Useful Kamal Command Lines for Rails Deployment

Kamal is the new hotness for Rails deployment and I will say the following:

• Yes it works
• I have rarely had a more frustrating professional censored experience in my life

Useful Command Lines

Launch a bash shell under your app code so you can run a Rake task:

kamal app exec bash -i

Deploy your code UNLESS you have wrapped a shell script around it:

kamal deploy

Deploy my Rails application where I have a shell script wrapped around it:

./deploy

My ./deploy routine looks like this:

 cat ./deploy
#!/bin/bash
source .env.production
kamal deploy
unset DATABASE_URL%

NOTE: If you don’t unset DATABASE_URL then your local Rails dev environment will be completely broken because Rails itself is hardwired to use DATABASE_URL over the local database.yml.

Launch a Rails shell in your code base:

kamal app exec bin/rails c -i

Run the database setup:

kamal app exec rails db:setup

Get help

kamal app --help

An interesting edge case using Kamal for deploy happened just today. The situation was this:

Do we have been deploying regularly and last night this happened:
./deploy
Log into image registry...
  INFO [bd94f1bc] Running docker --version && docker buildx version as sjohnson@localhost
  INFO [bd94f1bc] Finished in 0.362 seconds with exit status 0 (successful).
  INFO [dd4c864a] Running docker login -u [REDACTED] -p [REDACTED] as sjohnson@localhost
  INFO [dd4c864a] Finished in 1.230 seconds with exit status 0 (successful).
  INFO [c176535f] Running docker login -u [REDACTED] -p [REDACTED] on polly.indiana50501.org
  Finished all in 1.7 seconds
  ERROR (Socket::ResolutionError): Exception while executing on host polly.indiana50501.org: getaddrinfo: nodename nor servname provided, or not known

Trying multiple times to do ./deploy would not fix the problem. The ultimate solution was to do this:

kamal build deliver

Note: To make kamal build deliver work, I had to manually execute one of the commands in my ./deploy routine to make sure the environment variables were setup correctly so my actual step was this:

source .env.production
kamal build deliver

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Last Updated On: 2025-09-01 04:31:51 -0400