How Nginx computes the ETag header for files.

Curious how the ETag: header is generated in Nginx?

Turns out it's a combination of the last modified time and the content length:

etag->value.len = ngx_sprintf(etag->value.data, "\"%xT-%xO\"",
                                  r->headers_out.last_modified_time,
                                  r->headers_out.content_length_n)
                      - etag->value.data;

You can determine the last modified time in hex by using this Unix line:

printf "%x" $(stat -c%Y <file>)

The content length is determined here:

stat --format="%s" <file>