Prefer let x+=1 for incrementing counters

The `((x++))` syntax is shorthand for `let x++`. According to `help let`:

    If the last ARG evaluates to 0, let returns 1; 0 is returned
    otherwise.

Thus the exit status of the expression `x=0; let x++` is 1, since the post-increment `++` operator evaluates to the value of the variable before incrementing.

In Bash 4, this non-zero exit status properly triggers `set -e`'s error trap, but in Bash 3 it does not. That's why the tests were passing on OS X (Bash 3) but not Linux (Bash 4).

We can work around the problem by choosing an incrementation expression that never evaluates to 0, such as `+=` or the pre-increment `++` operator. For consistency and clarity, I've changed to `x+=1` everywhere.

Ref. #25, #27

libexec/bats

@@ -67,7 +67,7 @@ for arg in "$@"; do
       while option="${arg:$index:1}"; do
         [ -n "$option" ] || break
         options[${#options[*]}]="$option"
-        index=$(($index+1))
+        let index+=1
       done
     fi
   else

libexec/bats-exec-suite

@@ -17,7 +17,7 @@ trap "kill 0; exit 1" int
 
 count=0
 for filename in "$@"; do
-  count=$(($count + $(bats-exec-test -c "$filename")))
+  let count+="$(bats-exec-test -c "$filename")"
 done
 
 if [ -n "$count_only_flag" ]; then
@@ -35,11 +35,11 @@ for filename in "$@"; do
     while IFS= read -r line; do
       case "$line" in
       "begin "* )
-        index=$(($index + 1))
+        let index+=1
         echo "${line/ $index / $(($offset + $index)) }"
         ;;
       "ok "* | "not ok "* )
-        [ -n "$extended_syntax_flag" ] || index=$(($index + 1))
+        [ -n "$extended_syntax_flag" ] || let index+=1
         echo "${line/ $index / $(($offset + $index)) }"
         [ "${line:0:6}" != "not ok" ] || status=1
         ;;

libexec/bats-exec-test

@@ -97,7 +97,7 @@ bats_capture_stack_trace() {
       if [[ "$frame" = *"$test_pattern" ]]; then
         break
       else
-        ((index++))
+        let index+=1
       fi
     done
   fi
@@ -125,7 +125,7 @@ bats_print_stack_trace() {
       echo "from function \`$fn' in file $filename, line $line,"
     fi
 
-    ((index++))
+    let index+=1
   done
 }
 
@@ -197,7 +197,7 @@ bats_perform_tests() {
   status=0
   for test_name in "$@"; do
     "$0" $BATS_EXTENDED_SYNTAX "$BATS_TEST_FILENAME" "$test_name" "$test_number" || status=1
-    test_number=$(($test_number + 1))
+    let test_number+=1
   done
   exit "$status"
 }

libexec/bats-format-tap-stream

@@ -131,7 +131,7 @@ trap finish EXIT
 while IFS= read -r line; do
   case "$line" in
   "begin "* )
-    index=$(( $index + 1 ))
+    let index+=1
     name="${line#* $index }"
     buffer begin
     flush
@@ -145,11 +145,10 @@ while IFS= read -r line; do
     fi
     ;;
   "not ok "* )
-    failures=$(( $failures + 1 ))
+    let failures+=1
     buffer fail
     ;;
   "# "* )
-
     buffer log "${line:2}"
     ;;
   esac

libexec/bats-preprocess

@@ -33,7 +33,7 @@ tests=()
 index=0
 
 while IFS= read -r line; do
-  index=$(($index + 1))
+  let index+=1
   quoted_name="$(expr "$line" : ' *@test  *\([^ ].*\)  *{ *$' || true)"
 
   if [ -n "$quoted_name" ]; then