The error occurs because the return statement in Bash expects a numeric exit status (0 for success, 1-255 for failure), but you are trying to return the result of a conditional expression ([[ "$a" == "b" ]]), which is a boolean (true or false), not a number.

How to Fix It

In Bash, [[ ... ]] is a conditional expression that returns true (0) or false (1) as an exit status. If you want to return the result of the comparison, you can directly use the exit status of the [[ ... ]] expression. However, return expects a numeric value, so you need to ensure you are returning a number.

Here’s the corrected version:

my_test() {
    a='c'
    if [[ "$a" == "b" ]]; then
        return 0  # true
    else
        return 1  # false
    fi
}
my_test

Or, more concisely, since [[ ... ]] already sets the exit status:

my_test() {
    a='c'
    [[ "$a" == "b" ]]
}
my_test

In this version, the function will return the exit status of the [[ ... ]] command, which is 0 (true) if the condition is met, and 1 (false) otherwise.

Explanation

• return in Bash must be followed by a number (0-255).
• [[ ... ]] is a command that returns 0 (true) or 1 (false) as its exit status.
• If you want to return the result of a condition, you can either:
  • Use if and explicitly return 0 or return 1.
  • Let the function return the exit status of the last command (in this case, [[ ... ]]).