打开一个shell将会发生什么？

登录和登出一个Linux终端，系统都将会根据系统或者用户配置文件规定的事情。

环境变量都有哪些？

shell配置文件有：

文件含义/bin/bashbash可执行程序/etc/profile系统执行的，登陆shell执行的文件/etc/bash.bashrc系统执行的，每个shell都执行的文件/etc/bash.bash.logout系统执行的，登出shell都会执行的清理文件~/.bash_profile个人执行的，登陆shell执行的文件~/.bashrc个人执行的，每个shell都会执行的文件~/.bash_logout个人执行的，登出shell都会执行的清理文件~/.inputrc文字命令的初始化文件

对于个人使用，只需要明白~/.bash_profile是个人的配置文件，当然由于历史原因，配置也可能存在于~/.bash_login和~/.profile两个等效文件中。

那么可以怎么查看环境变量？env推荐或export。

使用env查看当前环境变量。export也可以查看环境变量，不过其形式都是declare -x声明的形式。

第一步：系统整体配置文件读取/etc/profile

/etc/profile是系统的整体配置，一般不作修改。Ubuntu16.04下内容如下：

# /etc/profile: system-wide .profile file for the Bourne shell (sh(1))
# and Bourne compatible shells (bash(1), ksh(1), ash(1), ...).

if [ "$PS1" ]; then
  if [ "$BASH" ] && [ "$BASH" != "/bin/sh" ]; then
    # The file bash.bashrc already sets the default PS1.
    # PS1='\h:\w\$ '
    if [ -f /etc/bash.bashrc ]; then
      . /etc/bash.bashrc
    fi
  else
    if [ "`id -u`" -eq 0 ]; then
      PS1='# '
    else
      PS1='$ '
    fi
  fi
fi

if [ -d /etc/profile.d ]; then
  for i in /etc/profile.d/*.sh; do
    if [ -r $i ]; then
      . $i
    fi
  done
  unset i
fi

脚本主要做了两件事情：

• . /etc/bash.bashrc：重读bash.bashrc文件内容
• 执行/etc/profile.d下的所有.sh脚本

bash.bashrc完成了一些提示性语言，和终端外观控制等，其内容如下：

# System-wide .bashrc file for interactive bash(1) shells.

# To enable the settings / commands in this file for login shells as well,
# this file has to be sourced in /etc/profile.

# If not running interactively, don't do anything
[ -z "$PS1" ] && return

# check the window size after each command and, if necessary,
# update the values of LINES and COLUMNS.
shopt -s checkwinsize

# set variable identifying the chroot you work in (used in the prompt below)
if [ -z "${debian_chroot:-}" ] && [ -r /etc/debian_chroot ]; then
    debian_chroot=$(cat /etc/debian_chroot)
fi

# set a fancy prompt (non-color, overwrite the one in /etc/profile)
PS1='${debian_chroot:+($debian_chroot)}\u@\h:\w\$ '

# Commented out, don't overwrite xterm -T "title" -n "icontitle" by default.
# If this is an xterm set the title to user@host:dir
#case "$TERM" in
#xterm*|rxvt*)
#    PROMPT_COMMAND='echo -ne "\033]0;${USER}@${HOSTNAME}: ${PWD}\007"'
#    ;;
#*)
#    ;;
#esac

# enable bash completion in interactive shells
#if ! shopt -oq posix; then
#  if [ -f /usr/share/bash-completion/bash_completion ]; then
#    . /usr/share/bash-completion/bash_completion
#  elif [ -f /etc/bash_completion ]; then
#    . /etc/bash_completion
#  fi
#fi

# sudo hint
if [ ! -e "$HOME/.sudo_as_admin_successful" ] && [ ! -e "$HOME/.hushlogin" ] ; then
    case " $(groups) " in *\ admin\ *|*\ sudo\ *)
    if [ -x /usr/bin/sudo ]; then
	cat &2
		   return 127
		fi
	}
fi

第二步 用户配置读取

按一定顺序依次读取不同习惯用户配置文件~/.bash_profile or ~./bash_login or ~/.profile。他们都是家目录下的隐藏文件，对ubuntu而言，只有默认的~/.profile。以这个文件说明用户配置都有哪些。

# ~/.profile: executed by the command interpreter for login shells.
# This file is not read by bash(1), if ~/.bash_profile or ~/.bash_login
# exists.
# see /usr/share/doc/bash/examples/startup-files for examples.
# the files are located in the bash-doc package.

