Linux环境变量

1. Linux环境变量

bash shell使用环境变量来存储shell会话和工作环境的相关信息，如系统信息、存储临时数据和配置信息等。
bash shell有两种环境变量：

• 全局变量：所有进程都可以访问的环境变量
• 局部变量：只能在定义它的进程中访问。

1.1 printenv 查看全局变量

使用printenv 来查看全局环境变量

$ printenv			#查看所有全局环境变量
SHELL=/bin/bash
WSL2_GUI_APPS_ENABLED=1
WSL_DISTRO_NAME=Ubuntu-20.04
NAME=GodOfWar
...

#查看环境变量SHELL
$ printenv SHELL 	#等于echo $SHELL
/bin/bash

1.2 设置自定义局部环境变量

设置局部环境变量格式为
环境变量名=“环境变量值”(注意变量名、等号和值之间没有空格)：

$ my_env="Hello"
$ echo $my_env
Hello

1.3 export 设置全局环境变量

使用export可以将局部环境变量升级为全局环境变量
注意 先创建局部环境变量，再将其导出到全局环境中

$ export my_env
$ echo $my_env
Hello

1.4 修改环境变量的值

修改环境变量的值与自定义环境变量一样，直接赋值即可

$ my_env="Hello World!"
$ echo $my_env
Hello World!

1.5 unset删除局部环境变量

使用unset命令删除局部环境变量

$ unset my_env
$ echo $my_env

注意：关于$符号的使用，使用变量时，变量名前面加$；操作变量时则不加$。这里printenv是特例。

1.6 PATH环境变量

PATH环境变量定义用于查找命令和程序目录,PATH中的目录之间用冒号分隔。

$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:
...

1.7 环境变量持久化

登录Linux系统时，bash shell会作为登录shell启动，通常从5个不同的启动文件中读取命令

• /etc/profile
• $HOME/.bash_profile
• $HOME/.bashrc
• $HOME/.bash_login
• $HOME/.profile

下面分别介绍这5个文件

1.7.1 /etc/profile

/etc/profile文件是系统中默认bash shell的启动文件，系统中每个用户登录时都会执行该启动文件。

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

if [ -d /etc/profile.d ]; then
  for i in /etc/profile.d/*.sh; do		#迭代/etc/profile.d下的所有.sh文件
    if [ -r $i ]; then
      . $i
    fi
  done
  unset i
fi

1.7.2 $HOME目录下的启动文件

• $HOME/.bash_profile
• $HOME/.bashrc
• $HOME/.bash_login
• $HOME/.profile

$HOME下的启动文件大多数Linux系统只用其中的一两个。这些文件以.开头，表示为隐藏文件，其中的环境变量在每次启动bash shell时生效。

$ cat $HOME/.bashrc
# ~/.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'

# 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

export PATH=~/bin:$PATH

.bashrc会做两件事：

• 检查/etc目录下的通用bashrc文件
• 为用户提供一个定制自己命令别名和脚本函数的地方

综上所述，推荐将需要持久化的环境变量保存在$HOME/.bashrc文件中。

1.8 数组环境变量

$ my_env=(zhangsan lisi wangwu)
$ echo $my_env
zhangsan
$ echo ${my_env[*]}
zhangsan lisi wangwu

$ unset my_env[1]		#删除my_env数组中第2个环境变量
$ echo ${my_env[*]}	#查看my_env所有环境变量
zhangsan wangwu
$ unset my_env			#删除my_env整个数组环境变量
$ echo ${my_env[*]}