Dockerfile if else condition with external arguments

Question

It might not look that clean but you can have your Dockerfile (conditional) as follow:

FROM centos:7
ARG arg
RUN if [[ -z "$arg" ]] ; then echo Argument not provided ; else echo Argument is $arg ; fi

and then build the image as:

docker build -t my_docker . --build-arg arg=45

or

docker build -t my_docker .

There is an interesting alternative to the proposed solutions, that works with a single Dockerfile, require only a single call to docker build per conditional build and avoids bash.

Solution:

The following Dockerfile solves that problem. Copy-paste it and try it yourself.

ARG my_arg

FROM centos:7 AS base
RUN echo "do stuff with the centos image"

FROM base AS branch-version-1
RUN echo "this is the stage that sets VAR=TRUE"
ENV VAR=TRUE

FROM base AS branch-version-2
RUN echo "this is the stage that sets VAR=FALSE"
ENV VAR=FALSE

FROM branch-version-${my_arg} AS final
RUN echo "VAR is equal to ${VAR}"

Explanation of Dockerfile:

We first get a base image (centos:7 in your case) and put it into its own stage. The base stage should contain things that you want to do before the condition. After that, we have two more stages, representing the branches of our condition: branch-version-1 and branch-version-2. We build both of them. The final stage than chooses one of these stages, based on my_arg. Conditional Dockerfile. There you go.

Output when running:

(I abbreviated this a little...)

my_arg==2

docker build --build-arg my_arg=2 .
Step 1/12 : ARG my_arg
Step 2/12 : ARG ENV
Step 3/12 : FROM centos:7 AS base
Step 4/12 : RUN echo "do stuff with the centos image"
do stuff with the centos image
Step 5/12 : FROM base AS branch-version-1
Step 6/12 : RUN echo "this is the stage that sets VAR=TRUE"
this is the stage that sets VAR=TRUE
Step 7/12 : ENV VAR=TRUE
Step 8/12 : FROM base AS branch-version-2
Step 9/12 : RUN echo "this is the stage that sets VAR=FALSE"
this is the stage that sets VAR=FALSE
Step 10/12 : ENV VAR=FALSE
Step 11/12 : FROM branch-version-${my_arg}
Step 12/12 : RUN echo "VAR is equal to ${VAR}"
VAR is equal to FALSE

my_arg==1

docker build --build-arg my_arg=1 .
...
Step 11/12 : FROM branch-version-${my_arg}
Step 12/12 : RUN echo "VAR is equal to ${VAR}"
VAR is equal to TRUE

Thanks to Tõnis for this amazing idea!

From some reason most of the answers here didn't help me (maybe it's related to my FROM image in the Dockerfile)

So I preferred to create a bash script in my workspace combined with --build-arg in order to handle if statement while Docker build by checking if the argument is empty or not

Bash script:

#!/bin/bash -x

if test -z $1 ; then 
    echo "The arg is empty"
    ....do something....
else 
    echo "The arg is not empty: $1"
    ....do something else....
fi

Dockerfile:

FROM ...
....
ARG arg
COPY bash.sh /tmp/  
RUN chmod u+x /tmp/bash.sh && /tmp/bash.sh $arg
....

Docker Build:

docker build --pull -f "Dockerfile" -t $SERVICE_NAME --build-arg arg="yes" .

Remark: This will go to the else (false) in the bash script

docker build --pull -f "Dockerfile" -t $SERVICE_NAME .

Remark: This will go to the if (true)

Edit 1:

After several tries I have found the following article and this one which helped me to understand 2 things:

1) ARG before FROM is outside of the build

2) The default shell is /bin/sh which means that the if else is working a little bit different in the docker build. for example you need only one "=" instead of "==" to compare strings.

So you can do this inside the Dockerfile

ARG argname=false   #default argument when not provided in the --build-arg
RUN if [ "$argname" = "false" ] ; then echo 'false'; else echo 'true'; fi

and in the docker build:

docker build --pull -f "Dockerfile" --label "service_name=${SERVICE_NAME}" -t $SERVICE_NAME --build-arg argname=true .

The accepted answer may solve the question, but if you want multiline if conditions in the dockerfile, you can do that placing \ at the end of each line (similar to how you would do in a shell script) and ending each command with ;. You can even define someting like set -eux as the 1st command.

Example:

RUN set -eux; \
  if [ -f /path/to/file ]; then \
    mv /path/to/file /dest; \
  fi; \
  if [ -d /path/to/dir ]; then \
    mv /path/to/dir /dest; \
  fi

In your case:

FROM centos:7
ARG arg
RUN if [ -z "$arg" ] ; then \
    echo Argument not provided; \
  else \
    echo Argument is $arg; \
  fi

Then build with:

docker build -t my_docker . --build-arg arg=42

Just use the "test" binary directly to do this. You also should use the noop command ":" if you don't want to specify an "else" condition, so docker does not stop with a non zero return value error.

RUN test -z "$YOURVAR" || echo "var is set" && echo "var is not set"
RUN test -z "$YOURVAR" && echo "var is not set" || :
RUN test -z "$YOURVAR" || echo "var is set" && :