# the default umask is set in /etc/profile; for setting the umask
# for ssh logins, install and configure the libpam-umask package.
#umask 022

# if running bash
if [ -n "$BASH_VERSION" ]; then
    # include .bashrc if it exists
    if [ -f "$HOME/.bashrc" ]; then
	. "$HOME/.bashrc"
    fi
fi

# set PATH so it includes user's private bin directories
PATH="$HOME/bin:$HOME/.local/bin:$PATH"

• 刷新形式读取~/.bashrc文件内容
• 设置PATH变量私有用户命令（$HOME/bin）

~/.bashrc内容如下：

# ~/.bashrc: executed by bash(1) for non-login shells.
# see /usr/share/doc/bash/examples/startup-files (in the package bash-doc)
# for examples

# If not running interactively, don't do anything
case $- in
    *i*) ;;
      *) return;;
esac

# don't put duplicate lines or lines starting with space in the history.
# See bash(1) for more options
HISTCONTROL=ignoreboth

# append to the history file, don't overwrite it
shopt -s histappend

# for setting history length see HISTSIZE and HISTFILESIZE in bash(1)
HISTSIZE=1000
HISTFILESIZE=2000

# check the window size after each command and, if necessary,
# update the values of LINES and COLUMNS.
shopt -s checkwinsize

# If set, the pattern "**" used in a pathname expansion context will
# match all files and zero or more directories and subdirectories.
#shopt -s globstar

# make less more friendly for non-text input files, see lesspipe(1)
[ -x /usr/bin/lesspipe ] && eval "$(SHELL=/bin/sh lesspipe)"

# set variable identifying the chroot you work in (used in the prompt below)
if [ -z "${debian_chroot:-}" ] && [ -r /etc/debian_chroot ]; then
    debian_chroot=$(cat /etc/debian_chroot)
fi

# set a fancy prompt (non-color, unless we know we "want" color)
case "$TERM" in
    xterm-color|*-256color) color_prompt=yes;;
esac

# uncomment for a colored prompt, if the terminal has the capability; turned
# off by default to not distract the user: the focus in a terminal window
# should be on the output of commands, not on the prompt
#force_color_prompt=yes

if [ -n "$force_color_prompt" ]; then
    if [ -x /usr/bin/tput ] && tput setaf 1 >&/dev/null; then
	# We have color support; assume it's compliant with Ecma-48
	# (ISO/IEC-6429). (Lack of such support is extremely rare, and such
	# a case would tend to support setf rather than setaf.)
	color_prompt=yes
    else
	color_prompt=
    fi
fi

if [ "$color_prompt" = yes ]; then
    PS1='${debian_chroot:+($debian_chroot)}\[\033[01;32m\]\u@\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\$ '
else
    PS1='${debian_chroot:+($debian_chroot)}\u@\h:\w\$ '
fi
unset color_prompt force_color_prompt

# If this is an xterm set the title to user@host:dir
case "$TERM" in
xterm*|rxvt*)
    PS1="\[\e]0;${debian_chroot:+($debian_chroot)}\u@\h: \w\a\]$PS1"
    ;;
*)
    ;;
esac

# enable color support of ls and also add handy aliases
if [ -x /usr/bin/dircolors ]; then
    test -r ~/.dircolors && eval "$(dircolors -b ~/.dircolors)" || eval "$(dircolors -b)"
    alias ls='ls --color=auto'
    #alias dir='dir --color=auto'
    #alias vdir='vdir --color=auto'

    alias grep='grep --color=auto'
    alias fgrep='fgrep --color=auto'
    alias egrep='egrep --color=auto'
fi

