Today I Learned

A Hashrocket project

"experience uses an unsupported version of Expo"


If this happens when developing a React Native app with Expo and trying to test it in the iOS Simulator, it means the version of Expo on the iOS Simulator is out of date.

To fix that try the following:

  1. Quit the Expo app on the simulator
  2. Uninstall the Expo app on the simulator
  3. Lunch the app again in the simulator from the Expo XDE

This will cause the Expo app on the simulator to reinstall with the latest version.

Move window (tab) in tmux

To move a new window to the left do this:

tmux-prefix, :swap-window -t -1

To move window to the right

tmux-prefix, :swap-window -t +1

You can also bind a key to that command:

bind-key S-Left swap-window -t -1
bind-key S-Right swap-window -t +1

Now it will be tmux-prefix, Shift + Left and tmux-prefix, Shift + Right

Require local package in mix.exs

In Elixir as you are writing your application it is recommended to split parts of it into smaller applications (also can be called micro-services if you want to be buzzword compliant).

You don’t however need to publish those dependencies to the Hex package manager in order to load them, instead you can use the path argument when defining a dependency.

In this example we have our main application Foo and in the directory above it we have an application called Bar.

To make the Bar module available in Foo we can do it like so:


defp deps do
    {:bar, path: "../bar"},

List Merged Branches

Today I learned a new Git trick: to show branches already merged into a branch, try this command:

$ git branch --merged master

For an open source project like Today I Learned, this is an interesting way to get a sense of the project.

Module attribute constants nil in Elixir

Module attributes in elixir (@something) can be used as constants and assigned a value, however one must make sure that the value assigned to the constant is available at compile time.

For example when using a constant that loads environment variables if the value is not available in compile time they will resolve to nil.

@salt System.get_env(:your_app_name, "AUTH_SALT")

So either export the environment variables before compiling, or don’t use module attributes, instead you can use a function:

defp salt do
  System.get_env(:your_app_name, "AUTH_SALT")


  • The downside to using a function is that it will be re-evaluated each time.
  • The downside of exporting env-vars before compile is that they you might forget and your app will crash in production. You can circumvent that by writing a script for compilation.
  • You can also call System.get_env from your config.exs but make sure to run mix clean after doing so since the compiler seems to cache the config compilation.

Weekly jobs in crontab

The syntax for crontab entries is confusing at best. Fortunately, cron provides easy to read special strings for common cases.

If you want a job to run once a week your crontab would look like this:

@weekly /usr/bin/my_job

And thats it! @weekly in this case means 0 0 * * 0. Every Sunday at midnight.

You can check other crontab special strings with man 5 crontab.

H/T Dorian Karter

Authenticate With Username or Email

Today I learned a way to implement a login form that accepts email or username as the login. I’ve been on the other side of a form like this many times, but had never written one myself.

Here’s one solution, with ActiveRecord:

User.where('username = ? or email = ?', "jwworth", "")

title or slug are represented by the same parameter, and either can be nil.

login = params.fetch('username_or_email')
User.where('username = ? or email = ?', login, login)

Vim move cursor within a visual selection

When expanding a visual selection you might want to increase both sides. For instance, if you have made a selection from line 20 to line 40 you may want to expand the selection to line 18 to line 42. The visual selection can only be expanded with the cursor and the cursor is only on one side, but the cursor can be moved to the other side of the selection with o.

Read more about it with: :help visual-change.

Convert nested JSON object to nested OpenStructs

If you are parsing a nested JSON string such as this:

   "vendor": {
       "company_name": "Basket Clowns Inc",
       "website": ""

And want to access it with dot notation, simply doing:

will not do!

Turns out there is a cool option on JSON.parse called object_class:

JSON.parse(json_str, object_class: OpenStruct)

Now you can access the resulting object with dot notation all the way down: #=> ""

Toggle line numbers in VIM

I often find myself needing to turn line numbers on and off in VIM. I’ve typically typed out the full

off - set nonumber and on - set number

Theres also abbreviations:

off - set nonu and on - set nu

TIL you can also toggle quickly between on and off by using:

set nu!

h/t Jason Cummings

Allow an Empty Commit

Today my pair did something I’d never seen: create an empty commit. Out goal was to test a third-party integration, but we didn’t have a meaningful change to contribute at the time. In this situation, I think an empty commit is a good compromise, because it’s honest about our intent— to simply kick off a remote process.

Here was the command:

$ git commit --allow-empty

h/t Dorian Karter

Fork A Heroku Project

The Heroku CLI has a useful command I recently learned about, called heroku fork. Here’s how it works:

$ heroku fork --from tilex-staging --to tilex

As the example suggests, I’ve used this tool to clone a tricky-to-set-up Heroku staging application as a production application. It’s great.

