How to get the hostname of the node in Bash?

In Bash, how to get the hostname of the node running the program?

In Bash, you can get the hostname of the node in at least 2 ways:

• Use the variable $HOSTNAME.
• Get the hostname by command hostname.
• Get the content of /proc/sys/kernel/hostname

Example:

$ echo $HOSTNAME
host01
$ hs=`hostname`
$ echo $hs
host01
$ cat /proc/sys/kernel/hostname
host01