Created SC2015 (markdown)

SC2015.md

@@ -0,0 +1,29 @@
+# Note that A && B || C is not if-then-else. C may run when A is true.
+
+### Problematic code:
+
+    [[ $dryrun ]] && echo "Would delete file" || rm file
+
+### Correct code:
+
+    if [[ $dryrun ]]
+    then
+      echo "Would delete file"
+    else
+      rm file
+    fi
+
+### Rationale:
+
+It's common to use `A && B` to run `B` when `A` is true, and `A || C` to run `C` is `A` is false.
+
+However, combining them into `A && B || C` is not the same as `if A then B else C`.
+
+In this case, if `A` is true but `B` is false, `C` will run. 
+
+For the code sample above, if the script was run with stdout closed for any reason (such as explicitly running `script --dryrun >&-`), echo would fail and return false and the file would be deleted, even though `$dryrun` was set!
+
+If an `if` clause is used instead, this problem is avoided.
+
+### Contraindications
+Ignore this warning when you actually do intend to run C when either A or B fails.