fix option parsing, add missing help (#45)

In light of the discussion in #44, option parsing needs to be more
strict. We shouldn't allow an orphaned config at the end of a line that
looks like an option to another argument. This commit ensures that the
config is either passed FIRST, or via the -c/--config option. It also
does full argument validation before calling showStatus or showHelp.

README.md

@@ -53,7 +53,7 @@ is not met.
 ### Available Command Line Options
 
 ```text
-Usage: ./zfs-replicate.sh [options] [config]
+Usage: ./zfs-replicate.sh [config] [options]
 
 POSIX shell script to automate ZFS Replication
 

zfs-replicate.sh

@@ -446,7 +446,7 @@ showStatus() {
 
 ## show usage and exit
 showHelp() {
-  printf "Usage: %s [options] [config]\n\n" "${SCRIPT}"
+  printf "Usage: %s [config] [options]\n\n" "${SCRIPT}"
   printf "POSIX shell script to automate ZFS Replication\n\n"
   printf "Options:\n"
   printf "  -c, --config <configFile>    configuration file\n"
@@ -459,9 +459,15 @@ showHelp() {
 loadConfig() {
   configFile=""
   status=0
+  help=0
   ## sub macros for logging
   TAG="$(subTags "$TAG")"
   LOG_FILE="$(subTags "$LOG_FILE")"
+  ## check for config file as first argument for backwards compatibility
+  if [ $# -gt 0 ] && [ -f "$1" ]; then
+    configFile="$1"
+    shift
+  fi
   ## process command-line options
   while [ $# -gt 0 ]; do
     if [ "$1" = "-c" ] || [ "$1" = "--config" ]; then
@@ -470,20 +476,23 @@ loadConfig() {
       shift
       continue
     fi
-    if [ "$1" = "-s" ] || [ "$1" = "--status" ]; then
-      status=1
+    if [ "$1" = "-h" ] || [ "$1" = "--help" ]; then
+      help=1
       shift
       continue
     fi
-    ## unknown option - check for config file for backwards compatibility
-    if [ -z "$configFile" ] && [ -f "$1" ]; then
-      configFile="$1"
+    if [ "$1" = "-s" ] || [ "$1" = "--status" ]; then
+      status=1
       shift
       continue
     fi
-    ## nothing left, error out
+    ## unknown option
     writeLog "ERROR: illegal option ${1}" && exit 1
   done
+  ## someone ask for help?
+  if [ "$help" -eq 1 ]; then
+    showHelp
+  fi
   ## attempt to load configuration
   if [ -f "$configFile" ]; then
     # shellcheck disable=SC1090