Can anybody explain why this doesn't work:
# docker run -ti --rm golang echo $GOPATH
# docker run -ti --rm golang echo \$GOPATH
$GOPATH
when this works:
# docker run -ti --rm golang bash -c "echo \$GOPATH"
/go
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1 Answer
1)
docker run -ti --rm golang echo $GOPATH
docker run -ti golang echo $GOPATH
/Users/me/go
I removed the --rm flag to be able to inspect the container. Then, I did docker inspect container-id:
"Env": [
"no_proxy=*.local, 169.254/16",
"PATH=/go/bin:/usr/local/go/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
"GOLANG_VERSION=1.8.1",
"GOPATH=/go"
],
"Cmd": [
"echo",
"/Users/me/go"
],
$GOPATH is expanded before the container starts. It's expanded by your shell, indeed before the docker command is called.
2)
docker run -ti --rm golang echo \$GOPATH
docker run -ti golang echo \$GOPATH
$GOPATH
docker inspect container-id:
"Env": [
"no_proxy=*.local, 169.254/16",
"PATH=/go/bin:/usr/local/go/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
"GOLANG_VERSION=1.8.1",
"GOPATH=/go"
],
"Cmd": [
"echo",
"$GOPATH"
],
Here the $GOPATH is a literal string and doesn't get expanded by the shell because you escaped the $, then echo receives the literal var name instead of its value.
3)
docker run -ti --rm golang bash -c "echo \$GOPATH"
docker run -ti golang bash -c "echo \$GOPATH"
/go
docker inspect container-id:
"Env": [
"no_proxy=*.local, 169.254/16",
"PATH=/go/bin:/usr/local/go/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
"GOLANG_VERSION=1.8.1",
"GOPATH=/go"
],
"Cmd": [
"bash",
"-c",
"echo $GOPATH"
],
Here, your shell runs docker with no variable expansion too, due to the \$, but there is another shell that will catch it and expand it: bash will do that but inside the container this time, where the GOPATH is provided by docker as environment variable defined in the golang image (GOPATH=/go). See the Dockerfile where that is defined.
