Shell and Environment Variables

Current shell

Where a program is executed.

Sub-shell (child shell)

created within a shell to run a program.
two types of variables: local (or shell) and environment. local variable
- Private to the shell in which it is created.
- Only used by programs that are started in that shell. environment variable
- Inherited from the current shell to the sub-shell during the execution of a program
- Value stored in an environment variable is accessible to the program, as well as any sub-programs that it spawns during its lifecycle.
- Any environment variable set in a sub-shell is lost when the sub-shell terminates.
- env or the printenv command to view predefined environment variables.
- Common predefined environment variables:
  - DISPLAY
    - Stores the hostname or IP address for graphical terminal sessions
  - HISTFILE
    - Defines the file for storing the history of executed commands
  - HISTSIZE
    - Defines the maximum size for the HISTFILE
  - HOME
    - Sets the home directory path LOGNAME Retains the login name
  - MAIL
    - Contains the path to the user mail directory
  - PATH
    - Directories to be searched when executing a command. Eliminates the need to specify the absolute path of a command to run it.
  - PPID
    - Holds the identifier number for the parent program
  - PS1
    - Defines the primary command prompt PS2 Defines the secondary command prompt
  - PWD
    - Stores the current directory location
  - SHELL
    - Holds the absolute path to the primary primary shell file
  - TERM
    - Holds the terminal type value
  - UID
    - Holds the logged-in user’s UID
  - USER
    - Retains the name of the logged-in user

Setting and unsetting variables

export, unset, and echo to define and undefine environment variables
Use uppercase for variables

echo command

Restricted to showing the value of a specific variable

env command

Displays the environment variables only.

[root@localhost ~]# env
SHELL=/bin/bash
HISTCONTROL=ignoredups
HISTSIZE=1000
HOSTNAME=localhost
PWD=/root
LOGNAME=root
XDG_SESSION_TYPE=tty
MOTD_SHOWN=pam
HOME=/root
LANG=en_US.UTF-8
LS_COLORS=rs=0:di=01;34:ln=01;36:mh=00:pi=40;33:so=01;35:do=01;35:bd=40;33;01:cd=40;33;01:or=40;31;01:mi=01;37;41:su=37;41:sg=30;43:ca=30;41:tw=30;42:ow=34;42:st=37;44:ex=01;32:*.tar=01;31:*.tgz=01;31:*.arc=01;31:*.arj=01;31:*.taz=01;31:*.lha=01;31:*.lz4=01;31:*.lzh=01;31:*.lzma=01;31:*.tlz=01;31:*.txz=01;31:*.tzo=01;31:*.t7z=01;31:*.zip=01;31:*.z=01;31:*.dz=01;31:*.gz=01;31:*.lrz=01;31:*.lz=01;31:*.lzo=01;31:*.xz=01;31:*.zst=01;31:*.tzst=01;31:*.bz2=01;31:*.bz=01;31:*.tbz=01;31:*.tbz2=01;31:*.tz=01;31:*.deb=01;31:*.rpm=01;31:*.jar=01;31:*.war=01;31:*.ear=01;31:*.sar=01;31:*.rar=01;31:*.alz=01;31:*.ace=01;31:*.zoo=01;31:*.cpio=01;31:*.7z=01;31:*.rz=01;31:*.cab=01;31:*.wim=01;31:*.swm=01;31:*.dwm=01;31:*.esd=01;31:*.jpg=01;35:*.jpeg=01;35:*.mjpg=01;35:*.mjpeg=01;35:*.gif=01;35:*.bmp=01;35:*.pbm=01;35:*.pgm=01;35:*.ppm=01;35:*.tga=01;35:*.xbm=01;35:*.xpm=01;35:*.tif=01;35:*.tiff=01;35:*.png=01;35:*.svg=01;35:*.svgz=01;35:*.mng=01;35:*.pcx=01;35:*.mov=01;35:*.mpg=01;35:*.mpeg=01;35:*.m2v=01;35:*.mkv=01;35:*.webm=01;35:*.webp=01;35:*.ogm=01;35:*.mp4=01;35:*.m4v=01;35:*.mp4v=01;35:*.vob=01;35:*.qt=01;35:*.nuv=01;35:*.wmv=01;35:*.asf=01;35:*.rm=01;35:*.rmvb=01;35:*.flc=01;35:*.avi=01;35:*.fli=01;35:*.flv=01;35:*.gl=01;35:*.dl=01;35:*.xcf=01;35:*.xwd=01;35:*.yuv=01;35:*.cgm=01;35:*.emf=01;35:*.ogv=01;35:*.ogx=01;35:*.aac=01;36:*.au=01;36:*.flac=01;36:*.m4a=01;36:*.mid=01;36:*.midi=01;36:*.mka=01;36:*.mp3=01;36:*.mpc=01;36:*.ogg=01;36:*.ra=01;36:*.wav=01;36:*.oga=01;36:*.opus=01;36:*.spx=01;36:*.xspf=01;36:
SSH_CONNECTION=192.168.0.233 56990 192.168.0.169 22
XDG_SESSION_CLASS=user
SELINUX_ROLE_REQUESTED=
TERM=xterm-256color
LESSOPEN=||/usr/bin/lesspipe.sh %s
USER=root
SELINUX_USE_CURRENT_RANGE=
SHLVL=1
XDG_SESSION_ID=1
XDG_RUNTIME_DIR=/run/user/0
SSH_CLIENT=192.168.0.233 56990 22
which_declare=declare -f
PATH=/root/.local/bin:/root/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin
SELINUX_LEVEL_REQUESTED=
DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/0/bus
MAIL=/var/spool/mail/root
SSH_TTY=/dev/pts/0
BASH_FUNC_which%%=() {  ( alias;
 eval ${which_declare} ) | /usr/bin/which --tty-only --read-alias --read-functions --show-tilde --show-dot $@
}
_=/usr/bin/env
OLDPWD=/dev/vg200

printenv command

Displays the environment variables only.

set command

View both local and environment variables.

Command and Variable substitutions

PS1 Environment variable sets what the prompt looks like.
Default value is \u@\h \W\$
- \u
  - logged-in user name
- \h
  - System hostname
- \W
  - Working directory
- \$
  - End of command prompt
Command whose output you want assigned to a variable must be encapsulated within either backticks hostname or parentheses $(hostname).