Generate the App Store Connect API private key

According to the official Apple documentation:

To generate keys, you must have an Admin account in App Store Connect. You may generate multiple API keys with any roles you choose.

1. To generate an API key to use with the App Store Connect API, log in to App Store Connect.
2. Select Users and Access, and then select the API Keys tab.
3. Click Generate API Key or the Add (+) button.
4. Enter a name for the key. The name is for your reference only and is not part of the key itself.
5. Under Access, select the role for the key.
6. Click Generate.
7. The new key's name, key ID, a download link, and other information appears on the page.

Name you key App Store Connect API key, download it and persist it somewhere safe. It should be a AuthKey_<key_id>.p8 file.

Also, remember the key identifier and your Issuer ID for later use.

Store the key at the right location

Then, store a copy of your private API key under the path ~/.appstoreconnect/private_keys/. This step is very important for altool to locate your API key.

Use altool, the xcrun subcommand replacing Application Loader

You can now use the following to upload your .ipa to App Store Connect:

xcrun altool --upload-app -f <PATH_TO_IPA>.ipa --apiKey <KEY_ID> --apiIssuer <ISSUER_ID>

Make it a script for simpler use

Open a new file named upload_ipa.sh and paste the following:

#!/bin/bash

echo "Uploading IPA to App Store Connect"
xcrun altool --upload-app -f $1 \
  --apiKey <KEY_ID> \
  --apiIssuer <ISSUER_ID>

Save the file somewhere handy, and make it executable with this command:

chmod +x upload_ipa.sh

Use the script

Finally, you can upload your freshly built .ipa to App Store Connect with your brand new script:

./upload_ipa.sh <PATH_TO_IPA>.ipa