Configure and manage the environment

printenv

• This command is used to print environment variables.
• Usage: printenv [OPTION]... [VARIABLE]...
• If used without any argument it prints all the environment variables.
• Examples:
  • printenv
  • printenv HOME
  • printenv USER
  • printenv PWD SHELL
• You can also print the value of environment variables using echo.
• Example:
  • echo $HOME

set - Set shell options

• The set command, when used without options or arguments, will display both the shell and environment variables, as well as any defined shell functions.
• Example:
  • set

Modifying environment variables:

• You can modify variables by assigning them a new value using the assignment operator.
• Example:
  • PATH=$PATH:$HOME/bin
  • foo="Value"

export

• The export command tells the shell to make the contents of PATH available to child processes of this shell.
• Usage: export variable
• Example:
  • export foo
  • export (all exported variables)

Login shells

• A Login shell is created after a successful login of user. For example, when you login t a Linux system via terminal, SSH or switch to user with su - command.

Non-login shells

• Non Login Shell is the shell, which is started by the login shell. For example, A shell which you started from another shell or started by a program etc.

Startup Files for Login Shell Sessions

• /etc/profile - A global configuration script that applies to all users.
• ~/.bash_profile - A user’s personal startup file. It can be used to extend or override settings in the global configuration script.
• ~/.bash_login - If ~/.bash_profile is not found, bash attempts to read this script.
• /.profile - If neither ~/.bash_profile nor ~/.bash_login is found, bash attempts to read this file. This is the default in Debian-based distributions, such as Ubuntu.

Startup Files for Non-Login Shell Sessions

• /etc/bash.bashrc - A global configuration script that applies to all users.
• ~/.bashrc - A user’s personal startup file. It can be used to extend or override settings in the global configuration script.

Check if you run a login shell or non-login shell

• echo $0
• If the above command outputs a string starting with hyphen(-), then it is a login shell else it is a non-login shell
• Examples:
  • -bash (login shell output)
  • bash (non-login shell output)

Which Files Should We Modify?

• As a general rule, to add directories to your PATH or define additional environment variables, place those changes in .profile
• For everything else, place the changes in .bashrc.

source

• The changes we have made to our .bashrc will not take effect until we close our terminal session and start a new one because the .bashrc file is read only at the beginning of a session. However, we can force bash to reread the modified .bashrc file with the following command.
• Example:
  • source ~/.bashrc