Created SC2084 (markdown)

SC2084.md

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+## Remove '$' or use '_=$((expr))' to avoid executing output.
+
+### Problematic code:
+
+    i=4
+    $(( i++ ))
+
+### Correct code:
+
+Bash, Ksh:
+
+    i=4
+    (( i++ ))
+
+POSIX (assuming `++` is supported):
+
+    i=4
+    _=$(( i++ ))
+
+Alternative POSIX version that does not preserve the exit code:
+
+    : $(( i++ ))
+
+### Rationale:
+
+`$((..))` expands to a number. If it's the only word on the line, the shell will try to execute this number as a command name:
+
+    $ i=4
+    $ $(( i++ ))
+    4: command not found
+    $ echo $i
+    5
+
+To avoid trying to execute the number as a command name, use one of the methods mentioned:
+
+    $ i=4
+    $ _=$(( i++ ))
+    $ echo $i
+    5
+
+### Exceptions:
+
+None.