How to add a header and/or footer to a sed or awk stream?

This works, as indicated by jasonwryan:

awk 'BEGIN{print "START"}; {print}; END{print "END"}'

This can be done in sed with

sed -e $'1i\\\nSTART' -e $'$a\\\nEND'

1i means insert before line 1; $a means append after the last line.  The $'…' syntax is bash-specific.  In other shells, you should be able to do this with:

sed -e '1i\Enter
START' -e '$a\Enter
END'Enter

If you're already using sed, you can use 1 to match the first line and $ to match the last line (see Scott's answer). If you're already using awk, you can use a BEGIN block to run code before the first line and an END block to run code after the last line (see Michael Durrant's answer).

If all you need to do is add a header and a footer, just use echo and cat.

echo START
cat
echo END

In a pipeline, to run multiple commands, use { … } to tell the parser that they're one single compound command.

content-generator |
{ echo START; cat; echo END; } |
postprocessor