anton niklasson

System commands in vim

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TL;DR: CtrlR + any register inputs the contents of the register in insert mode.

You can run system commands in vim using the expression register, available on =. Pasting from the expression register involves writing an expression that evaluates to a string, and it supports a function system() to run commands.

Ctrl R = then system('ls -la') inputs the output of ls -la in your current vim buffer.

From time to time I find myself wanting to run some kind of system command and capture the output in the file I am working on at the moment. For one-off commands I usually go with something like :!yarn add axios from inside vim to quickly install another dependency. That runs a system command “from” vim, but the result is not captured in the buffer.

For a project I am working on at the moment I had a json file with about 100 posts. Each post had a set of attributes such as title, body, tags, excerpt etc. But none of them had a unique id. So I set out to add a unique id to each of the posts.

There is a command on a mac perfect for this: uuidgen.

I could see this change as a vim macro. I wanted to perform the following steps for each post:

  1. Find the title attribute
  2. Insert a new line above that line
  3. Add id: "",
  4. Generate a unique id inside the quotes

The first few steps are quite straight forward:

  1. /"title": to find the next occurance of the title attribute
  2. O to insert a new line above it, then just add the id attribute

Generating the uuid involves a special kind of register in vim known as the expression register. Registers in vim are basically just variables. You can read and write to and from them. The expression register is a bit different though: reading from the register involves you inputing a command that evaluates to a string.

On top of that nugget of knowledge; CtrlR + some register in insert mode inputs the contens from the register at the cursor position.

Here is how to run a system command in the expression register:

  1. Enter insert mode
  2. Ctrl R =
  3. Type system('<command-here>') and press enter

uuidgen unfortunately ends with a backspace. I just backspace to get rid of it in the macro, but I am sure there is an easier way ¯_(ツ)_/¯

Here is another practical use case for this:

I usually maintain a components folder in react projects. In this folder I might create folders for each components, or just a single file per component, depending on the current setup. When consuming these components I want to make the imports slightly more compact by exporting each component by their name from an index.js in the components folder. Then importing them is a bit more comfortable:

import { Button, Input, Modal } from "../components";

Last time I realised we needed an index file for our components we had about 25 components. So I decided to use this thing again. Worth noting is that we had each component in a separate folder, with an index file exporting each component as the default export.

The key command here is ls -la src/components:

  1. Create components/index.js
  2. Enter insert mode in that empty file
  3. Ctrl+R``= and type :system('ls -l src/components')

This inputs the following:

total 8
drwxr-xr-x  4 anton  staff  128 13 May 10:34 ContentBlock
drwxr-xr-x  4 anton  staff  128 13 May 10:34 ExpandableMenuItem
drwxr-xr-x  4 anton  staff  128 13 May 21:23 Footer
drwxr-xr-x  8 anton  staff  256 13 May 21:23 Header
drwxr-xr-x  4 anton  staff  128 13 May 10:34 Hyperlink
drwxr-xr-x  4 anton  staff  128 13 May 10:34 Icon
drwxr-xr-x  4 anton  staff  128 13 May 10:34 Link
drwxr-xr-x  4 anton  staff  128 13 May 21:23 Logo
drwxr-xr-x  4 anton  staff  128 13 May 10:34 MainNavigation
drwxr-xr-x  4 anton  staff  128 13 May 10:34 MobileNavigation
drwxr-xr-x  4 anton  staff  128 13 May 10:34 Player
drwxr-xr-x  4 anton  staff  128 13 May 10:34 PostMeta
drwxr-xr-x  4 anton  staff  128 13 May 10:34 SearchInterface
drwxr-xr-x  6 anton  staff  192 13 May 21:23 Teaser
drwxr-xr-x  4 anton  staff  128 13 May 10:34 Tooltip
-rw-r--r--  1 anton  staff  907 13 May 21:23 index.js

At this point it is just a matter of cleaning up the file:

  • dd the “total” line and the one with index.js
  • Visual block mode (Ctrlv) to get rid of the filesystem metadata on each line
ContentBlock
ExpandableMenuItem
Footer
Header
Hyperlink
Icon
Link
Logo
MainNavigation
MobileNavigation
Player
PostMeta
SearchInterface
Teaser
Tooltip

So now I have each component name on a single line, I would like that to read

export { default as Component } from './Component'

That is doable with a macro. Record a macro as an isolated change on a single line, and then use visual line mode and normal command to run that macro on the other lines independantly. Here is what that macro could look like:

  1. qq to record the macro to the q register
  2. 0 to jump to the beginning of the line
  3. yiw to yank to component name
  4. i and type export { default as
  5. A and type } from './';
  6. hP to paste the component name within the quotes

When you have the macro in place:

  1. V to enter visual line mode
  2. Select the other lines
  3. :norm @q to run the q macro on each of the selected lines