105.1 Customize and use the shell environment

Weight: 4

Candidates should be able to customize shell environments to meet users’ needs. Candidates should be able to modify global and user profiles.

Key Knowledge Areas

• Set environment variables (e.g. PATH) at login or when spawning a new shell
• Write Bash functions for frequently used sequences of commands
• Maintain skeleton directories for new user accounts
• Set command search path with the proper directory

Terms

• .
• source
• /etc/bash.bashrc
• /etc/profile
• env
• export
• set
• unset
• ~/.bash\_profile
• ~/.bash\_login
• ~/.profile
• ~/.bashrc
• ~/.bash\_logout
• function
• alias

Environment variables

Shell Environment Variables are special variables that contain information and configuration about the shell. System shell uses them when it needs to show you a prompt, run a command, or search what to run. Different operating system distributions have different environment variables and you can add yours if you need so; or want so!

env

can set, remove or display variables or even run a command in a modified environment.

Syntax
     env [OPTION]... [NAME=VALUE]... [COMMAND [ARGS]...]

Options

  -u NAME
  --unset=NAME
       Remove variable NAME from the environment, if it was in the
       environment.

  -
  -i
  --ignore-environment
       Start with an empty environment, ignoring the inherited
       environment.

You can also use the more general printenv command.

set

set allows you to change the values of shell options (variables) or to display the names and values of shell variables.

Using set we can configure how bash works. These are some samples:

setting environment variables

Bash environment variables can contain only letters (a to z or A to Z), numbers ( 0 to 9) or the underscore character _ and cannot start with numbers. Traditionally we use UPPERCASE letters in our variable names.

To set a new environment variable do as follow:

$ name=jadi
$ desc='A programmer who enjoys cycling and promotes freedom'
$ echo $name
jadi
$ echo $desc
A programmer who enjoys cycling and promotes freedom

Notes:

1. There should be no spaces around =.
2. Use the quotes or double-quotes when your value do have spaces (or other special characters) in it.
3. Use a dollar sign only when referring to a variable, and not when defining it.

unset

This command unsets a variable or a function.

$ name=jadi
$ echo $name
jadi
$ unset name
$ echo $name

export

When setting environment variables with a simple =, the variable is only available in the current shell. So even if you run a new command from the current shell the variable wont be valid there (because technically the new command will be run in a sub-shell). If you want your variable to be valid in the current shell and all its sub-shells, you have to export it.

$ export name=jadi
$ echo $name
jadi
$ bash
$ echo $name
jadi

. (and source)

Yes only a dot! This is a shortcut for the bash source command. You can find it in files like /etc/profile. It runs the executable in front of it as part of the current environment (and not in a sub-process).

Note: If you just execute a file (without source or dot) bash creates a child, runs the executable there and then closes it.

The source command is commonly used when you want to load new/updated environment variables or functions from a script.

Aliases

This is a way to define... um.. and alias. You can find them in your ~/.bashrc:

alias ll='ls -alF'
alias la='ls -A'
alias l='ls -CF'

It is a way to define a shortcut and you can add yours:

alias testnet='ping 4.2.2.4'

functions

Like larger programming languages, Bash has functions, though in a somewhat limited implementation. A function is a subroutine, a code block that implements a set of operations, a "black box" that performs a specified task. Wherever there is repetitive code, when a task repeats with only slight variations in procedure, then consider using a function.

funnyls () {
    ls -ltrh
    echo "This is a funny ls"
}

Different shell envs

The shell (here bash) can be started in 3 different ways:

1. When you login into the shell, say after a ssh or when you site behind a terminal and login into a Linux machine. This is an interactive login session
2. When you open a new terminal (say from a GUI env). This is also interactive but not a login shell.
3. When a shell spawns when you run a command. Yes.. technically when you run a new command from a shell, a sup-shell starts, runs the commands and then returns back to your shell. This is called "non interactive" shell.

We will see how different default environment variables can be set in each of these modes.

login shell

This is when you give your user and pass to enter a shell. In this case, these steps will define your shell environment:

1- /etc/profile is run

2- A line in /etc/profile runs whatever is in /etc/profile.d/

Now the global profile is loaded and system will go for user specific profiles:

3.1- /home/USERNAME/.bash_profile

3.2- /home/USERNAME/.bash_login

3.3- /home/USERNAME/.profile

Note that only one of the 3, 4 & 5 will run. The system will go for .bash_profile and IF IT IS NOT THERE will try .bash_login and IF IT IS NOT THERE will try to run .profile. If any of these exists, the system wont look any further. So if you have only 4 & 5, only the 4 will be run.

At the end, the system loads:

4- /home/USERNAME/.bashrc

Commonly you will add whatever personal config you want into the ~/.bashrc.

Interactive (non-login) shell

if you run a shell in an interactive mode (non-login) shell from a GUI or within another temrinal, only two file will handle the environment:

1. /etc/bash.bashrc (or /etc/bashrc in some systems)
2. /home/USERNAME/.bashrc

Non Interactive Shell

There is no specific file to be used in this case. Instead we have the BASH_ENV. When a new non-interactive shell starts, it looks into this variable and if it points to a file, runs it.

On most Linux distributions, this environment value is not set by default.

A few more files

/etc/skel

This directory contains files which will be used as a starting template for each new user.

.bash_logout

This runs when you logout from a login shell. In many distros it only clears the screen so the next person will not be able to watch what you were doing before you logout.