June 24, 2009

Emacs Registers and Bookmarks

Filed under: blogging,emacs,usability — jonEbird @ 5:02 pm

Every once and a while I like to re-read manuals about pieces of software which I already feel quite comfortable in using in hopes to learn a new trick or two. Today, I was browsing the freely available GNU Emacs Manual and was breezing through it until I hit the registers section.

Have you every copied a region of text in plans to yank it back in multiple locations within your current buffer but in between this work, you realize you need to do some intermediate killing and yanking and therefore have overwritten your original region of text you had copied? Well, using registers is one way in solving that problem.

Bookmarks have a similar function as with registers within emacs. I am actually grouping them together in this weblog because their key sequences are so very similar. Bookmarks are, like the name implies, a way to keep track of position within a buffer. They can be saved and later referenced to not only re-open the particular file you were editing but take you back to the position within that file as well.

I can not seem to remember anything, from emacs commands or keystrokes to basic shell commands, without coming up with some mnemonic for memorizing it. I have just came up with one such mnemonic for using Registers and Bookmarks and I thought I’d share it with my bots (machines which read this weblog).

Register and Bookmark Mnemonics

Each key sequence starts with:
C-x r – Think “r” for register.
The general pattern is:
C-x r <key> <register> – “register” is any single digit or letter.

“key” is what I’m calling each category of register usage. Let’s explore them:

  • “<space>” – mark – Just like C-spc to set the mark, this asks that the mark be set in our register.
  • “s” – save – save the region’s text into the register.
  • “n” – number – save a particular number into the register.
  • “m” – bookmark – We’ll explore bookmarks further, but notice how you set bookmarks with what I consider the register key sequence.

Those are some of the basic storing actions, but with the same C-x r prefix you can perform other actions with the contents of the registers:

  • “+” – increment – When the register is a number, this increments it’s value. Convenient to use with macros.
  • “i” – insert – This action can be used for numbers, regions of text and even marks!
  • “j” – jump – jump to a mark. This assumes you have already stored a mark in the particular register.

We’ll shift a bit into bookmarks but stay succinct within the mnemonics section here.

  • “m” – bookmark – Repeated from above. Note that this is a interactive function which allows you to name your bookmark with an intelligent name. The default will be the basename of your current buffer’s filename. So, if you’re editing /path/to/emacs_notes.txt it will default to store the bookmark under “emacs_notes.txt” but maybe you want it to be called “emacs notes”. If so, go ahead and type that out and hit RET.
  • “l” – list bookmarks – This opens a new pseudo buffer with the list of all of your bookmarks.
  • “b” – bookmark jump – Jump to the named bookmark. This is a interactive function as well.

Bringing it Together – Examples

Save current mark to register “l”:
C-x r <space> l
Move to mark saved in register “l”:
C-x r j l
Save number in register “n”:
C-x r n n
Now, increment that number and then insert it:
C-x r + n C-x r i n

Love the emacs notes you’re editing, bookmark it:
C-x r m emacsnotes RET
Buried into multiple install READMEs for a particular product and want to return later:
C-x r m installreadme RET

Finally, the prettiest of the commands, let’s review our bookmarks:
C-x r l

Final Note on Registers and Bookmarks

There is a variable named bookmark-save-flag which when set to the value of “1″ will have the action of automatically updating your ~/.emacs.bmk file with any updates to your bookmarks. I recommend setting this in your ~/.emacs file so you don’t have to “M-x bookmark-save” periodically. Add the following to your ~/.emacs file:
(set-variable (quote bookmark-save-flag) 1 nil)

Finally, I’d be remiss if I didn’t mention how I generated that one-liner emacs-lisp line, even if it’s off topic for this weblog entry. I like to use various interactive functions and then capture their effective execution, in emacs-lisp form, via the “repeat-complex-command” function which is bound to “C-x M-:“. In this situation, I used the “set-variable” interactive function to set “bookmark-save-flag” to “1″ and then punched in “C-x M-:” and copied the one-liner for my ~/.emacs file. So, there you go, a bonus tip for those who’ve read this far.

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