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Environment variables

CS155 Environment

Introduction to Shells

• The shell manages communication between the user and Unix.
• Shells also provide facilities for completing tasks more efficiently
  • Environment Variables
  • Shell Variables
  • History
  • Scripting Language

Environment Variables

% date
Sat Nov 23 05:57:07 MST 2024

• How did the date command know what time zone I’m in?
• A variable is a symbol with an associated value.
• Environment variables are special variables that are accessible globally (i.e., by all programs).
• Environment variables are used to customize the behavior of the OS and many programs.
• They are also accessible to all shells attached to the current login session.

Environment Variables

% printenv
USER=applin
PATH=/bin:/usr/bin:/usr/bin/X11:.
HOME=/s/parsons/d/fac/applin
PWD=/s/parsons/d/fac/applin/tmp
GROUP=fac
HOST=blackthorn
LANG=en_US.UTF-8
HOSTNAME=blackthorn
PAGER=less
TZ=America/Denver

• List environment variables with printenv or env
• An environment variable has a name and a value.

Environment Variables

% echo 'I am $USER and my group is $GROUP'
I am $USER and my group is $GROUP
% echo "I am $USER and my group is $GROUP"
I am applin and my group is fac
% echo I am $USER and my group is $GROUP
I am applin and my group is fac
% echo $HOSTNAME
blackthorn
% echo $HOSTNAME | wc
      1       1      11

• Use $ to reference one of the variables
• Use double-quotes or no quotes

Setting environment variables

• We haven’t discussed how to change an environment variable.
• That’s because it depends on what shell you’re using.