The bad news is that Heroku us sunsetting fork as a core CLI command. Use it while you can. After that, you can fork the Github repo and use it as CLI plugin.

Set filetype/settings for a specific file in Vim

Given a file with a weird extension but an underlying known filetype e.g. yaml file with a .config extension. It is possible to force vim to set the filetype to yaml for that file:

At the top/bottom of the file add a comment (with the filetypes acceptable comment syntax - for yaml it is #):

# vi: ft=yaml
  foo: 'bar'

Re-open the file and vim will automatically set the filetype to yaml for that file.

This can also be used for setting other setting such as shiftwidth, tabstop etc

I’ve been using this trick for a while but keep forgetting it the exact syntax, usually using # vim: instead of # vi:. Hopefully my wording will make it more easily duckduckgoable.

Ruby Object Reference in blocks

Usually when I want to set the value of a variable by yielding to a block, I will explicity set it

  x = 'something'
  x = yield(x)

Turns out ruby will treat different types of objects different when yielded to a block (remember everything is an object). If you are yielding an Integer, or String for (a contrived) example:

def do_stuff_by_val
  do_block_stuff_by_val do |name|
    name += 'rocket'

def do_block_stuff_by_val
  company_name = 'hash'
  puts company_name #prints 'hash', not 'hashrocket'

If we wanted the company name to change to “hashrocket”, we would need to explicitly set the variable to the result of yielding:

company_name = yield(company_name)

Other types of objects do not behave this way (hash or array). If we yield a hash and set a key, the hash in the calling method will be updated:

def do_stuff_by_object_reference
  do_block_stuff_by_object_reference do |company|
    company[:name] = 'hashrocket'

def do_block_stuff_by_object_reference
  company = {}
  puts company # { name: 'hashrocket'}

Typing card suits in vim ♢♡♣♠

Digraphs are two letter combinations that provide a easy way to type and remember non-ascii characters in vim. In insert mode you can type a digraph with Ctrl-k and then two letters that map to the unicode character.

For card suits the pneumonic is lower case c for card and upper case C for clubs, D for diamonds, H for hearts, and S for spades.

cD ♢
cC ♣
cH ♡
cS ♠

You can see all the digraphs with :digraphs.

Now you have all the necessary unicode characters to easily create card games in the terminal!

Show Tables That Match A Pattern In MySQL

An unfamiliar MySQL database with tons of tables can be a difficult thing to navigate. You may have an idea of the kind of table you are looking for based on a domain concept you’ve seen elsewhere.

You can pare down the results returned by show tables by including a like clause with a pattern. For example, this statement will show me only tables that have the word user in them:

> show tables like '%user%';
| Tables_in_jbranchaud (%user%) |
| admin_users                   |
| users                         |

Resetting the bundler --path

When running bundler install you can pass an option --path <dir>:

bundle install --path ./gems

Which will specify where to install the gems. This is a one time option. Everytime you invoke bundle from here on in you’ll see that its using the directory you specificed with the —path variable as the gem directory.

Bundler keeps track of this option in the .bundler/config file in the folder from where bundler was initially run. That file looks like this:

BUNDLE_PATH: "./gems"

To reset the gems path back to the default, remove the BUNDLE_PATH line from the bundle/config file.f

Rename A Remote

If you just added a remote (git remote add ...) and messed up the name or just need to rename some existing remote, you can do so with the rename command.

First, let’s see the remotes we have:

$ git remote -v
origin (fetch)
origin (push)

To then rename origin to destination, for example, we can issue the following command:

$ git remote rename origin destination

See man git-remote for more details.

Override ssh command for git

Git supports a number of environment variables one of them being GIT_SSH. You can override the default ssh command (ssh of course) with that environment variable.

GIT_SSH=./my_ssh git pull origin master

In this case my_ssh is a bash executable file that looks like this:


ssh -i ~/.ssh/id_rsa $1 $2

An idea I got from this stack overflow answer.

Clearly the private key file I used is the default private key file so a bit redundant but its a way to add any options that might be necessary for the ssh connection, although specifying a host configuration in the ssh_config file might be simpler.

Dump A MySQL Database To A File

The mysqldump client is a handy tool for creating a backup or snapshot of a MySQL database. The standard use of this command produces an alphabetical series of statements that comprise the structure and data of the specified database. It directs all of this to stdout. You’ll likely want to redirect it to a file.

$ mysqldump my_database > my_database_backup.sql

The output will include special comments with MySQL directives that disable things like constraint checking. This is what allows the output to be in alphabetical order without necessarily violating any foreign key constraints.

