ed(1) is The Right Tool (III)
===================================

A todo-list is useful if you start working on the items and you are
eventually able to remove some items from the list, sooner or later...

Our current list looks like this:

  ,n
  1       TODO LIST
  2       - phlog about ed(1)
  3       - put rubbish bins outside
  4       - buy some coffee
  5       - buy sugar
  6       - make a coffee
  7       - check postgrey hiccup

Now it is clear I have started phlogging about ed(1), so the first item
should somehow be updated. I have come up with a simple convention to
identify the status of items in a todo-list, by using the first
character of the item: a "-" indicates an outstanding item, a "+"
indicates something I have started working on, and a "*" marks a
completed item. In this case, I would like to change the "-" in the
first item with a "+". Let's start searching for the line where I
mentioned "phlog":

/phlog/
- phlog about ed(1)

Yep. An expression like "/expr/" is used in ed(1) to search for a
pattern. ed(1) search for the first occurrence of that pattern and
prints it. Most versions of ed(1) will also wrap the search around and
start from the beginning if they reach the end of the file. Now the
current line (the so-called ".") is the one containing the item I want
to edit. Let's "substitute" the first "-" in the line with a "+":

s/-/+/
p
+ phlog about ed(1)

Yeah, it's that simple. The command "s" (for "substitute") replaces the
first occurrence of the pattern between the first pair of "/" following
it with the pattern between the second pair of "/" following it. In this
case, we simply replaced "-" with "+". 

The command "s" accepts a line (or a range of lines) to work on. So if
we are indeed working on all the items in the todo-list, you can change
their state with a single command:

1,$s/-/+/
,p
TODO LIST
+ phlog about ed(1)
+ put rubbish bins outside
+ buy some coffee
+ buy sugar
+ make a coffee
+ check postgrey hiccup

Well, that's powerful, right? But in my case, well, it's not true, since
I have not put the rubbish outside and some other items are in a
different state. Let's revert out last change:

u
,p
TODO LIST
+ phlog about ed(1)
- put rubbish bins outside
- buy some coffee
- buy sugar
- make a coffee
- check postgrey hiccup

The command "u" (for "undo") will do that (i.e., revert the effect of
the last command). Try to see what happens if you keep pressing "u" :)

The cool thing about "s" is that the line it has to act upon can be
expressed with a pattern. For instance, I have solved the hiccup in
postgrey, so I can change the status of the line containing "postgrey"
to "done" by replacing the leading "-" with a "*". But I don't remember
which line is it, so I can use the command:

/postgrey/s/-/*/
,p
TODO LIST
+ phlog about ed(1)
- put rubbish bins outside
- buy some coffee
- buy sugar
- make a coffee
* check postgrey hiccup

Remember that the "." is set at the last edited line:

.
* check postgrey hiccup

Now that we have completed an item, we can finally celebrate: hurray!
Well, there is still a bunch of stuff to do anyway. How do we jump to
the next outstanding item in the list? Easy: we just look for a "-":

/-/
- put rubbish bins outside

Yeah, I know, but this is not rubbish-day. Let's look for the next
outstanding task:

//
- buy some coffee

Yep! The quite quick combination "//" repeats the last search you did!
And we can keep going:

//
- buy sugar
//
- make a coffee

That's handy, especially since "/" is normally placed in a quite
comfortable position in many keyboard layouts. The last example: I have
to be honest with you: I can't drink any coffee, so I would like to
replace any occurrence of "coffee" with "tea":

/coffe/s/coffee/tea/
p
- buy some tea

The command "/coffee/s/coffee/tea/" is to be read as "look for the next
line containing 'coffee' and replace 'coffee' with 'tea' on that line.
So what happened here? Let's have a look at the whole file:

,n
1       TODO LIST
2       + phlog about ed(1)
3       - put rubbish bins outside
4       - buy some tea
5       - buy sugar
6       - make a coffee
7       * check postgrey hiccup

So, the last search we performed ("//") had set the dot to line 6
("-make a coffee"). When we issued the substitution command, ed(1) had
to look for the next occurrence of "coffee". It wrapped the search
around (siince there is no line containing "coffee" after line 6), found
"coffee" on line 4, and replaced "coffee" with "tea" on that line. We
would like to repeat the last "s" command then, to replace the other
occurrence of "coffee"

/coffe/s/coffee/tea/
.
- make a tea

OK, that's powerful, but re-entering the command is not fun. Anyway, we
can't learn everything in a single phlog, and this one is already quite
long. Let's not forget to save our work:

w
121

To be honest with you, I really would like to get rid of that line with
"sugar", since I never put any sugar in my tea. But that's a quest for
another day :)