control ZFS pool with ZPOOL envvar; simplify code

zfsbk.sh

@@ -1,11 +1,13 @@
 #! /bin/sh
 
-# change this to the name of your ZFS pool.
-zpool="zroot"
+# change this to the name of your ZFS pool. Or set ZPOOL envvar at runtime
+zpool=${ZPOOL:-"zroot"}
 
+# name of backup to take
+bkname=${1:-"default"}
 # make a backup collection having 1 full and N-1 incremental backups.
 # set to 1 to avoid incrementals and always take full dumps.
-max_incremental=4
+max_incremental=${2:-"4"}
 
 usage () {
     echo "Usage:"
@@ -18,23 +20,21 @@ usage () {
     echo "UPLOAD_PATH   upload generated backup to this path. scp:// or rsync://"
 }
 
-# first argument: name of backup set
-if [ $# -lt 1 ]
-then
+failmsg () {
+    echo $*
+    echo
     usage
     exit 1
-fi
+}
 
-bkname=$1
-if ! echo $bkname | grep -qiE '^[a-z0-9-]+$'
-then
-    echo "Given bkname is invalid. Want alphanumeric and hyphens."
-    exit 1
-fi
-if echo "$2" | grep -qE '^[1-9][0-9]*$'
-then
-    max_incremental=$2
-fi
+verify_zpool () {
+    zpool status $zpool >/dev/null 2>&1 || failmsg "ZFS pool '$zpool' not found. Fix your \$ZPOOL envvar."
+}
+
+verify_zpool
+
+echo $bkname | grep -qiE '^[a-z0-9]+$' || failmsg "Given bkname is invalid. Want alphanumeric."
+echo "$max_incremental" | grep -qE '^[1-9][0-9]*$' || failmsg "Invalid number of backups sequence steps, want positive integer."
 
 export PATH=$PATH:/usr/local/bin