ed(1) is the right tool (VII)
===================================

The nice thing about TODO-lists is that you will eventually be able to
get through them and pencil-out all the completed tasks. But sometimes
it is good to keep track of the many things you have accomplished by
going through a TODO-list. So I normally just save the finised tasks in
a "WELL-DONE" file, to give myself a reassuring (although fictitious)
pat in the shoulder.  Let's have a look at our current TODO-list:

  $ ed TODO.txt
  176
  1,$p
  TODO LIST
  - buy some tomatoes
  - buy some tea
  + configure Unix V7 outpost
  + read Schismatrix
  * phlog about ed(1)
  * put rubbish bins outside
  * make a tea
  * check postgrey hiccup

As per our choice, all the lines starting with "*" denote completed
tasks. If we want to copy all of them to a file named "WELL-DONE", ed(1)
knows a useful trick or two:

  6,9w WELL-DONE
  84

We should be able to figure out what might have happened here using our
current ed-fu. "6,9" specifies a set of lines (all the lines between 6
and 9, both included), while "w" is the command to "write" ed(1) buffers
to a file. We have also seen than "w" followed by a file name will
actually dump all the current content of ed(1)'s buffer on that file.
Well, this is exactly what happened here, only we told ed(1) to dump a
range of lines (6-to-9) instead of the whole buffer, and the destination
file is called WELL-DONE. We can check if our intuition is correct by
exiting ed(1) and using cat(1) on the file WELL-DONE. The good news is
that we don't need to exit ed(1) at all:

  !cat WELL-DONE
  * phlog about ed(1)
  * put rubbish bins outside
  * make a tea
  * check postgrey hiccup
  !

Wait, what was that? Let's give it another shot:

  !pwd
  /home/katolaz/tmp
  !

this can't be what we think it is, right?

  !ls -l
  total 8
  -rw-r--r--  1 katolaz  katolaz  176 Oct 10 10:39 TODO.txt
  -rw-r--r--  1 katolaz  katolaz   84 Oct 11 05:35 WELL-DONE
  !

OK, OK, we got it! ed(1) uses the command "!" to "shell out" another
command, i.e., to execute a command of our choice as if we were typing
it at the shell's prompt. The "!" command will execute the command you
type, show its output on the screed (beware, ONLY on the screen, without
affecting your current buffer), and then finish with a single "!" on a
line. At that point, we know that the external command has finished, and
ed(1) is ready to receive more commands:

  1,$p
  TODO LIST
  - buy some tomatoes
  - buy some tea
  + configure Unix V7 outpost
  + read Schismatrix
  * phlog about ed(1)
  * put rubbish bins outside
  * make a tea
  * check postgrey hiccup

Quite handy, right? Notice again that the invocation of external
commands through "!" does *not* modify the current buffer.

So we have successfully written lines 6-9 to the file called "WELL-DONE"
in the current directory. Now we remove them from TODO.txt:

  g/^\* /d
  1,$p
  TODO LIST
  - buy some tomatoes
  - buy some tea
  + configure Unix V7 outpost
  + read Schismatrix

to keep our TODO-list in order. I also did my shopping yesterday, so:

  g/buy/s/^-/\*/
  1,$p
  TODO LIST
  * buy some tomatoes
  * buy some tea
  + configure Unix V7 outpost
  + read Schismatrix

but now I would like to append the newly-completed tasks to the file
"WELL-DONE". If we used (don't do it!):

  2,3w WELL-DONE

ed(1) would silently destroy the previous content of WELL-DONE, and put
there the two tasks we just completed. But this is not what we want:
remember, keeping track of the stuff you have done is a good boost for
yout morale! The ed(1) command to "append" lines at the end of an
existing file is instead "W":

  2,3W WELL-DONE
  35
  !cat WELL-DONE
  * phlog about ed(1)
  * put rubbish bins outside
  * make a tea
  * check postgrey hiccup
  * buy some tomatoes
  * buy some tea
  !

We can now remove the completed tasks and save our current TODO.txt
file:

  2,3d
  w
  57
  1,$p
  TODO LIST
  + configure Unix V7 outpost
  + read Schismatrix

What if we want to re-insert our old (completed) tasks somewhere in our
TODO.txt file, e.g., after line 1? Simple:

  1r WELL-DONE
  119
  1,$p
  TODO LIST
  * phlog about ed(1)
  * put rubbish bins outside
  * make a tea
  * check postgrey hiccup
  * buy some tomatoes
  * buy some tea
  + configure Unix V7 outpost
  + read Schismatrix

And our old (completed) tasks are back. But no, we really don't need to
see them here: 
  
  u
  1,$p
  TODO LIST
  + configure Unix V7 outpost
  + read Schismatrix
  w
  57
  q

Note that in the versions of ed(1) currently available in most unix
operating systems (including Linux, OpenBSD, FreeBSD, NetBSD), the "r"
command can also read lines from an external command into an ed(1)
buffer.  For instance

  $ ed filelist
  filelist: No such file or directory
  r ! ls -l
  125
  1,$p
  total 8
  -rw-r--r--  1 katolaz  katolaz   57 Oct 11 06:01 TODO.txt
  -rw-r--r--  1 katolaz  katolaz  119 Oct 11 06:00 WELL-DONE

which is sometimes very handy. This option was not available to the
original versions of ed(1) up to UNIXv7. In that case, you would use a
simple trick:

  $ ed filelist
  filelist: No such file or directory
  ! ls -l > tmp
  !
  r tmp
  178
  1,$p
  total 8
  -rw-r--r--  1 katolaz  katolaz   57 Oct 11 06:01 TODO.txt
  -rw-r--r--  1 katolaz  katolaz  119 Oct 11 06:00 WELL-DONE
  -rw-r--r--  1 katolaz  katolaz    0 Oct 11 06:06 tmp

which (almost) does the job.

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