Generate GitHub App JWT and Token

github_app_jwt.sh

#!/usr/bin/env bash

# Combination of generating the JWT and the token
# https://docs.github.com/en/apps/creating-github-apps/authenticating-with-a-github-app/generating-a-json-web-token-jwt-for-a-github-app#generating-a-json-web-token-jwt
# https://docs.github.com/en/apps/creating-github-apps/authenticating-with-a-github-app/authenticating-as-a-github-app#using-a-json-web-token-jwt-to-authenticate-as-a-github-app
# https://docs.github.com/en/apps/creating-github-apps/authenticating-with-a-github-app/generating-an-installation-access-token-for-a-github-app#generating-an-installation-access-token
# https://docs.github.com/en/apps/creating-github-apps/authenticating-with-a-github-app/authenticating-as-a-github-app-installation

set -o pipefail

client_id=$GITHUB_APP_ID
install_id=$GITHUB_INSTALL_ID
pem=$GITHUB_APP_PEM

now=$(date +%s)
iat=$((${now} - 60)) # Issues 60 seconds in the past
exp=$((${now} + 600)) # Expires 10 minutes in the future

b64enc() { openssl base64 | tr -d '=' | tr '/+' '_-' | tr -d '\n'; }

# JWT header
header_json='{
    "typ":"JWT",
    "alg":"RS256"
}'
header=$( echo -n "${header_json}" | b64enc )
payload_json="{
    \"iat\":${iat},
    \"exp\":${exp},
    \"iss\":\"${client_id}\"
}"
payload=$( echo -n "${payload_json}" | b64enc )
header_payload="${header}"."${payload}"
signature=$(
    openssl dgst -sha256 -sign <(echo -n "${pem}") \
    <(echo -n "${header_payload}") | b64enc
)

# Create JWT
JWT="${header_payload}"."${signature}"

curl_status=$(curl --request GET --silent --output /dev/null \
--write-out "%{http_code}" \
--url "https://api.github.com/app/installations" \
--header "Accept: application/vnd.github+json" \
--header "Authorization: Bearer $jwt" \
--header "X-GitHub-Api-Version: 2022-11-28")

echo "GitHub App JWT API test response code: ${curl_status}"
printf '%s\n' "JWT: $jwt"

# Create token (requires the JWT)
token=$(curl --request POST --silent \
--url "https://api.github.com/app/installations/$install_id/access_tokens" \
--header "Accept: application/vnd.github+json" \
--header "Authorization: Bearer $jwt" \
--header "X-GitHub-Api-Version: 2022-11-28" | jq .token --raw-output)

printf '%s\n' "Token: ${token}"