[Hadoop Wiki] Update of "UnixShellScriptProgrammingGuide" by SomeOtherAccount

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The "UnixShellScriptProgrammingGuide" page has been changed by SomeOtherAccount:
https://wiki.apache.org/hadoop/UnixShellScriptProgrammingGuide?action=diff&rev1=17&rev2=18

  
   * Avoid adding more globals or project specific globals and/or entries in *-env.sh and/or a comment at the bottom here.  In a lot of cases, there is pre-existing functionality that already does what you might need to do.  Additionally, every configuration option makes it that much harder for end users. If you do need to add a new global variable for additional functionality, start it with HADOOP_ for common, HDFS_ for HDFS, YARN_ for YARN, and MAPRED_ for MapReduce.  It should be documented in either *-env.sh (for user overridable parts) or hadoop-functions.sh (for internal-only globals). This helps prevents our variables from clobbering other people.
  
-  * Remember that abc_xyz_OPTS can and should act as a catch-all for Java daemon options.  Custom heap environment variables add unnecessary complexity for both the user and us.  They should be avoided.
+  * Remember that abc_xyz_OPTS can and should act as a catch-all for Java daemon options.  Custom heap environment variables and other custom daemon variables add unnecessary complexity for both the user and us.  They should be avoided.  In almost every case, it is better to have a global and apply it to all daemons to have a universal default.  Users can/will override that variables as necessary in their init scripts.  This also helps cover the case when functionality starts in one chunk of Hadoop but ends up in multiple places.
  
   * Avoid mutli-level `if`'s where the comparisons are static strings.  Use case statements instead, as they are easier to read.