All UNIX-like operating systems such as FreeBSD, Redhat, CentOS, Debian allows you to set environment variables. This variables are used by system applications. To find the current values of all your environment variables and functions you can any shell you’d like: sh, zsh, bash, etc…
Below the list of system variables:
PATH - Display lists directories the shell searches, for the commands.
HOME - User's home directory to store files.
TERM - Set terminal emulator being used by UNIX.
PS1 - Display shell prompt in the Bourne shell and variants.
MAIL - Path to user's mailbox.
TEMP - Path to where processes can store temporary files.
JAVA_HOME - Sun (now Oracle) JDK path.
ORACLE_HOME - Oracle database installation path.
TZ - Timezone settings
PWD - Path to the current directory.
HISTFILE - The name of the file in which command history is saved
HISTFILESIZE -The maximum number of lines contained in the history file
HOSTNAME -The system's host name
LD_LIBRARY_PATH -It is a colon-separated set of directories where libraries should be searched for.
USER -Current logged in user's name.
DISPLAY -Network name of the X11 display to connect to, if available.
SHELL -The current shell.
TERMCAP - Database entry of the terminal escape codes to perform various terminal functions.
OSTYPE - Type of operating system.
MACHTYPE - The CPU architecture that the system is running on.
EDITOR - The user's preferred text editor.
PAGER - The user's preferred text pager.
MANPATH - Colon separated list of directories to search for manual pages.
Shell builtin commands are commands that can be executed within the running shell’s process. Note that, in the case of csh(1) builtin commands, the command is executed in a subshell if it occurs as any component of a pipeline except the last.
Print out the environment
Set environment and execute command, or print environment