use eval "echo \${$FOO}"
#!/bin/sh
VALUE1="happy!" # target value
ENV_VAR=VALUE1 # variable bname
echo $VALUE1 # => happy!
echo $ENV_VAR # => VALUE1
echo ${ENV_VAR} # => VALUE1
echo \${$ENV_VAR} # => ${VALUE1}
#=== ash or other shells ===#
eval "echo \${$ENV_VAR}" # => happy!
#=== bash supports original indirect expantion, but not ash on Alpine linux ===#
echo ${!ENV_VAR}
#=== example for "if" "test" conditions ===#
test -n $ENV_VAR \ # (1)
&& test "$(eval "echo \${$ENV_VAR}")" == "happy!" \ # (2)
&& echo "I'm Happy!!"
# (1) if ENV_VAR is set,
# (2) and if $VALUE1 == "happy!"