105.1. Customize and use the shell environment

105.1 Customize and use the shell environment
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Description: 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
The following is a partial list of the used files, terms and utilities:
source
/etc/bash.bashrc
/etc/profile
env
export
set
unset
~/.bash_profile
~/.bash_login
~/.profile
~/.bashrc
~/.bash_logout
function
alias
lists
​
We have already talk about environment variables in section "103-1". In this section we want to talk about user profiles and system wide profile , and how they interact when user log on.
Login shell and Non login shell
The shell program, for example Bash, uses a collection of startup scripts to create an environment. Each script has a specific use and affects the login environment differently. Every subsequent script executed can override the values assigned by previous scripts.
Startup is configured differently for Login shells and Non login shells. 
1.Login shells : If you open a shell or terminal (or switch to one), and it asks you to log in (Username? Password?) before it gives you a prompt, it's a login shell.
2.Non login shells : If it doesn't ask you log in (like gnome-terminal), and lets you use it straight away, it's a non-login shell (GUI)
Adding global configuration for login shell
/etc/profile
/etc/profile contains Linux system wide environment and startup programs. It is used by all users with bash, ksh, sh shell. It only runs for login shell.  
/etc/profile.d
if you need to customize the login environment for all users on your system,  you can use /etc/profile, but as  the distribution files in /etc, such as /etc/profile, can be modified by system updates, so it is better not to edit them directly. Rather, create additional files in /etc/profile.d.
adding global configs for non-login (interactive) shell
/etc/bash.bashrc &  /etc/bashrc
You can use /etc/bash.bashrc ( or /etc/bashrc )[based on your distro] for adding global configs and global  aliases.
user specfic configs
/home/user/.bash_profile  &  /home/user/.bashrc
 ~/.bash_profile and ~/.bashrc are shell scripts that contain shell commands. These files are executed in a user's context when a new shell opens or when a user logs in so that their environment is set correctly. As we mentioned~/.bash_profile is executed for login shells and ~/.bashrc is executed for interactive non-login shells.
 This means that when a user logs in (via username and password) to the console (either locally or remotely via something like SSH), the ~/.bash_profile script is executed before the initial command prompt is returned to the user. After that, every time a new shell is opened, the ~/.bashrc script is executed. 
Most of the time PATH and env vars go into the in ~/.bash_profile and aliases go into the ~/.bashrc.
Aliases
In bash, you can define aliases for commands. You use aliases to provide an alternative name for a command, to provide default parameters for the command, or sometimes to construct a new or more-complex command.
 You set aliases or list aliases with the alias command and remove them with the unalias command.
### list aliases
[email protected]:~# alias 
alias egrep='egrep --color=auto'
alias fgrep='fgrep --color=auto'
alias grep='grep --color=auto'
alias l='ls -CF'
alias la='ls -A'
alias ll='ls -alF'
alias ls='ls --color=auto'
​
### unalias with unalias command
[email protected]:~# unalias ll
[email protected]:~# ll
ll: command not found
​
### setting an alias
[email protected]:~# alias ll='ls -alF'
[email protected]:~# ll
total 472
...
in order to make aliases permanent for user they are defined in /home/user/.bashrc :
...
# some more ls aliases
alias ll='ls -alF'
alias la='ls -A'
alias l='ls -CF'
...
 Another common use of aliases is for the root user. The cp, rm, and mv commands are usually aliased to include the -i parameter, to help prevent accidental destruction of files.
/etc/skel
You might be wondering how files like ~/.bash_profile, ~/.bashrc, or ~/.bash_logout got created in your home directory. These are skeleton files that are copied from /etc/skel.
[email protected]:~# ls -1a /etc/skel/
.
..
.bash_logout
.bashrc
examples.desktop
.profile
.bash_logout
The file .bash_logout is read and executed every time a login shell exits. It clears the screen whenever you log out. Without .bash_logout whatever you were working on could be visible for the next user!
[email protected]:~# cat /etc/skel/.bash_logout 
# ~/.bash_logout: executed by bash(1) when login shell exits.
​
# when leaving the console clear the screen to increase privacy
​
if [ "$SHLVL" = 1 ]; then
    [ -x /usr/bin/clear_console ] && /usr/bin/clear_console -q
fi
. and source
 If a script runs in a child shell, any variables it might export are lost when it returns to the parent.
[email protected]:~# cat .profile 
# ~/.profile: executed by Bourne-compatible login shells.
​
if [ "$BASH" ]; then
  if [ -f ~/.bashrc ]; then
    . ~/.bashrc
  fi
fi
​
mesg n || true
 So if .profile runs the ~/.bashrc script, why aren’t the variables and functions that are exported from ~/.bashrc lost? The answer is that you run the script in the current environment by using the source (or .) command. 
Shell functions
Aliases are useful, but what happens if you want to handle parameters? Aliases expand only the first word, and everything else on the command line is appended to the expansion. If you want to run a command with some parameters and then process the output somehow, you are out of luck with an alias. In this case, you need a shell function.
myfunc() {
    echo "hello $1"
}
​
# Same as above (alternate syntax)
function myfunc() {
    echo "hello $1"
}
$1 returns the first argument
[email protected]:~# myfunc "payam"
hello payam
Shell functions have a couple of advantages over aliases:
You can handle parameters.
You can use programming constructs, such as testing and looping, to enhance your processing.
We can use unset command inorder to unset our defined function.
lists
Shell supports a different type of variable called an array variable. This can hold multiple values at the same time. Arrays provide a method of grouping a set of variables. Instead of creating a new name for each variable that is required, you can use a single array variable that stores all the other variables.
array_name=(value1 ... valuen)
​
### example:
[email protected]:~# mylist=(lets learn linux)
​
### for accessing value:  {array_name[index]}
​
### example:
[email protected]:~# echo ${mylist[2]}
linux
Another example:
[email protected]:~# mylist2=("we are" "learning" "lpic 1-102 exam")
[email protected]:~# echo ${mylist2[2]}
lpic 1-101 exam
Keep your eyes on syntax!
.
.
.
​http://howtolamp.com/articles/difference-between-login-and-non-login-shell/​
​https://askubuntu.com/questions/155865/what-are-login-and-non-login-shells​
​https://www.cyberciti.biz/faq/set-environment-variable-linux/​
​https://developer.ibm.com/tutorials/l-lpic1-105-1/​
​https://jadi.gitbooks.io/lpic1/content/1051_customize_and_use_the_shell_environment.html​
​https://www.tutorialspoint.com/unix/unix-using-arrays.htm​
With the special thanks of shawn powers.
.

104.7. Find system files and place files in the correct location

105.2. Customize or write simple scripts

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105.1 Customize and use the shell environment

/etc/profile

/etc/profile.d

/etc/bash.bashrc & /etc/bashrc

/home/user/.bash_profile & /home/user/.bashrc