bash pipe construct to prepend something to the stdoutput of previous command

Question

I want to use sendmail to send me stuff and want to do it in a oneliner.

echo "mail content" | sendmail emailataddres.com 

Sends it without subject.

The subject line must come before the Mail content, so I am looking for something along the lines of:

echo "mail content" | prepend "Subject: All that matters" | sendmail emailataddres.com 

sed and awk tend to be really awkward to use and remember.

EDIT:Just to clarify: echo "Mail content" is just an illustrating example. I need to be able to prepend stuff to stdout streams from any source. e.g.: ifconfig, zcat, etc..

• Daniel Andersson: echo -e "Subject: All that matters\nmail content"? Or more platform agnostic: printf 'Subject: %s\n%s\n' "All that matters" "mail content". You could also write a small script that just takes the two string arguments to make an even simpler one-liner.
• AndreasT: Well if you add this as an answer I might accept it... :)
• AndreasT: ah, no I don't, sorry. Please regard my edit.
• allquixotic: What should prepend do? In order to know how to prepend something, you have to wait until the upstream command (e.g. echo) sends the EOF, so that you can insert your data ahead of it in the stream before passing it to stdout to get piped to sendmail. Sounds like a task for a shortish Ruby or Python script.
• Wim Lewis: A slightly different approach would be to use the mail command (sometimes called bsdmail) instead of sendmail directly. mail takes an optional -s subject option, e.g.: ifconfig -a | mail -s 'Current ifconfig output' [email protected]

Answer 1

$ echo 1 | (echo 2 && cat)
2
1

I am pretty sure that there is a nicer solution, but this should do.

• Hennes: Simple, functional and demonstrated. +1
• VenomFangs: Is there an easy way to do it without the break return?
• Trendfischer: @VenomFangs Use printf instead of the second echo and you should be fine
• kyb: >Is there an easy way to do it without the break return? - echo -n

Answer 2

Either use what Claudius said or make your own:

~/bin/prepend:

#!/bin/sh
echo -en "$@"
cat -

e.g.:

$ echo "Splendid SUPANINJA! Let's do it!" |\
 prepend "Subject: Venetian Snares\n"
Subject: Venetian Snares
Splendid SUPANINJA! Lets do it!

Answer 3

This is inspired by Claudius answer.

If you don't want a break return between your outputs, add the -n param. This will look like:

$ echo 1 | (echo -n 2 && cat)

Which will give:

21

• AndreasT: Actually, no it won't. It gives "2\n1". You have to attach the "-n" to the second echo. Still +1. If you Correct that, I might accept it. I am not sure what's fair, since Claudius contributed the most to the solution.
• VenomFangs: Sorry, think I put the -n in the wrong place. Anyway, The update should give "21\n" if you want to get rid of the \n then just add a -n before the 1.

Answer 4

You can prepend to piped output using process substitution:

$ echo "mail content" | cat <(echo "Subject: All that matters") -
Subject: All that matters
mail content

Answer 5

From the pieces I've gathered... you could do something like this:

echo "Subject: All that matters
`echo "mail content"`" | sendmail blah@blahblah

Notice that I did not close the quotes on the first line... because not all shells translate the \n into a newline character... but I have yet to find one that wont process an actual newline inside the quotes.

When a command is enclosed in the ` character, it will be executed and the output will be injected in-place. Keep in mind that this bit of code is a bit dangerous as it is possible to inject additional commands inline that could easily compromise your system...

****edit**** Following advise of Claudius, a cleaner option would look like this:

echo -e "Subject: All that matters \n $(echo "mail content") |sendmail blah@blahblah

Even with that template, it could be exploited.

• Claudius: It is usually nicer to use $() rather than `` as it is supposed to be more portable (and also more readable). Furthermore, you can use echo’s -e option to make it translate newlines (echo -e "1\n2") or just use printf as suggested by Daniel in the first comment.
• TheCompWiz: Immediately after posting my answer... I read yours & like it better. Good points.