Docker get a list of possible environment variables

command linedockerdocker-composedocker-registryenvironment-variables

Hello! After intensive research I decided to bring this question up to the community. I started learning docker & docker-compose not long ago.

My question is:

How can I get a list of the possible environment variables in the console. They are listed on the docker hub but how can I get if from the image itself? For example, the image nextcloud has listed this environment variables on the website:

PostgreSQL:

POSTGRES_DB Name of the database using postgres.
POSTGRES_USER Username for the database using postgres.
POSTGRES_PASSWORD Password for the database user using postgres.
POSTGRES_HOST Hostname of the database server using postgres.
If you set any values, they will not be asked in the install page on first run. With a complete configuration by using all variables for your database type, you can additionally configure your Nextcloud instance by setting admin user and password (only works if you set both):

NEXTCLOUD_ADMIN_USER Name of the Nextcloud admin user.


  NEXTCLOUD_ADMIN_PASSWORD Password for the Nextcloud admin user.
    If you want, you can set the data directory, otherwise.......
.................................................................

But when I do

docker inspect nextcloud |  jq '.[] | .Config.Env'

I only get this:

[
  "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
  "PHPIZE_DEPS=autoconf \t\tdpkg-dev \t\tfile \t\tg++ \t\tgcc \t\tlibc-dev \t\tmake \t\tpkg-config \t\tre2c",
  "PHP_INI_DIR=/usr/local/etc/php",
  "APACHE_CONFDIR=/etc/apache2",
  "APACHE_ENVVARS=/etc/apache2/envvars",
  "PHP_EXTRA_BUILD_DEPS=apache2-dev",
  "PHP_EXTRA_CONFIGURE_ARGS=--with-apxs2 --disable-cgi",
  "PHP_CFLAGS=-fstack-protector-strong -fpic -fpie -O2 -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64",
  "PHP_CPPFLAGS=-fstack-protector-strong -fpic -fpie -O2 -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64",
  "PHP_LDFLAGS=-Wl,-O1 -pie",
  "GPG_KEYS=42670A7FE4D0441C8E4632349E4FDC074A4EF02D 5A52880781F755608BF815FC910DEB46F53EA312",
  "PHP_VERSION=7.4.22",
  "PHP_URL=https://www.php.net/distributions/php-7.4.22.tar.xz",
  "PHP_ASC_URL=https://www.php.net/distributions/php-7.4.22.tar.xz.asc",
  "PHP_SHA256=8e078cd7d2f49ac3fcff902490a5bb1addc885e7e3b0d8dd068f42c68297bde8",
  "PHP_MEMORY_LIMIT=512M",
  "PHP_UPLOAD_LIMIT=512M",
  "NEXTCLOUD_VERSION=22.0.0"
]

Which is a lot less important than what is listed on the hub. How can I also get those variables?

Best Answer

Docker won't know it. For example, if I made a script that says:

#!/bin/sh

if [ -n "$SUPER_SECRET_VAR" ]; then
  echo "Super secret mode enabled"
else
  echo "Hello world"
fi

And packaged that inside of a container, there's nothing in the docker packaging that requires you to tell docker about $SUPER_SECRET_VAR and since docker can package apps written in lots of languages, there's no universal way to parse every possible application to extract this.

You're left with:

  1. Reading the docs on the image/application.
  2. Reading the scripts, particularly the entrypoint script.
  3. Reading the code.
Related Question