You create a temporary auto-submitting HTML page, point the browser to that page, and after a couple of seconds, you remove the temporary HTML file as it is no longer needed. In script form:
#!/bin/bash
# Create an autodeleted temporary directory.
Work="$(mktemp -d)" || exit 1
trap "cd / ; rm -rf '$Work'" EXIT
# Create a HTML page with the POST data fields,
# and have it auto-submit when the page loads.
cat > "$Work/load.html" <<END
<!DOCTYPE html>
<html>
<head>
<title>…</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<script type="text/javascript">
function dosubmit() { document.forms[0].submit(); }
</script>
</head>
<body onload="dosubmit();">
<form action="https://www.startpage.com/do/asearch" method="POST" accept-charset="utf-8">
<input type="hidden" name="cat" value="web">
<input type="hidden" name="cmd" value="process_search">
<input type="hidden" name="language" value="english">
<input type="hidden" name="engine0" value="v1all">
<input type="hidden" name="query" value=""Nominal Animal"">
</form>
</body>
</html>
END
# Load the generated file in the browser.
firefox "file://$Work/load.html"
# Firefox returns immediately, so we want to give it a couple
# of seconds to actually read the page we generated,
# before we exit (and our page vanishes).
sleep 2
Let's change the above, so that we do a StartPage search on whatever string(s) are supplied on the command line:
#!/bin/bash
# Create an autodeleted temporary directory.
Work="$(mktemp -d)" || exit 1
trap "cd / ; rm -rf '$Work'" EXIT
# Convert all command-line attributes to a single query,
# escaping the important characters.
rawAmp='&' ; escAmp='&'
rawLt='<' ; escLt='<'
rawGt='>' ; escGt='>'
rawQuote='"' ; escQuote='"'
QUERY="$*"
QUERY="${QUERY//$rawAmp/$escAmp}"
QUERY="${QUERY//$rawQuote/$escQuote}"
QUERY="${QUERY//$rawLt/$escLt}"
QUERY="${QUERY//$rawGt/$escGt}"
# Create a HTML page with the POST data fields,
# and have it auto-submit when the page loads.
cat > "$Work/load.html" <<END
<!DOCTYPE html>
<html>
<head>
<title>…</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<script type="text/javascript">
function dosubmit() { document.forms[0].submit(); }
</script>
</head>
<body onload="dosubmit();">
<form action="https://www.startpage.com/do/asearch" method="POST" accept-charset="utf-8">
<input type="hidden" name="cat" value="web">
<input type="hidden" name="cmd" value="process_search">
<input type="hidden" name="language" value="english">
<input type="hidden" name="engine0" value="v1all">
<input type="hidden" name="query" value="$QUERY">
</form>
</body>
</html>
END
# Load the generated file in the browser.
firefox "file://$Work/load.html"
# Firefox returns immediately, so we want to give it a couple
# of seconds to actually read the page we generated,
# before we exit (and our page vanishes).
sleep 2
All that changes is the chunk where we use Bash string operations to replace each & with &, each " with ", each < with <, and each > with >, so that the query string can be safely written as the value attribute of the hidden input named query. (Those four suffice. It is also important to do the ampersand first, because the subsequent replacements contain ampersands. Since we emit this as the value of a hidden input, the query string is not url-encoded; it's just normal HTML content, but without doublequotes (because the value itself is in doublequotes).)
The downside of auto-submitting POST requests is that you may need to update your auto-submitting HTML page every now and then, simply because the site can change the POST variable naming and internal URLs whenever they want.
