The ngx_http_sub_module module is a filter that modifies a response by replacing one specified string by another.

This module is not built by default, it should be enabled with the --with-http_sub_module configuration parameter.

location / {
    sub_filter      </head>
        '</head><script language="javascript" src="$script"></script>';
    sub_filter_once on;
}

Sets a string that needs to be changed, and a replacement string. The string to be replaced is matched ignoring the case. A replacement string can contain variables.

sub_filter_last_modified off;

Allows to preserve the “Last-Modified” header field from the original response during replacement to facilitate response caching.

By default, the header field is removed as contents of the response are modified during processing.

sub_filter_once on;

Determines how many times to look for a string to be replaced, once or several times.

sub_filter_types text/html;

Enables string replacement in responses with the specified MIME types in addition to “text/html”. The special value “*” matches any MIME type (0.8.29).