Differences

This shows you the differences between two versions of the page.

--- bash:math:error_with_expression [2021/01/26 12:02] – created peter
+++ bash:math:error_with_expression [2021/01/26 12:04] (current) – peter
@@ Line 1: / Line 1: @@
 ====== 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>
+</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!  Do not use **<nowiki>((expression))</nowiki>** here.
+</WRAP>