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..

`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. — Daniel Andersson, Nov 13 '12 at 16:09
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. — allquixotic, Nov 13 '12 at 16:48
I don't have to wait. It can be inserted at the beginning of the stream. — AndreasT, Nov 13 '12 at 16:56
sed worked great for me. From this [answer](http://stackoverflow.com/a/13884279/878969). `echo "input" | awk '{print "Subject: All that matters"$1}'` — funroll, Sep 11 '14 at 17:41
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' me@someaddr.com` — Wim Lewis, Aug 16 '15 at 21:41

score 54 · Answer 1 · answered Nov 13 '12 at 16:51

54

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

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

answered Nov 13 '12 at 16:51

Claudius

8,808
1
15
13

2

Simple, functional and demonstrated. +1 – Hennes Dec 30 '13 at 14:14
Is there an easy way to do it without the break return? – VenomFangs Jul 21 '15 at 16:18
2

@VenomFangs Use `printf` instead of the second `echo` and you should be fine – Trendfischer Jul 24 '15 at 09:47
*>Is there an easy way to do it without the break return?* - `echo -n` – kyb Apr 06 '18 at 11:00

Ярослав Рахматуллин · Answer 2 · 2013-12-30T13:08:09.473

9

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!

edited Dec 30 '13 at 13:08

answered Nov 19 '12 at 04:39

Ярослав Рахматуллин

11,096
5
42
74

VenomFangs · Answer 3 · 2015-07-23T15:18:39.527

6

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

edited Jul 23 '15 at 15:18

answered Jul 21 '15 at 16:21

VenomFangs

463
4
16

1

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. – AndreasT Jul 23 '15 at 14:36
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. – VenomFangs Jul 23 '15 at 15:20

score 2 · Answer 4 · answered Sep 12 '19 at 01:05

2

You can prepend to piped output using process substitution:

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

answered Sep 12 '19 at 01:05

aparkerlue

206
1
5

TheCompWiz · Answer 5 · 2012-11-13T17:20:34.867

1

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.

edited Nov 13 '12 at 17:20

answered Nov 13 '12 at 16:54

TheCompWiz

10,602
1
23
21

1

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. – Claudius Nov 13 '12 at 16:56
Immediately after posting my answer... I read yours & like it better. Good points. – TheCompWiz Nov 13 '12 at 16:58

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

5 Answers5

~/bin/prepend:

e.g.: