Today I Learned

A Hashrocket project

Custom Browser with Create React App

Sometimes Google Chrome isn’t your default browser for surfing the internet, but it is your default browser during development. With create-react-app you can specify an alternate browser to automatically open (or no browser at all), to avoid opening the wrong development browser all the time.

Just pre-pend the browser to the start script:

{
  "name": "custom-browser",
  "version": "0.1.0",
  ...
  
  "scripts": {
    "start": "BROWSER='Google Chrome' react-scripts start",
    ...
  }
}

Search Github Faster with Shortcuts

Last year, I wrote about Github repository shortcuts for code, issues, and pull requests.

Now Github has a projects tab on each repository. Ever the completist, here’s the shortcut for that tab: gb. Typing this on a Github repo’s page loads the projects tab.

Researching this led me to an even better shortcut, s. This is site-wide and it focuses the Github search bar.

Here’s the impressive full list of Github hotkeys.

Elixir Maps With String/Colon Keys Become Atoms

While running the Elixir 1.6 formatter against Today I Learned, this change caught my eye. The formatter changes maps like this:

%{ "short_name": "TIL" }

To this:

%{ short_name: "TIL" }

This change makes sense because Elixir automatically coerces strings keys in a map, when followed by colons:

iex(1)> %{"foo": "bar"}
%{foo: "bar"}

And so, the conversion is implied. Elixir 1.6’s formatter forces us to be explicit and skip the conversion.

String Interpolation With Template Literals

ES6 adds support for template literals. Template literals make it much easier to compose strings of content — string interpolation. They allow for single and double quotes without having to fuss with escaping. Embedded expressions are also supported which means you can avoid awkward-to-type string concatenation with the + operator.

Here is an example:

> let movie = 'it'
undefined
> `What's up, I just saw "${movie.toUpperCase()}".`
"What's up, I just saw "IT"."

Proxy To An API Server In Development With CRA

create-react-app is a great way to bootstrap a React project, especially if you are building a single-page app. When building an SPA, you more likely than not will have a backend API that you interact with.

You can set up your React app to interact with that backend API server in development using the proxy configuration in package.json.

// package.json
  ...
  "proxy": "http://localhost:4000",
}

This will allow you to keep your API calls nice and clean.

fetch("/api/session", ...

No need to manage some sort of host URL environment variable.

Additionally, this will remove an CORS issues because the webpackDevServer will be proxying any paths that it doesn’t recognize to the host and port that you’ve specified.

See the create-react-app docs for more details.

Split The Current Window

Generally when I want to open up a buffer in a window split, I use :sp or :vsp and type out the filename. It may be a bit more convenient to use window commands to create the splits.

To open a horizontal split:

Ctrl-w s

To open a vertical split:

Ctrl-w v

With the split made, you can use Ctrl-o/Ctrl-i, BufExplorer, or whatever way you like to navigate between buffers.

h/t Ryan Messner

Turn Off HTML5 Form Validations

HTML5 added a variety of input types such as date, email, and tel (telephone number). These make entering certain kinds of information easier by providing tools such as a datepickers, range sliders, and optimized keyboards on mobile devices.

These input types also introduce custom validations on some browsers, which are useful, but can potentially clash with your application’s look and feel.

To keep the input type, but disable the validations, add novalidate to your form tag:

<form novalidate />

h/t Dillon Hafer

docs

Make console.log stand out with custom css style

I know your browser console is full of messages because you are debugging something, and that creates a lot of noise. Now you are adding a new console.log, and you need it to stand out above the rest.

Maybe you are like facebook and just want to warn your users from pasting in code in the browser in social engineering attacks.

facebook

To style a console.log message use the %c interpolation and pass it a css style. e.g.

console.log('%c%s', 'color:red;font-size:5em', alert)

In the example above %s means inerpolate the object into the output string.

preview

Compatibility: tested to work on Firefox, Chrome, and Safari.

h/t Dillon Hafer

Storing recurring schedules in #Rails + #Postgres

If you have a scheduling component to your Rails application you may need to store the day of week and time of day in the database.

One way to store the day of week is to use an integer column with a check constraint that will check that the value is between 0 and 6.

create table schedules (
  id serial primary key,
  day_of_week integer not null check(day_of_week in (0,1,2,3,4,5,6)),
  beg_time time not null,
  end_time time not null
);

Then when you read it back from the database and need to convert it back to day name you can use Date::DAYNAMES. e.g.:

[2] pry(main)> require 'date'
=> true
[3] pry(main)> Date::DAYNAMES
=> ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"]
[4] pry(main)> Date::DAYNAMES[0]
=> "Sunday"
[5] pry(main)>

If you need to store time of day as entered (in a time without timezone column - as specified above) check out the wonderful Tod gem by Jack Christensen

Checking that an association is loaded

Ecto will NOT load associations automatically, that is something that you must do explicitly and sometimes you might expect an association to be loaded but because of the code path travelled, it is not.

You can check to see if an association is loaded with Ecto.assoc_loaded?

case Ecto.assoc_loaded?(post.channel) do
  true -> IO.puts('yep its loaded')
  false -> IO.puts('you do not have the data')
end

Ruby Binding Class

Ruby’s Binding class allows you to access classes, variables, and methods outside of the current scope.

class Foo
    def bar
        my_var = 20
        binding()
    end
end

Normally, if you made reference to my_var outside of that method, you’d get an error. Binding objects allow you to do this:

my_foo = Foo.new

foo_bar = my_foo.bar
puts foo_bar.eval('my_var')

# 20

Legacy Refs vs Ref Callbacks

Reactjs provides a way to get references to dom elements that react is rendering through jsx. Previously, it was through what are now legacy refs:

componentWillUpdate() {
  this.refs.thing.tagName == "div";
}

render() {
  return (
    <div ref="thing"/>
  );
}

Where you can assign an element an identifier and react would keep a refs hash up to date with references to the dom for that element.

The react docs say this about the previous system.

We advise against it because string refs have some issues, are considered legacy, and are likely to be removed in one of the future releases.

The new system uses callbacks

render() {
  return (
    <div ref={(div) => { console.log('tag name:', div.tagName); }} />
  );
}

This callback is called when the component mounts with a reference to the dom element as an argument. Importantly, when the component unmounts the callback is called again but this time with null as an argument.

Install a Homebrew Formula at Master

We’ve been testing out the forthcoming Elixir 1.6 formatter on Tilex’s codebase, which requires a version of the langage that has not yet been officially released.

There’s a few ways to do this. Since Elixir is installed with Homebrew on my machine, here’s how to do it with Homebrew:

$ brew install elixir --HEAD

From the Homebrew man page:

If --HEAD is passed, and formula defines it, install the HEAD version, aka master, trunk, unstable.

Welcome to the bleeding edge!

h/t Brian Dunn

Stacking HEREDOCS

I ran across this in a co-workers code yesterday. It made my head hurt for a second. Let’s say you need a a function that takes 4 lines as an argument and outputs each of those lines to stdout.

def print_stanza(line1, line2, line3, line4)
  puts line1, line2, line3, line4
end

print_stanza(<<-LINEA, <<-LINEB, <<-LINEC, <<-LINED)
  You can get all hung up
LINEA
  in a prickle-ly perch.
LINEB
  And your gang will fly on.
LINEC
  You'll be left in a Lurch.
LINED

The second HEREDOC starts where this first one ends, the third HEREDOC starts where the second one ends, etc. Its all valid Ruby and starts to make sense if you look at it long enough.

Most editors I’ve seen haven’t been able to highlight the syntax correctly though. Sorta leaves you in a Lurch.

H/T Brian Dunn

Show File Diffs When Viewing Git Log

Include the -p flag with the git log command to include the diffs along with the rest of the information for each commit. Here is an example of running this against my TIL repository.

$ git log -p
commit b9809a329acd8150b2474168f8faaf008f376e35
Author: jbranchaud <jbranchaud@gmail.com>
Date:   Wed Oct 11 07:27:53 2017 -0500

    Add Inline Style Attributes Should Be Camel Cased as a react til

diff --git a/README.md b/README.md
index c982f8e..6ee7d32 100644
--- a/README.md
+++ b/README.md
@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).

-_574 TILs and counting..._
+_575 TILs and counting..._

 ---

See man git-log for more details.

.babelrc ignores babel settings in package.json

There are three ways to provide babel configuration listed on the usage page for babelrc. You can provide a .babelrc file, you can place a babel section in your package.json or you can declare configuration with an env option.

If you have both a .babelrc in your root dir AND a babel section in your package.json file then the settings in the package.json will be completely ignored.

This happened to me when needing to declare an extra plugin for babel that create-react-app did not provide. I ejected from create-react-app, I added a .babelrc that declared the extra plugin, and this broke the build for my app. The babel configuration for an ejected create-react-app is in the package.json file.

Convert to BigDecimal with `to_d` w/ActiveSupport

Ruby provides the BigDecimal method to convert to BigDecimal.

> require 'bigdecimal'
> BigDecimal("123.45")
#<BigDecimal:56236cc3cab8,'0.12345E3',18(18)>

But you can’t convert a float without a precision

> BigDecimal(123.12)
ArgumentError: can't omit precision for a Float.
> BigDecimal(123.12, 5).to_s
"0.12312E3"

When using Rails, and specifically with ActiveSupport required, you can use the to_d method converts to BigDecimal.

> require 'active_support'
> 123.to_d
#<BigDecimal:55ebd7800ea8,'0.123E3',9(27)>
> "123".to_d
#<BigDecimal:55ebd7800ea8,'0.123E3',9(27)>

And for floats provides a default precision of Float::DIG+1 which for me is 16. DIG is described as

The number of decimal digits in a double-precision floating point.

> 123.45.to_d
#<BigDecimal:55ebd2d6cfb8,'0.12345E3',18(36)>
> 123.45.to_d.to_s
"123.45"

Note, to_s in ActiveSupport outputs a more human readable number. Also Note, nil is not convertable with to_d

> require 'active_support'
> nil.to_d
NoMethodError: undefined method `to_d' for nil:NilClass
> BigDecimal(nil)
TypeError: no implicit conversion of nil into String

Inline Style Attributes Should Be Camel Cased

When adding a few quick styles to React components, you can add it directly on the tags in the JSX. To do this, use the style tag with a plain old JavaScript object of styles.

<div style={{ padding: "1em", color: "#fff" }}>

If you are using a CSS attribute that is normally hyphenated like padding-top or background-color, you’ll need to camel case it in the JSX.

<div style={{ paddingTop: "1em", backgroundColor: "#fff" }}>

This is because our styles now need to conform to JavaScript syntax rules since they are in the form of a POJO.

Read the documentation for more details.

Clean Up Autojump

autojump is a command-line tool for faster directory changing. It’s a must-have on my workstation, because it lets me jump to deeply nested directories like this:

:~/$ j cli
/Users/jwworth/projects/2017/twitter-killer/client
:~/jwworth/projects/2017/twitter-killer/client$

There are two ways we can clean up autojump once it’s been running for a while. First, purge non-existant directories from the jump database:

$ j --purge
Purged 8 entries.

Second, edit the file ~/Library/autojump/autojump.txt (OSX), which stores the jump database. Here you can remove directories that should never be jumped to, or are weighted too highly because of frequent use in the past.

Happy jumping!

Passing Props Down To React-Router Route

When using react-router, you’ll often use the component prop to have a certain component rendered.

<Route
  path="/my/path"
  component={MyComponent}
/>

If, however, you need to pass props down into MyComponent, then you’ll want to use the render prop with an inline function.

<Route
  path="/my/path"
  render={(routeProps) => (
    <MyComponent {...routeProps} {...props} />
  )}
/>

The two spread operator statements are essential. They will pass down the route props that Route would have passed down plus the additional set of props that you want to pass down.

`requestAnimationFrame` should call itself

This style of animation is useful when you’re making small changes via javascript. When you pass requestAnimationFrame a callback, the callback is called before a browser repaint, or about 60 times a second. To make sure that you’re getting 60 callbacks a second, you must call requestAnimationFrame from within your callback.

function animate() {
  makeSomeSmallChangeToHtmlOrCss();
  requestAnimationFrame(animate);
}

This is a recursive function, so without an exit condition, it will recurse infinitely.

H/T Brian Dunn

Deleting Words in Vim

As a recent vim convert, I learn new commands everyday from my fellow rocketeers. Here’s some cool ones for deleting words:

  • Use dw to delete word. Cursor placement is important! If your cursor is not on the first character, it will only delete from your cursor to the end of the word.
  • Use diw to delete inside word. Deletes the entire word that your cursor resides in.
  • Use dt<char> to delete to character. Deletes from your cursor to the specified character.

Tidy your Reducers with `combineReducers()`

Today I learned by necessity the value of the Redux combineReducers(reducers) function.

combineReducers() supports, in a pretty neat way, the crucial Redux task of delegating to reducing functions their own slice of the state.

The documentation example looks like this:

combineReducers({ todos: myTodosReducer, counter: myCounterReducer });

Which we can improve by naming the reducer functions after the state slices they manage, allowing ES6 shorthand notation:

combineReducers({ counter, todos });

This creates a state object like so:

{
  counter: ...,
  todos: ...,
}

Break Current tmux Pane Out To Separate Window

You have a split pane open in your current window. Perhaps it is running a local server or has a connection to a database. The pane is taking up too much space in the current window, but it is important so you don’t want to just kill it. If that is the case, break it out into a separate window.

With that pane in focus, enter into command mode (<prefix>:) and then issue the break-pane command.

Alternatively, you can trigger this with a keybinding: <prefix>!.

See man tmux for more details.

Jump To The Next Misspelled Word

If you turn on spelling with set spell, Vim will highlight any words it considers misspelled. For a large document it can be tedious to scroll through and find all of the spelling errors.

Fortunately, Vim makes it easy to jump to the next misspelled word. Hit ]s and your cursor will be transported to that word.

Likewise, hit [s to jump backwards in the document to the closest misspelled word behind the cursor.

See h ]s for more details.

Upgrading npm when using `asdf` with `reshim`

The version of npm that comes with nodejs when installed with asdf may not be the latest. In my case I had npm 5.3.0 installed and the newest version is 5.4.2. I upgraded npm with npm install -g npm and saw output that made me think everything installed successfully, but when I ran npm -v I still got 5.3.0.

The answer is to use asdf’s reshim command.

> asdf help reshim
asdf reshim <name> <version>    Recreate shims for version of a package

I ran the following command:

> npm -v
5.3.0
> asdf reshim nodejs
> npm -v
5.4.2

And now I have the latest version and everything is great!

Show Create Statement For A Table In MySQL

In MySQL, you can get a quick rundown of a table using describe users. An alternative to this approach is to have MySQL show the create statement for a table.

> show create table users\G
*************************** 1. row ***************************
       Table: users
Create Table: CREATE TABLE `users` (
  `id` mediumint(8) unsigned NOT NULL AUTO_INCREMENT,
  `first_name` varchar(80) NOT NULL,
  `last_name` varchar(80) NOT NULL,
  `email` varchar(80) NOT NULL,
  `middle_initial` varchar(80) NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `unique_email` (`email`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1

This includes some additional information like primary key and index information. It is also a great way to study the SQL that it takes to create all the facets of a table.

See the show create table docs for more details.

h/t Jake Worth

What you had before you saved w/`previous_changes`

When you set values into the ActiveRecord object the previous values are still available with changes, but when you save, however you save, those changes are wiped out. You can access what those values were before saving with previous_changes.

> thing = Thing.create({color: 'blue', status: 'active'})
> thing.color = 'red'
> puts thing.changes
{"color" => ['blue', 'red']}
> puts thing.previous_changes
{}
> thing.save
> puts thing.changes
{}
> puts thing.previous_changes
{"color" => ['blue', 'red']}

Call a program one time for each argument w/ xargs

Generally, I’ve used xargs in combination with programs like kill or echo both of which accept a variable number of arguments. Some programs only accept one argument.

For lack of a better example, lets try adding 1 to 10 numbers. In shell environments you can add with the expr command.

> expr 1 + 1
2

I can combine this with seq and pass the piped values from seq to expr with xargs.

> seq 10 | xargs expr 1 + 
expr: syntax error

In the above, instead of adding 1 to 1 and then 1 to 2, it trys to run:

expr 1 + 1 2 3 4 5 6 7 8 9 0

Syntax Error!

We can use the -n flag to ensure that only one argument is applied at time and the command runs 10 times.

> seq 10 | xargs -n1 expr 1 +
2
3
4
5
6
7
8
9
10
11

For more insight into what’s being called, use the -t flag to see the commands.

Use dotenv In A Non-Rails Project

Up to now I’ve only used dotenv in a Rails context. It can just as easily be used in a plain old Ruby project.

Install the non-Rails version of the gem.

