What's the behavior of ${SOME_VAR+x} ? Could It fail in a .sh file?

Question

I'm debugging a sh script. What's the behavior of these lines of code?

SOME_VAR=$(env)

if [ ! -z ${SOME_VAR+x} ]; then
    echo "SOMETHING HAPPENED "
fi

What does +x mean in this case? It turns out that code works fine with bash, but no with sh.

EDIT: I'm working with the deployment of an Eclipse Che local instance like it's explained here.

I installed the Eclipse Che CLI that is a .sh script:

$ curl -sL https://raw.githubusercontent.com/eclipse/che/master/che.sh > /usr/local/bin/che
$ chmod +x /usr/local/bin/che

To make it work, I need to run it using bash:

$ bash che start

If I debug the script...

$ sh -x che start

The script fails here:

get_list_of_che_system_environment_variables() {

    [...]

    if [ ! -z ${CHE_VARIABLES+x} ]; then
      env | grep CHE_ >> $DOCKER_ENV
      RETURN=$DOCKER_ENV
    fi

Something is not working. If you say the substitution should work in .sh too, what would be the problem?

The code should work identically in any POSIX-compatible shell. — chepner, Aug 25 '16 at 00:05
What means that the code doesn't work with `sh`? All the commands are valid in `sh`. — , Aug 25 '16 at 01:17
@redmm I edited my question to show you what could be the problem — JonDoe297, Aug 25 '16 at 14:07

score 2 · Answer 1 · answered Aug 25 '16 at 00:02

${SOME_VAR+x} expands to the empty string if SOME_VAR is not set. If it is set (even to the empty string), it expands to x.

Since SOME_VAR was set on the previous line, it will always expand to x. This makes the if statement irrelevant, as the code will always output SOMETHING HAPPENED.

score 1 · Answer 2 · answered Aug 25 '16 at 00:02

1

It is a form of parameter substitution which will yield x if SOME_VAR is set, or a null String if it isn't.

You can learn more about the different type of parameters substitution here.

answered Aug 25 '16 at 00:02

Aaron

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What's the behavior of ${SOME_VAR+x} ? Could It fail in a .sh file?

2 Answers2