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--- bash:arithmetic_evaluation_and_errexit_trap [2021/01/09 14:39] – peter
+++ bash:arithmetic_evaluation_and_errexit_trap [2021/01/26 12:00] (current) – removed peter
@@ Line 1: / Line 1: @@
-====== BASH - 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>
-</code>
-----
-===== Check the return code =====
-<code bash>
-echo $?
-</code>
-returns:
-<code bash>
-</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>
-</code>
-This time a failure.
-A definite trap to watch out for!
-</WRAP>