-table-data.sh >data.html
    $ fsub '<!-- TABLE DATA -->' data.html base.html >index.html

You can also use the special filename ‘-’ to represent the standard input,
allowing for usage in a pipeline:

    $ ./generate-table-data.sh
          | fsub '<!-- TABLE DATA -->' - base.html >index.html

This is just a very basic overview, for more information check the fsub(1)
manual page.