Expand environment variable from variable

问题:

I have a code that looks like this:

my_var="MY_FIRST_ENV_VAR"

My question is, how do I get the value from the environment variable MY_ENV_VAR.

I have tried a lot of thing, but the main problem is that my_var is now a string.

The reason I would like to do this is because I have some environment variables that have almost the same name.

MY_FIRST_ENV_VAR=R1.2.3
MY_SECOND_ENV_VAR=R2.3.4

for vari in FIRST SECOND; do
    branch=MY_$( echo $vari )_ENV_VAR;
    echo $branch;
    echo ${branch};
    echo "${branch};
done;

I have tried a couple of other things as well. I my code I just have access to the FIRST and SECOND strings, I need to construct the name of the variable first.

I have looked for quite some time, and maybe I am just looking for the wrong thing.

回答1:

Try this:

MY_FIRST_ENV_VAR=R1.2.3
MY_SECOND_ENV_VAR=R2.3.4

for vari in FIRST SECOND; do
    varName="MY_${vari}_ENV_VAR"
    branch=${!varName}
    echo "$branch"
done

Output:

R1.2.3
R2.3.4

${!varName} is known as indirect expansion and allows you to expand the variable called varName. Consult the bash man page for details.

---

If your shell does not support indirect expansion shown above, use eval instead:

MY_FIRST_ENV_VAR=R1.2.3
MY_SECOND_ENV_VAR=R2.3.4

for vari in FIRST SECOND; do
    branch=$(eval "echo \$MY_${vari}_ENV_VAR")
    echo "$branch"
done

回答2:

Let's show you

MY_FIRST_ENV_VAR=R1.2.3
MY_SECOND_ENV_VAR=R2.3.4

for vari in FIRST SECOND; do
    branch="MY_${vari}_ENV_VAR"
    echo "branch variable: $branch"
    echo "branch variable indirection: ${!branch}"
done

Output

branch variable: MY_FIRST_ENV_VAR
branch variable indirection: R1.2.3
branch variable: MY_SECOND_ENV_VAR
branch variable indirection: R2.3.4

Doc

${!branch} is a variable indirection