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====== BASH - Math - Error with ((expression)) ======

<WRAP important>
**WARNING:**  Be careful when using **<nowiki>((expression))</nowiki>**.
</WRAP>


----

====== Arithmetic evaluation and errexit trap ======

<code bash>
count=0
things="0 1 0 0 1"

for i in $things;
do
  if [ $i == "1" ]; then
    (( count++ ))
  fi
done

echo "Count is ${count}"
</code>

returns:

<code bash>
2
</code>

----

===== Check the return code =====

<code bash>
echo $?
</code>

returns:

<code bash>
0
</code>

<WRAP info>
**NOTE:**  A **0** indicates success.

A **1** indicates failure.

</WRAP>


<WRAP box>
**NOTE:**  This looks fine; but there is a small gotcha:

The **<nowiki>((expression))</nowiki>** is evaluated according to the Arithmetic Evaluation rules.

  * If the value of the expression is non-zero, the return status is 0; otherwise the return status is 1.
  * This is exactly equivalent to **let "expression"**.

But if you run this script with **-e** or enable errexit:

<code bash>
bash -e test.sh
</code>

then count++ is going to return 0 (post-increment) and the script will stop.

Checking the result:

<code bash>
echo $?
</code>

returns:

<code bash>
1
</code>

This time a failure.

A definite trap to watch out for!  Do not use **<nowiki>((expression))</nowiki>** here.
</WRAP>