If you need to dump multiple databases, include the --databases flag with a space-separated list of database names. Or dump all of them with --all-databases.

See man mysqldump for more details.

Reinstall all rubies with rvm

When compiling ruby the executable is attached to various C system libs like libreadline. If libreadline is updated then each ruby on the system attached to the old libreadline must be re-compiled. If you are using rvm then this can be done with one command:

rvm reinstall all --force

The --force flag will skip any question the process asks.

Be careful, if you are working on a machine that has installed many rubies and has had many projects for those rubies this could take an extraordinarily long time.

Specifying the private key to use in ssh

A private key is necessary in ssh to authenticate the connection securely. Generally ssh will look for a file in the ~/.ssh/ dir with a name of id_rsa (or if not using rsa a file beginning with id_ and the encryption algo name). That file should contain the private key.

You can, however, specify a different primary key at the command line with the -i flag like so:

ssh -i ~/.ssh/my_other_key.pem

Additionally, you may use the IdentityFile option the the ssh_config file to permanently configure the private key for a specific connection.

Display Output In A Vertical Format In MySQL

Output for tables with lots of columns can be hard to read and sometimes overflow the terminal window. Consider the output from Show Indexes For A Table:

> show indexes in users;
| Table | Non_unique | Key_name     | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
| users |          0 | PRIMARY      |            1 | id          | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
| users |          0 | unique_email |            1 | email       | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |

We can vertically orient the output of a statement by terminating it with \G instead of ; (or \g).

> show indexes in users\G
*************************** 1. row ***************************
        Table: users
   Non_unique: 0
     Key_name: PRIMARY
 Seq_in_index: 1
  Column_name: id
    Collation: A
  Cardinality: 0
     Sub_part: NULL
       Packed: NULL
   Index_type: BTREE
*************************** 2. row ***************************
        Table: users
   Non_unique: 0
     Key_name: unique_email
 Seq_in_index: 1
  Column_name: email
    Collation: A
  Cardinality: 0
     Sub_part: NULL
       Packed: NULL
   Index_type: BTREE

Show Indexes For A Table In MySQL

When describing a table, such as users:

> describe users;
| Field      | Type                  | Null | Key | Default | Extra          |
| id         | mediumint(8) unsigned | NO   | PRI | NULL    | auto_increment |
| first_name | varchar(80)           | NO   |     | NULL    |                |
| last_name  | varchar(80)           | NO   |     | NULL    |                |
| email      | varchar(80)           | NO   | UNI | NULL    |                |

We can see in the Key column that there’s a primary key and a unique key for this table on id and email, respectively.

These keys are indexes. To get more details about each of the indexes on this table, we can use the show indexes command.

> show indexes in users;
| Table | Non_unique | Key_name     | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type |
| users |          0 | PRIMARY      |            1 | id          | A         |           0 |     NULL | NULL   |      | BTREE      |
| users |          0 | unique_email |            1 | email       | A         |           0 |     NULL | NULL   |      | BTREE      |

Hack to size the container of a background image

An element that contains a background image cannot have its height and width derived from that background image. An element can have a height and width that is determined by the size of a child element however. Combine the two to ensure that the container of the background image has the height and width of the image itself.

<div style="background-image: url('')">
    <img src="">

List Databases And Tables In MySQL

If you’ve started a mysql session, but haven’t connected to a particular database yet, you can list the available databases like so:

> show databases;
| Database                    |
| information_schema          |
| my_app_dev                  |

If you are curious about the tables in a particular database, you can list them by specifying the database’s name:

> show tables in my_app_dev;
| Tables_in_my_app_dev         |
| pokemons                     |
| trainers                     |

Alternatively, you can connect to the database of interest and then there is no need to specify the name of the database going forward.

> use my_app_dev;
> show tables;
| Tables_in_my_app_dev         |
| pokemons                     |
| trainers                     |

Change The Nullability Of A Column

Do you have an existing table with a column that is exactly as you want it except that it needs to be changed to either null: false or null: true?

One option is to use ActiveRecord’s change_column_null method in your migration.

For example to change a nullable column to null: false, you’ll want a migration like the following:

def change
  change_column_null :posts, :title, false

Note, if you have existing records with null values in the title column, then you’ll need to deal with those before migrating.

If you want to make an existing column nullable, change that false to true:

def change
  change_column_null :posts, :title, true

`border-collapse` to collapse borders

There are two settings for the css property border-collapse, separate and collapse. Table cells and the table itself can both have borders. If the value separate is set, then each cell has its own border, with the space in between the borders governed by the border-spacing property.

