envsubst

What it does

How to write it

envsubst ['$VAR1 $VAR2'] < TEMPLATE > OUTPUT

Most used

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$ run as a normal user# run as root (or with sudo)

1. 1Fill a Kubernetes manifest in a pipeline

   $cat deploy.tpl.yaml
   image: ghcr.io/acme/shop:${TAG}
   replicas: ${REPLICAS}
   $export TAG=v1.4.0 REPLICAS=3
   $envsubst < deploy.tpl.yaml | kubectl apply -f -

2. 2Replace only some variables (keep nginx's own $host)

   $export SERVER_NAME=shop.example.com
   $envsubst '$SERVER_NAME' < nginx.conf.tpl > /etc/nginx/conf.d/shop.conf

   Without the list, $host and $remote_addr would become empty.

3. 3Container entrypoint that writes the config at start

   $cat entrypoint.sh
   #!/bin/sh
   envsubst '$API_URL' < /etc/app/config.tpl.json > /etc/app/config.json
   exec "$@"

4. 4The official nginx image does this for you

   $docker run -e SERVER_NAME=shop.example.com -v ./templates:/etc/nginx/templates nginx:1.27

   Files in /etc/nginx/templates/*.template are filled into /etc/nginx/conf.d/ at start.

5. 5A .env file per stage

   $export DB_HOST=db.prod.internal
   $envsubst < .env.tpl > .env.prod

6. 6See which variables a template uses

   $envsubst -v "$(cat deploy.tpl.yaml)"
   TAG
   REPLICAS

7. 7One line

   $NAME=world envsubst <<< 'Hello $NAME'
   Hello world

8. 8Fail if a variable is missing (check first)

   $for v in TAG REPLICAS; do [ -n "${!v}" ] || { echo "missing $v"; exit 1; }; done

9. 9Use with heredoc

   $export PORT=8080
   $envsubst <<'EOF2' > app.conf
   listen = 0.0.0.0:${PORT}
   EOF2

10. 10Fill all templates of a folder

    $for f in k8s/*.tpl.yaml; do envsubst < "$f" > "${f%.tpl.yaml}.yaml"; done

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