spm

Add help command and usage function

diff --git a/README.pod b/README.pod
@@ -4,7 +4,7 @@ tpm - tiny password manager
 
 =head1 SYNOPSIS
 
-B<tpm> I<COMMAND> I<ENTRY>
+B<tpm> I<COMMAND> [I<ENTRY>]
 
 =head1 DESCRIPTION
 

@@ -18,7 +18,8 @@ Invoking tpm consists of specifying a command either I<insert> or
 I<show> and supplying one entry as a target. If I<insert> is specified
 as a command tpm will create a new entry and prompt for the
 corresponding password. If you specify I<show> as a command tpm will
-write the password for the given entry to standard output.
+write the password for the given entry to standard output. Specifying
+I<help> as command without entry will show how to use tpm.
 
 =head1 ENVIRONMENT
 

diff --git a/tpm b/tpm
@@ -52,6 +52,10 @@ readpw() {
   [ -t 0 ] && stty echo
 }
 
+usage() {
+  echo "USAGE: tpm (show|insert|help) [ENTRY]"
+}
+
 ##
 # Commands
 ##

@@ -113,8 +117,11 @@ case "${1}" in
   "insert")
     insert  "${2}"
     ;;
+  "help")
+    usage
+    ;;
   *)
-    abort   "USAGE: tpm COMMAND ENTRY"
+    abort   "You didn't specify a command. See 'tpm help'."
     ;;
 esac