There are many different versions of mail around. When you go beyond mail -s subject to1@address1 to2@address2 <body (for sending, that's all POSIX guarantees — and even -s didn't exist in the old days), they tend to have different command line options. Adding an additional header isn't always easy.
With some mailx implementations, e.g. from mailutils on Ubuntu or Debian's bsd-mailx, it's easy, because there's an option for that.
mailx -a 'Content-Type: text/html' -s "Subject" to@address <test.html
With the Heirloom mailx, there's no convenient way. One possibility to insert arbitrary headers is to set editheaders=1 and use an external editor (which can be a script).
## Prepare a temporary script that will serve as an editor.
## This script will be passed to ed.
temp_script=$(mktemp)
cat <<'EOF' >>"$temp_script"
1a
Content-Type: text/html
.
$r test.html
w
q
EOF
## Call mailx, and tell it to invoke the editor script
EDITOR="ed -s $temp_script" heirloom-mailx -S editheaders=1 -s "Subject" to@address <<EOF
~e
.
EOF
rm -f "$temp_script"
With a general POSIX mailx, I don't know how to get at headers.
If you're going to use any mail or mailx, keep in mind that
- This isn't portable even within a given Linux distribution. For example, both Ubuntu and Debian have several alternatives for
mail and mailx.
- When composing a message,
mail and mailx treats lines beginning with ~ as commands. If you pipe text into mail, you need to arrange for this text not to contain lines beginning with ~.
If you're going to install software anyway, you might as well install something more predictable than mail/Mail/mailx. For example, mutt. With Mutt, you can supply most headers in the input with the -H option, but not Content-Type, which needs to be set via a mutt option.
mutt -e 'set content_type=text/html' -s 'hello' 'to@address' <test.html
Or you can invoke sendmail directly. There are several versions of sendmail out there, but they all support sendmail -t to send a mail in the simplest fashion, reading the list of recipients from the mail. (I think they don't all support Bcc:.) On most systems, sendmail isn't in the usual $PATH, it's in /usr/sbin or /usr/lib.
cat <<'EOF' - test.html | /usr/sbin/sendmail -t
To: to@address
Subject: hello
Content-Type: text/html
EOF