$ gem install dotenv

Then add the following lines wherever you want dotenv included and loaded. In my case, I want it pulled in as part of my RSpec setup in spec_helper.rb.

require 'dotenv'
Dotenv.load

Your environment variables declared in .env are now accessible via fetches against the ENV object.

ENV.fetch('my_env_var')

Mix tasks accessing the db with `Mix.Ecto`

When running reports or one-off data operations it might be necessary to create a mix task that can access the database. Ecto provides convenience functions in the Mix.Ecto module to help facilitate setting up and starting the Ecto repos.

The function parse_repo(args) will process the arguments used when calling the mix task. It looks specifically for -r MyApp.Repo but if you don’t pass anything it will return all the repos from the configuration.

The function ensure_started(repo) takes the repo as an argument ensures that the Repo application has been started. Without calling this function the Repo will throw an error when used.

Put it all together:

defmodule Mix.Tasks.MyApp.SayHi do
  use Mix.Task
  import Mix.Ecto

  def run(args) do
    repo = parse_repo(args) |> hd

    ensure_started(repo)

    result = repo.query("select 'hi!';")

    result.rows
    |> hd
    |> hd
    |> IO.puts
  end
end

Force A Component To Only Have One Child

A component can normally have an arbitrary number of elements nested directly inside it. React’s Children.only function can be used to force it to a single direct child.

import React, { Children, Component } from "react";

class App extends Component {
  render() {
    return (
      <SingleChildContainer>
        <span>There can only be one!</span>
      </SingleChildContainer>
    );
  }
}

const SingleChildContainer = props => {
  return Children.only(props.children);
};

export default App;

The React docs describe the behavior of Children.only as such, “Returns the only child in children. Throws otherwise.”.

If you modify the return in App to contain the following JSX

<SingleChildContainer>
  <span>There can only be one!</span>
  <div>What about me?!</div>
</SingleChildContainer>

then an error will be thrown (React.Children.only expected to receive a single React element child).

The Provider component in react-redux is an example of where this is used.

Use `source /dev/stdin` to execute commands

Let’s say there’s a command in a file, like a README file, and you don’t have any copy or paste tools handy. You can get the command out of the README file with:

> cat README.md | grep "^sed"
sed -ie "s/\(.*\)/Plug '\1'/" .vimbundle.local

Great! Now how do we run it? The source is generally used to read and execute commands in files, and really /dev/stdin behaves like a file.

You can use the pipe operator to place the command into stdin and then source will read from stdin.

> cat README.md | grep "^sed" | source /dev/stdin

A simpler example can be constructed with echoing

> echo "echo 'hi there'"
echo 'hi there'

And

> echo "echo 'hi there'" | source /dev/stdin
hi there

From Ruby Variables To JavaScript Variables

I sometimes find myself writing so much Ruby that as soon as I am back in a JavaScript file, my code starts looking like this:

const my_javascript_var = 123;

It would be easy enough to hit caw to the delete the entire word and then retype it as camel case. I happen to have the Abolish.vim plugin installed, so there is an even quicker way.

If I hit crc over the variable, it will be coerced to camel case.

const myJavascriptVar = 123;

If I hit crs then it will be coerced back to snake case. Hit crc one more time and I can get back to writing some JavaScript.

See :h abolish-coercion for more details.

Escaping String Literals With Dollar Quoting

String literals in PostgreSQL are defined by surrounding the content with the ' character. For string literals that contain the ' character, you may have seen it escaped with a preceding '.

> select 'Isn''t this nice?';
     ?column?
------------------
 Isn't this nice?

This is easy enough to do, but can be error prone and doesn’t work well if SQL is being programmatically generated. A great workaround is to escape string literals using what is called dollar quoting.

> select $$Isn't this even nicer?$$;
        ?column?
------------------------
 Isn't this even nicer?

Just wrap both ends in $$ instead of '.

source

Print Information about an Elixir Data Type

IEx ships with a neat feature, def i(term \\ v(-1)), which provides information about an argument. Here are two examples:

iex(1)> i ["one", "two"]
Term
  ["one", "two"]
Data type
  List
Reference modules
  List
Implemented protocols
  IEx.Info, Collectable, Enumerable, Inspect, List.Chars, String.Chars
iex(1)> i "three"
Term
  "three"
Data type
  BitString
Byte size
  5
Description
  This is a string: a UTF-8 encoded binary. It's printed surrounded by
  "double quotes" because all UTF-8 encoded codepoints in it are printable.
Raw representation
  <<116, 104, 114, 101, 101>>
Reference modules
  String, :binary
Implemented protocols
  IEx.Info, Collectable, Inspect, List.Chars, String.Chars

Use this to inspect an item, or learn more about Elixir.