# colored GCC warnings and errors
#export GCC_COLORS='error=01;31:warning=01;35:note=01;36:caret=01;32:locus=01:quote=01'

# some more ls aliases
alias ll='ls -alF'
alias la='ls -A'
alias l='ls -CF'
alias rd='rm -rf'
# Add an "alert" alias for long running commands.  Use like so:
#   sleep 10; alert
alias alert='notify-send --urgency=low -i "$([ $? = 0 ] && echo terminal || echo error)" "$(history|tail -n1|sed -e '\''s/^\s*[0-9]\+\s*//;s/[;&|]\s*alert$//'\'')"'

# Alias definitions.
# You may want to put all your additions into a separate file like
# ~/.bash_aliases, instead of adding them here directly.
# See /usr/share/doc/bash-doc/examples in the bash-doc package.

if [ -f ~/.bash_aliases ]; then
    . ~/.bash_aliases
fi

# enable programmable completion features (you don't need to enable
# this, if it's already enabled in /etc/bash.bashrc and /etc/profile
# sources /etc/bash.bashrc).
if ! shopt -oq posix; then
  if [ -f /usr/share/bash-completion/bash_completion ]; then
    . /usr/share/bash-completion/bash_completion
  elif [ -f /etc/bash_completion ]; then
    . /etc/bash_completion
  fi
fi

主要完成以下内容：

• 别名。简单的别名alias ls='ls --color=auto'和别名配置文件调用. ~/.bash_aliases
• 补全。如，. /usr/share/bash-completion/bash_completion . /etc/bash_completion

在文件中所有的补全脚本程序都会被执行，如git-prompt，几乎所有的程序的配置文件（或文件夹）都会放置在~/.*下，像是vim git thunder cmake vscode等。像是git补全程序就放在. /etc/bash_completion中由文件~/.bashrc调用。

三、配置并刷新方法举例

用户安装的软件配置文件一般会被放在HOME目录下的与软件名有关的隐藏文件中，登录时将会读取这些配置并输出至环境变量中。如VIM和GIT设置：

我们常用的vim行号开启就是在~/.vimrc中写入。

• ~/.vimrc
• ~/.gitconfig

通常配置文件需要用户启动时才生效，因为此时已经登录，所以只能通过重新登陆或者source or .来实现。

• VIM常用配置

set number
set tabstop=4 # 一个TAB多少空格
set shiftwidth=4 # 级间缩进空格数
syntax on
set tabstop=4
set softtabstop=4
set shiftwidth=4
set autoindent
set nu
set nocompatible
filetype plugin indent on 75 hi Comment ctermfg =blue  ##解决蓝色注释看不清的问题

• git别名选项别名配置

[user]
    name = yourname
    email = yourname@email.com
[color]
    ui = true
    diff = true
    status = true
    branch = true
    interactive = true
[alias]    //配置别名
    st = status
    ss = status -s
    ci = commit
    ca = commit --amend
    co = checkout
    ll = log --graph --color --format=format:'%C(bold)%h%C(reset) -%C(bold)%d%C(reset) %C(white)%s%C(reset) %C(bold red)- %an' --abbrev-commit
    ft = fetch
    pl = pull --rebase
    br = branch
    cp = cherry-pick
    mg = merge
    rb = rebase
    dci = dcommit
    sbu = submodule update 

git别名是选项别名，不是我们环境变量中的别名，这要注意。

• 添加环境变量 对于个人用户只需要在~/.bashrc中最后一行添加export PATH=$PATH:/usr/local/arm/XXX

matlab添加到环境变量方便直接在终端敲入matlab即可执行，添加到以下语句： export/usr/local/MATLAB/R2018a/bin添加命令所在的目录即可，一般是XXX/bin

• 命令别名 为了简化命令操作，常常会用到别名。别名可以直接在终端输入alias 简化命令='长或复杂命令'”，要想一直生效把这一段命令放在.bashrc或者等效配置文件中即可。