CODEWITHSUNDEEP
bashdockerubuntuenvironment-variablesalpine-linux
daverona
daverona

Reputation: 43

Pass ENV to Docker container running single command

This prints blank:

docker run --rm --env HELLO="world" ubuntu:18.04 bash -c "echo $HELLO"

However this works:

docker run --rm -it --env HELLO="world" ubuntu:18.04 bash
# in the container
echo $HELLO

HELLO seems passed to the container though:

docker run --rm --env HELLO="world" ubuntu:18.04 env

Why is the first command not seeing HELLO? What am I missing?

Upvotes: 3

Views: 4054

Answers (2)

Mostafa Hussein
Mostafa Hussein

Reputation: 11980

Because of the double quotes, $HELLO will be evaluated by the docker host itself once the command got executed before going inside the container. So you need to either escape the dollar sign ($) using Backslash (\) which tells the bash that the $ is a part of the command itself and no need to be evaluated by the current shell (which is the docker host in our case) or use single quotes ('') like this:

Using single quotes

$ docker run --rm --env HELLO="world" ubuntu:18.04 bash -c 'echo $HELLO'
world

Using Backslash to escape

$ docker run --rm --env HELLO="world" ubuntu:18.04 bash -c "echo \$HELLO"
world

Upvotes: 7

omajid
omajid

Reputation: 15223

The reason you are not seeing what you expect is because things are being evaluated before you expect them to be evaluated.

When you run:

docker run --rm --env HELLO="world" ubuntu:18.04 bash -c "echo $HELLO"

The "echo $HELLO" really isn't any different to bash than:

echo "echo $HELLO"

The shell (bash) parses double quotes (") and things inside them. It sees "echo $HELLO" and replaces the variable $HOME with it's value. If $HOME is not defined, this evaluates to echo.

So,

echo "echo $HELLO"

is parsed and evaluted by your shell. Which then just runs this at the end:

echo "echo "

So the "echo $HELLO" in your docker command is evaluated to "echo " and that's what gets passed to the docker command.

What you want to do is to prevent your shell from evaluating the variable. You can do it a couple of ways:

  1. You can use single quotes instead of double quotes. Your shell doesn't parse it; it will be passed to the bash inside the container as is:

    docker run --rm --env HELLO="world" ubuntu:18.04 bash -c 'echo $HELLO'

  2. You can escape the $ to avoid evaluating it in this shell and let the bash inside the docker container evaluate it:

    docker run --rm --env HELLO="world" ubuntu:18.04 bash -c "echo \$HELLO"

Upvotes: 0

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