Bash – [: =: unary operator expected during trap execution

I am trying to use trap in order to run a clean-app jar in case the main app jar exists successfully (0 exit code) :

trap "exit_code=$?; if [ "${exit_code}" = "0" ]; then java -jar /clean-app.jar; fi" EXIT

java -jar /main-app.jar

but I am getting the following error and not sure I’m getting the reason behind it :

/bin/bash: line 1: [: =: unary operator expected

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Could someone share a pointer please ? Thank you

>Solution :

You can’t nest double quotes like this. Here, you can switch to single quotes for the outer layer. You’ll want to do this anyway because you need to prevent $? from being expanded before trap is called.

trap 'exit_code=$?; if [ "${exit_code}" = "0" ]; then java -jar /clean-app.jar; fi' EXIT

Better yet, define a function first.

handler () {
    exit_code=$?
    if [ "$exit_code" = 0 ]; then java -jar /clean-app.jar; fi
}

trap handler EXIT

In either case, you don’t really need exit_code, as you don’t need the original exit code after the test. You can also use -eq, since $? is guaranteed to be an integer.

handler () {
    [ $? -eq 0 ] && java -jar /clean-app.jar
}