If collapse is set, then the table cells and the surrounding table will share the borders surrounding them.

`any` and `all` special Postgres constructs

To determine if a value exists in a subquery or scalar value list you can use the in operator like:

> select 1 in (1, 2)

any and all go a step further by allowing for an operator in the special construct which is used against each value in the subquery or array (no scalar value list for these constructs).

any examples:

> select 1 = any(Array[1, 2, 3])
> select 1 > any(Array[1, 2, 3])

all examples

> select 17 <> any(Array[2, 4, 8])
> select 17 < any(Array[2, 4, 8])

some is a synonym for any.

Remove, Get, And Compile a dependency

To get a better feel for how a dependency works I’ll do some IO.puts style debugging, but when I’m done I need to clean up all those IO.puts statements.

mix deps.clean ecto

The above command will remove two directories, the _build/dev/lib/ecto dir and the deps/ecto dir.

mix deps.get ecto

This command will resolve the dependency tree and get the appropriate version of ecto based on the information in your mix.exs file. It will not reach out to the internet if the package is already cached as a tar file in the .hex/packages/hexpm dir. It will unpack the tar file into the local deps/ecto dir but it will not compile it.

mix deps.compile ecto

Will compile ecto to the _build/dev/lib/ecto directory.

Refresh A Twitter Summary Card Image

Twitter provides a feature called summary card images: if you’ve ever seen a tweet with a rectangular image included, that’s it. We can specify this image via HTML meta tags. Most tweets from the @hashrocketil follow this pattern.

Changing the image is tricky, however, because Twitter caches summary cards for an unknown amount of time (I’ve read one week). Which means past and future Tweets could include an outdated image for a while.

One solution is to use Twitter’s card validator on your site. Running the site it through the validator seems to refresh the cache for your summary cards.

What file is this?

If you are working in an environment where the status line is not displayed and you don’t know what file you are currently editing, you can use the :file command to give you that information.

"app/models/post.rb" line 43 of 139 --30%-- col 4

:f is the abbreviated version

Fingerpring of an ssh key in multiple formats

You might need to confirm which keys you have attached to your github account. Github provides a list of your keys with the associated fingerprints. If you have a public key on your machine you can see what fingerprint it has with:

> ssh-keygen -lf ~/.ssh/
4096 SHA256:/6Rat4zVZ0auHEwWtF4QG8NA0j4NAKWisFuXV0ZP5zk (RSA)

Github displays an md5 fingerprint rather than a sha256 fingerprint. You can use the -E flag to choose a specific hash type.

> ssh-keygen -E md5 -lf ~/.ssh/
4096 MD5:e3:19:85:1c:8a:31:0f:4b:de:cb:6d:a7:64:b4:4e:e7 (RSA)

Find host and port in development

Rails 5.1 is a little different the pre 5.1

require 'rails/commands/server/server_command'

In rails 5.0 or below

require 'rails/commands/server'
# lib/host_and_port.rb

def __host_and_port__
  options =!(ARGV)
  options.values_at(:Host, :Port)

You can then find the host and port for various configuration files.

# config/initializers/carrier_wave.rb
require Rails.root.join('lib/host_and_port').to_s

CarrierWave.configure do |config|
  config.asset_host = "http://" + __host_and_port__.join(":")


# config/environments/development.rb
require Rails.root.join('lib/host_and_port').to_s

host, port = __host_and_port__
config.action_mailer.default_url_options = { host: host, port: port }

Setting the desktop picture in Gnome

Gnome’s going to be the default desktop for Ubuntu as of version 17.10. To me this is exciting stuff and its never too early to start learning how to configure the gnome desktop.

Here’s a command line statement that will set the desktop image.

> gsettings set org.gnome.desktop.background picture-uri file:///home/myname/Downloads/Major_Oak.JPG

Compile time purge level

Elixir can conditionally remove log statements of the form Logger.debug/2 to Logger.error/2 at compile time.

config :logger, compile_time_purge_level: :info

In this configuration, logger statements like Logger.debug("something inconsequential happened") will be compiled out altogether.

Logging to a file in Elixir test environment

The default configuration for a logger in the test env is:

config :logger, level: :warn

That is, of the four logging levels (defug, info, warn and error) only log warnings and errors.

By default this logger logs to the console making the below redundant:

config :logger, level: :warn, backends: [:console]

:console is the only option available by default, but there are other backends available via hex package.

With hex package logger_file_backend you might have a different config:

config :logger, backends: [{LoggerFileBackend, :error_log}]

config :logger, :error_log, path: 'log/here.log'