The env and printenv commands can be used to list the current environment variables.

~]$ env
SHELL=/bin/bash
HISTCONTROL=ignoredups
HISTSIZE=5000
HOSTNAME=DLPRWEB011
JAVA_HOME=/usr/bin/java

echo can be used to print the value of an environment variable.

~]$ echo $SHELL
/bin/bash

Some environment variables are set in the /etc/profile file, such as HISTSIZE.

~]# cat /etc/profile
HISTSIZE=1000

Some environment variables are set in files in the /etc/profile.d directory, such as LANG.

~]# cat /etc/profile.d/lang.sh
LANG=en_US.UTF-8

~]$ env | grep LANG
LANG=en_US.UTF-8

You may also have user specific environment variables in your users /home/username/.bash_profile file.

~]$ cat .bash_profile 
export JAVA_HOME=/usr/bin/java

~]$ env | grep JAVA_HOME
JAVA_HOME=/usr/bin/java

export can be used to temporarily create a variable.

~]$ export FOO=bar
~]$ echo $FOO
bar

unset can be used to undefine a variable in your current shell session. Once you exit your shell session, your next shell session will only have the default environment variables.

unset FOO

It is noteworthy that /etc/security/pam_env.conf file can be used to override environment variables. For example, let's say you have the following XDG environment variables.

~]$ env | grep XDG
XDG_SESSION_TYPE=tty
XDG_SESSION_CLASS=user
XDG_SESSION_ID=3651
XDG_RUNTIME_DIR=/run/user/1002

You could add the following to /etc/security/pam_env.conf.

XDG_SESSION_TYPE  DEFAULT=""

And then the XDG_SESSION_TYPE should be empty.

~]$ env | grep XDG
XDG_SESSION_TYPE=
XDG_SESSION_CLASS=user
XDG_SESSION_ID=3651
XDG_RUNTIME_DIR=/run/user/1002