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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
Thu Nov 21 09:39:57 MST 2024

% export TZ=America/Chicago
% date
Thu Nov 21 10:39:57 CST 2024

% export TZ=Asia/Dubai
% date
Thu Nov 21 20:39:57 +04 2024

% export TZ=Europe/Paris
% date
Thu Nov 21 17:39:57 CET 2024

% export LANG=fr_FR.UTF-8
% date
jeu. nov. 21 17:39:57 CET 2024

• How does the date command know what time zone to use?
• 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

% env
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 env.
• An environment variable has a name and a value.

Environment Variables

% echo 'I am $USER on $HOSTNAME'
I am $USER on $HOSTNAME
% echo "I am $USER on $HOSTNAME"
I am cs155 on beethoven
% echo I am $USER on $HOSTNAME
I am cs155 on beethoven
% echo I am \$USER on \$HOSTNAME
I am $USER on $HOSTNAME
% echo $HOSTNAME
beethoven
% echo $HOSTNAME | wc
      1       1      10
% cat $HOSTNAME
cat: beethoven: No such file or directory

• 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.

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