Bash: Features of Arithmetic Operations - Increment ((expression++))

When testing the host availability check script I mentioned last time, I encountered an interesting Bash feature when performing arithmetic operations💪.

👨💻What is the point: My script uses the check_count variable, which contains the current number of failed checks. This counter is increased with the increment command: ((check_count++)). The key word here is command. For further understanding, it is worth adding a bit of context😒

🤯Context: In Bash, the ((expression)) syntax performs two functions:

1. Arithmetic evaluation.
2. Logical check of the result (return code, also known as return code, rc).

Return code features of arithmetic operations:

• if the expression result is greater than 0, return code 0 is returned (success)
• if the result is equal to 0, return code 1 is returned (error)

And an operation of the form ((expression++)) first returns the current value of the variable, and then increments it🍿

So, if the initial value is check_count=0, then on the first execution of the ((check_count++)) command, Bash first returns code 1, which is interpreted as an error😵💫

By default, Bash ignores any errors and continues execution. Therefore, in my scripts I often use the shell option set -e so that the script exits on any error - this makes its behavior more predictable. And it seems obvious: got an error - exited, but the script has asynchronous tasks (&) running in subprocesses🤔

Example from the check_hosts.sh script:

for host in "${CHECK_HOSTS[@]}"; do
    monitor_host "$host" &
done

This block launches monitor_host in parallel for each host from the specified list. If a subprocess entered the else branch and reached ((check_count++)), where check_count was equal to 0, it exited with an error because of set -e. At the same time, the main script and other subprocesses continued working💪

This led to non-obvious behavior: some processes exited, while others continued running. Without detailed checks, you might not notice it. To catch this moment, I had to strain my brain a little🤯

👌Solution: I added a logical “or” || true so that the command always exits successfully:

((check_count++)) || true

Another possible (and probably more correct) way is to use the prefix form:

((++check_count))

In this case, Bash immediately returns the new value, and if it is greater than 0 (which it will be), the return code is 0.

Or, as I was told, explicitly add one:

check_count=$((check_count+1))

I knew about this nuance in Bash arithmetic, but this was the first time I encountered it in the context of asynchronous commands.

👨‍💻And…

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💬 All the best ✌️

That should be it. If not, check the logs 🙂