Emacs Tips

In this short post we’ll review a few features of Emacs that are not well known, but really worth knowing.

Repeating commands

The simplest way to repeat a command is to type Control-a number, then the command. For example if I type C-6 C-0 ~ (that’s 6 then 0 with the Control key down, then the tilde character), I get a line with 60 tilde:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Another way is to use the universal argument: we saw how to write an interactive Lisp function that accepts a numeric argument in the previous article. forward-line is such a function; it goes down N lines (by default one line). For example C-u 1 0 DOWN (Control-u then 1 then 0 then the down arrow key) goes down 10 lines. You can do the same by calling the function explicitly with C-u 1 0 M-x forward-line.

Registers

Registers are used for saving temporary data in the current session. They can store positions or bookmarks, text, numbers etc. Registers have a name which is a letter or a number: a to z, A to Z (so case-sensitive) and 1 to 9, which gives you a total of 62 registers. That’s more than enough.

C-x r is the prefix for all register operations. Generally you type this prefix, then a key to specify what operation you want (e.g. save or read), then the register name which is one extra key.

Buffer positions

For example, open any file you have in one of your projects, go to some position in the file, and hit C-x r SPC a (where SPC is the space bar). This command stores the buffer position in register a. Now go to a different buffer such as the scratch buffer, and type C-x r j a: this command will jump back to the buffer and the position you just stored in register a.

Note that if you close the file and then retype the same command, Emacs will ask you if you want to reopen the file, and it will bring you back to the same position again. This can be useful if you want to keep bookmarks in a large project, for example if you keep going to the same files and bits within these files, but you don’t want to keep them open all the time.

You can view the content of register a with command M-x view-register then a at the prompt. It will show a window saying that this register stores a position in your buffer (note that the position is a number, which represents the offset from the beginning of the buffer). Type C-x 1, or q in the other window, to dismiss it.

Bookmarks

A bookmark is similar to a buffer position but with a twist: you give it a name and it is persisted between Emacs sessions. Note that bookmarks are not actually associated with registers, but they use the same C-x r prefix.

The command is C-x r m (mark) which prompts for a name. By default it proposes the current buffer name but you can choose whatever you want. You can view the list of bookmarks with C-x r l (list). Just click on a bookmark to jump to it. You can also jump to a bookmark with C-x r b (bookmark) which prompts for the name using auto-complete. The nice thing about bookmarks is that they are saved on the filesystem when you exit Emacs, and they are available when you restart it. You can also force save with M-x bookmark-save.

Text

Sometimes you want to save a snippet of text somewhere, so you can paste it later. One way is to use the kill ring: M-w to save the selected text (which is what Emacs calls the region), then C-y to paste. The kill ring saves all copy operations you have done so far, so if you want to paste the second previous thing you copied, type C-y followed by M-y (repeat M-y to go back in history).

Another way is to save the region in a register, which you do with something like C-x r s a (save in register a). Now if you want to insert the content of a register at the current point, type C-x r i a (insert the content of a).

Windows

This is less useful than the above, but you can also save a window configuration in a register. For example, split the screen horizontally with C-x 2, then split the current window vertically with C-x 3. You now have 3 windows displayed, which you can resize as you see fit.

Let’s save this layout in register a with C-r w a. Now dismiss all other windows than the current one using C-x 1. If you want to restore the layout, type C-r j a (it is the same key for jumping to a buffer position). Note that this only saves window configurations and not buffers content: if you close one of the buffers Emacs will not reopen it for you.

Summary

The table below summarizes the keys we just learned.

Macros

Editing macros are a very powerful feature of Emacs. After all, Emacs stands for “Editing Macros” ¹.

There are several keys for macros, but really you only need to remember two of them: F3 and F4. The first one records a new macro. The second one terminates the recording, if you were recording a macro; otherwise it executes the macro. If you make an mistake while recording a macro, hit C-g to abort, and start over.

Let’s take an example. Suppose I have this text:

the quick brown fox
jumps over
the lazy dog

Now suppose I want to make each line start with a capital letter and end with a period. I could edit the text manually because it is only 3 lines, but just imagine that it is much longer for argument’s sake, in order to make the use of a macro more compelling.

The way to do this with a macro is simple: fix the first line, while recording a macro. Then execute the macro N times, one time per remaining line. To record the macro do the following:

• Move the cursor to the beginning (e.g. M-<).
• F3 to start recording.
• M-c to capitalize the first word (The).
• C-e to go to the end of the line.
• . to insert a period. Now the first line is good.
• C-a to go to the beginning of the line (where we started), and the down arrow to go to the next line.

Now type F4 to stop recording. Then F4 again to run the macro on the second line. Then F4 again to run the macro on the 3rd line. You’re done!

The quick brown fox.
Jumps over.
The lazy dog.

You could also run the macro N times using Control-a number then F4, as we saw earlier. You can also apply the macro to a whole region by selecting the region and running M-x apply-macro-to-region-lines, which is neat.

If you want to see macros at their best (and incidentally Emacs beat Vim at its own game), check out this quick video:

The entire series of Emacs Rocks is worth watching.

That’s it for today. Lots more to come. Stay tuned!