Access record from ActiveRecord::RecordInvalid

You can pass an array of hashes to Thing.create! in ActiveRecord. If one of those records is invalid, then an ActiveRecord::RecordInvalid error is thrown. You might need to know which record threw the error, in which case you can get the record from the error with record_invalid_error.record

bad_record = nil

begin
  Things.create!([{value: 'bad'}, {value: 'good'}])
rescue ActiveRecord::RecordInvalid => record_invalid_error
  bad_record = record_invalid_error.record
end

if bad_record
  puts "got a bad record with value: #{bad_record.value}"
end

Require Additional JS Libraries In Postman

When writing pre-request scripts and test scripts as part of a Postman request, you aren’t limited to vanilla JavaScript. There are a handful of libraries that can be required where needed.

This short list of available libraries includes cherrio, lodash, and moment.

To pull one of these into a particular script, use the standard require feature:

var moment = require('moment');

var now = moment();

For a full list of what is available, check out Postman’s Sandbox API Reference.

Array of hashes `create` many ActiveRecord objects

Generally, you use the create method of ActiveRecord objects to create an object by passing a hash of attributes as the argument.

Thing.create(color: 'green', status: 'active')

You can also pass an array of hashes to create:

things = [
  {
    color: 'blue',
    status: 'pending'
  },
  {
    color: 'green',
    status: 'active'
]

created_things = Thing.create(things)

One disappointing thing is that this does not batch the insert statements. It is still just one insert statement per object, but it might make your code simpler in some cases.

Render An Array Of Elements With React 16

React 16 was released today. Among many exciting features and updates is support for rendering an array of elements.

This can look as simple as this example:

return [
  <li key="1">One</li>,
  <li key="2">Two</li>,
  <li key="3">Three</li>
];

It really shines in the case of generating elements from an array of data.

let data = [
  { value: "One", key: "1" },
  { value: "Two", key: "2" },
  { value: "Three", key: "3" }
];
return data.map(item => {
  return (
    <li key={item.key}>
      {item.value}
    </li>
  );
});

No need to wrap the result in a <div>!

source

Environmental Variables and Chaining with Postman

Postman is a tool for building APIs. We’ve been digging into it today; it’s got some great features.

Today I learned we can chain requests, passing the result from one API interaction to all subsequent interactions.

Let’s say one API call gets a token on success, a token that all other calls require. When we run the first request, we can add a test that pulls out and sets the token:

var data = JSON.parse(responseBody);
postman.setEnvironmentVariable("token", data.token);

Now, other tests can reference {{token}} in their headers. We can even chain calls in Postman’s automated test runner, ensuring that the token-getting request always runs first.

more info

`github` as source block in Gemfile

source blocks in a Ruby Gemfile help group gems together that come from the same source. In addition, the Gemfile supports a github block for multiple gems that are coming from the same github repository. In my specific case, there are two gemspecs in the Brian Dunn’s flatware repo.

github 'briandunn/flatware', branch: master do
  gem 'flatware-rspec'
  gem 'flatware-cucumber'
end

With this example, only one change is needed to change the branch that both of those gems will come from.

H/T Brian Dunn

Split large file into multiple smaller files

Bash has a handy tool to split files into multiple pieces. This can be useful as a precursor to some parallelized processing of a large file. Lets say you have gigabytes of log files you need to search through, splitting the files into smaller chunks is one way to approach the problem.

> seq 10 > large_file.txt
> split -l2 large_file.txt smaller_file_
> ls -1
smaller_file_aa
smaller_file_ab
smaller_file_ac
smaller_file_ad
smaller_file_ae
large_file.txt

First, I created a “large” file with ten lines. Then, I split that file into files with the prefix smaller_file_. The -l2 option for split tells split to make every 2 lines a new file. For 10 lines, we’ll get 5 files. The suffix it adds (“aa”, “ab”, …) sorts lexigraphically so that we can reconstruct the file with cat and globs.

> cat smaller_file*
1
2
3
4
5
6
7
8
9
10