Environment Variables & Startup Files

Environment Variables

• The shell stores information in the environment variables.

• Each shell has its own values for these variables.

printenv

printenv
# Prints name and value pairs for all the environment variables
# VARIABLE_NAME=VALUE

printenv | grep PATH

• SHELL - user's shell is bash

• PWD - current working directory

• HOME - home directory for the current user

• PATH - list of the directories searched in order, to find commands to be executed.

  • Current working directory can be put first in this path, easier to execute current working directory programs but can be used for attacks.

There are two types of environment variables: global and local.

Global

• Global variables can be accessed by anything executed in that shell = Environment Variables

export / unset

# Create a global variable
export COUNT_GLOBAL=33
echo $COUNT_GLOBAL
	33

# Unset a variable
unset COUNT_GLOBAL

Local

• Local variables are valid only for the current shell (and not for sub-shells) = Shell Variables

# Create a shell variable
COUNT_LOCAL=42
echo $COUNT_LOCAL
	42
# $ is used to tell the shell that it is a variable

# Change shell (create sub-shell) and check the variable doesn't exist
bash
echo $COUNT_LOCAL

Startup files

• When starting a new bash shell, the shell is configured using startup files.

• Interactive non-login shell - the type of shell opened by the Terminal

nano .bashrc
# There are various configurations for the shell and commands aliases
# Aliases can be added at the bottom of this file

• alias name='COMMAND_VALUE' - the alias uses the entire command enclosed in single quotes

• After adding an alias, a new shell must be created to make the alias work, or use the source .bashrc command in the same shell.

source .bashrc
# Re-runs all the commands in the .bashrc file in the current shell

• Using a shell other than bash, the startup file name will be different.

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