yaal.local[535]> use
        ANT_HOME      /Users/amiller/devtools/ant/current     -> ant-1.8.1
        CATALINA_HOME /Users/amiller/devtools/tomcat/current  -> tomcat-6.0.29
        EMACS_HOME    /Users/amiller/devtools/emacs/current   -> emacs-23.2.app
        GRAILS_HOME   /Users/amiller/devtools/grails/current  -> grails-1.3.4
        GROOVY_HOME   /Users/amiller/devtools/groovy/current  -> groovy-1.7.5
        JAVA_HOME     /Users/amiller/devtools/jdk/current     -> jdk-1.6.0
        TOMCAT_HOME   /Users/amiller/devtools/tomcat/current  -> tomcat-6.0.29
    yaal.local[536]>

#!/bin/sh
#
\# sets a symlink by convention...
#
\# For example,
\#   in a directory names ~/devtools/grails with
\#     ~/devtools/grails/grails-1.1.2 and
\#     ~/devtools/grails/grails-1.2,
\#   executing `use 1.2`
\#   will create a symlink ~/devtools/grails/current -> ~/devtools/grails/grails-1.2
#
\# You can pass in more than one component/version pair...
\#   `use grails 1.3 tomcat 6.0.29 maven 2.2.1`
#
\# Set your path to include ~/devtools/grails/current/bin.  Got it?
#
DEFAULT_TOOL_DIR=~/devtools  

#~ functions ----------------------------------------------------------------------------------  

setCurrentLink() {
    rm -f current
    ln -s $1 current
    # this will echo the link to the console...
    #ls -l current | awk '{ printf "%s %s %s", $9, $10, $11 }'
}  

useComponentVersion()
{
    COMPONENT=$1
    VERSION=$2  

    #echo "default_tool_dir = $DEFAULT_TOOL_DIR"
    cd $DEFAULT_TOOL_DIR/$COMPONENT  

    USE=$COMPONENT-$VERSION
    if [ ! -d $USE ]; then
        # maybe there is a missing .0 ?
        USE=$COMPONENT-$VERSION.0
    fi  

    if [ -d $USE ]; then
        if [ -h current ]; then
            CURRENT=`ls -l current | awk '{ print $11 }'`
            if [ "$CURRENT" = "$USE" ]; then
                echo "using $CURRENT"
            else
                echo "switching from $CURRENT to $USE"
                setCurrentLink $USE
            fi
        else
            echo "using $USE"
            setCurrentLink $USE
        fi
    else
        echo "ERROR: missing directory for $@"
        echo "ERROR: available version are..."
        find . -name $COMPONENT-* -depth 1  

        if [ -h current ]; then
            CURRENT=`ls -l current | awk '{ print $11 }'`
            echo "WARNING: keeping $CURRENT"
        else
            echo "WARNING: current not set."
        fi
        exit -1
    fi
}  

echoCurrentHomeVars() {
    #echo "The current _HOME vars are..."
    for home_var in `printenv | grep _HOME | sort`; do
        home_var_name=`echo $home_var | awk -F= '{ print $1 }'`
        home_var_value=`echo $home_var | awk -F= '{ print $2 }'`
        if [ -h $home_var_value ]; then
        # echo out what symbolic links are pointing to...
            echo "    $home_var_name    `ls -l $home_var_value | awk '{ printf "%s -> %sn", $9, $11 }'`"
        else
            echo "    $home_var_name    $home_var_value"
        fi
    done
}  

#~ main body of the script --------------------------------------------------------------------  

#echo  

if [ $# -eq 0 ]; then
    echoCurrentHomeVars
    exit -1
elif [ $# -eq 1 ]; then
    echo "WARNING: Assuming you are making a link in the current directory."
    useComponentVersion `basename $PWD` $1
else
    # use each component/version pair
    while [ $# -gt 0 ]; do
        useComponentVersion $1 $2
        shift
        shift
    done
fi  

#~ END OF FILE --------------------------------------------------------------------------------

~/devtools/
        ant/
            ant-1.7.1/
            ant-1.8.1/
            current -> ant-1.8.1
        jdk/
            jdk-1.5.0/
            jdk-1.6.0/
            current -> jdk-1.5.0
        grails/
            grails-1.2.2/
            grails-1.3.4/
            current -> grails-1.3.4
        ...

alias iaap="use jdk 1.5.0 grails 1.1.1; cd ~/opi/clients/mdh/iaap"
    alias buyer="use jdk 1.6.0 grails 1.2.0; cd ~/opi/clients/buyer"
    alias mtm="use jdk 1.6.0 grails 1.3.4; cd ~/dev/mtm"
    ...