chapters/28.markdown @ a8dd2fa6d24c

Execute
=======

The `execute` command is used to evaluate a string as if it were a Vimscript
command.  Run the following command:

    :::vim
    :execute "echom 'Hello, world!'"

Vim evaluates `echom 'Hello, world!'` as a command and dutifully echoes it to
the screen and message log.  Execute is a very powerful tool because it lets you
build commands out of arbitrary strings.

Let's try a more useful example.  Prepare by opening a file in Vim, then using
`:edit "foo.txt"` in the same window to open a new buffer.  Now run the
following command:

    :::vim
    :execute "rightbelow vsplit " . bufname("#")

Vim will open the first file in a vertical split to the right of the second
file.  What happened here?

First, Vim sees builds the command string by concatenating "rightbelow vsplit
" with the result of the `bufname("#")` call.

We'll look at the function more later, but for now just trust that it returns
the path of the previous buffer.  You can play with it using `echom` if you want
to see for yourself.

Once `bufname` is evaluated Vim has a string that looks something like
"rightbelow vsplit bar.txt".  The `execute` command evaluates this as
a Vimscript command which opens the split with the file.

Is Execute Dangerous?
---------------------

In most programming languages the use of such an "eval" construct to evaluate
strings as program code is frowned upon, to put it lightly.  Vimscript's
`execute` command doesn't bear the same stigma for two reasons.

First, most Vimscript code only ever takes input from a single person: the user.
If the user wants to input a tricky string that will cause an `execute` command
to do something bad, well, it's their computer!

Contrast this with other languages, where programs constantly take input from
untrusted users.  Vim is a unique environment where the normal security concerns
simple aren't common.

The second reason is that because Vimscript has sometimes arcane and tricky
syntax, `execute` is often the easiest, most straightforward way to get
something done.  In most other languages using an "eval" construct won't usually
save you much typing, but in Vimscript it can collapse many lines into a single
one.

Exercises
---------

Skim `:help execute` to get an idea of some of the things you can and can't use
`execute` for.  Don't dive too deeply yet -- we're going to revisit it very
soon.

Read `:help leftabove`, `:help rightbelow`, `:help :split`, and `:help :vsplit`
(notice the extra colon in the last two topics).

Add a mapping to your `~/.vimrc` file that opens the previous buffer in a split
of your choosing (vertical/horizontal, above/below/left/right).