How do I get cURL to not show the progress bar?

curl -s http://google.com > temp.html

works for curl version 7.19.5 on Ubuntu 9.10 (no progress bar). But if for some reason that does not work on your platform, you could always redirect stderr to /dev/null:

curl  http://google.com 2>/dev/null > temp.html

I found that with curl 7.18.2 the download progress bar is not hidden with:

curl -s http://google.com > temp.html

but it is with:

curl -ss http://google.com > temp.html

In curl version 7.22.0 on Ubuntu and 7.24.0 on OSX the solution to not show progress but to show errors is to use both -s (--silent) and -S (--show-error) like so:

curl -sS http://google.com > temp.html

This works for both redirected output > /some/file, piped output | less and outputting directly to the terminal for me.

Update: Since curl 7.67.0 there is a new option --no-progress-meter which does precisely this and nothing else, see clonejo's answer for more details.

Since curl 7.67.0 (2019-11-06) there is --no-progress-meter, which does exactly this, and nothing else. From the man page:

   --no-progress-meter
         Option to switch off the progress meter output without muting or
         otherwise affecting warning and informational messages like  -s,
         --silent does.

         Note  that  this  is the negated option name documented. You can
         thus use --progress-meter to enable the progress meter again.

         See also -v, --verbose and -s, --silent. Added in 7.67.0.

It's available in Ubuntu ≥20.04 and Debian ≥11 (Bullseye).

For a bit of history on curl's verbosity options, you can read Daniel Stenberg's blog post.