Bash/sh 'if else' statement

Note that if you want to determine if a variable is set, you probably do not want to use either if/else or test ([). It is more typical to do things like:

# Abort if JAVA_HOME is not set (or empty)
: ${JAVA_HOME:?JAVA_HOME is unset}

or

# report the value of JAVA_HOME, or a default value
echo ${JAVA_HOME:-default value}

or

# Assign JAVA_HOME if it is unset (or empty)
: ${JAVAHOME:=default value}

if [ -z $JAVA_HOME  ]  
then  
    echo $JAVA_HOME  
else  
    echo "NO JAVA HOME SET"  
fi

You're running into a stupid limitation of the way sh expands arguments. Line 3 of your script is being expanded to:

if [ != ]

Which sh can't figure out what to do with. Try this nasty hack on for size:

if [ x$JAVA_HOME != x ]

Both arguments have to be non-empty, so we'll just throw an x into both of them and see what happens.

Alternatively, there's a separate operator for testing if a string is non-empty:

if [ !-z $JAVA_HOME ]

(-z tests if the following string is empty.)

The -n and -z options are tests that should be used here:

if [ -n "$JAVAHOME" ]; then
    echo "$JAVAHOME";
else
    echo "\$JAVAHOME not set";
fi