Environment variables are not accessible in bash script

That's because the HADOOP_HOME variable isn't exported:

$ cat foo.sh
#!/bin/bash
echo "HADOOP_HOME: $HADOOP_HOME"


$ HADOOP_HOME=/home/me/dist/hadoop
$ echo $HADOOP_HOME
/home/me/dist/hadoop

$ foo.sh
HADOOP_HOME: 

$ export HADOOP_HOME
$ foo.sh
HADOOP_HOME: /home/me/dist/hadoop

When you run a shell script, that script will run in its own bash instance (that's what the #!/bin/bash does) that is a child shell of the current one. Variables are not passed to child shells by default, only if they are exported. Think of each bash session as independent (they largely are). You usually don't want variables defined in one to pollute the environment of another. For those cases where that is necessary, use export.