Reprinted from http://hi.baidu.com/cuit_6015/blog/item/8d61edec8934feddb21cb14c.html
Establishing a cross-compilation environment is an important step in embedded development. Installing a cross-compilation environment is to set environment variables for the cross-compilation tool. Here is an example of installing a cross-compilation environment: First: install the working directory. Copy the decompressed cross-compilation toolchain arm-linux-gcc 3.4.1 version to /usr/local/arm/3.4.1/ Actually: set environment variables. #gedit /root/.bashrc Edit the /root/.bashrc file, in the last line export PATH=$PATH:/usr/local/arm/3.4.1/bin At the end: Re-login to the Redhat system (do not restart the machine, start-> logout) to make the above settings take effect, enter arm-linux-gcc -v on the command line to check whether the cross-compilation environment is successfully established. Analyze the contents of the /root/bashrc file: .bashrc: ······ if [ -f /etc/bashrc ]; then . /etc/bashrc fi export PATH=$PATH:/usr/local/arm/3.4. The first three lines of 1/bin mean "if the file /etc/bashrc exists, then execute it". The last line is added by myself to modify the PATH variable. Here is the effect of setting the environment variable. Running arm-linux- will run the corresponding command in export PATH=$PATH:/usr/local/arm/3.4.1/bin. And every time I log in, the environment variable PATH is automatically modified to the value I need.
Question 1: Use the export command to set environment variables in the Linux operating system: for example, after export HOME=/home j, HOME in the script represents /home. How does the arm-linux-gcc command in the shell match /usr/local/arm/3.4 arm-linux-gcc in .1/bin associated? Can't see their relationship directly in the statement? Answer: It is true that the correlation between HOME and /home is not directly visible in the statement as in the example. That's because the environment variable PATH is blinding you. Use $echo $PATH to view the value of the environment variable PATH (env view all environment variable values)
Add a little knowledge here: The usage of special symbols is explained through the following content. export A=/q/jing:aaa/cc/ld export B=.:/liheng/wang export A=/cd/cdr:$A Everyone pay attention to the red symbol: : means parallel meaning, for example, there are multiple A variable values , separated by the : symbol. . Indicates the current directory you are working on. For example the pap command will look for the B environment variable. Type the pap command in /home, the system will first look for the content of B in the /home directory (ie the current path), if not in the /liheng/wang directory. $ represents the value of the variable before this definition, for example, $A represents /q/jing:aaa/cc/ld. That is to say, the role of the A=/cd/cdr:/q/jing:aaa/cc/ld PATH environment variable is to determine which directories the shell will search for commands or programs. arm-linu-gcc is a command. export PATH=$PATH:/usr/local/arm/3.4.1/bin is not difficult to understand. It means that the previous value of PATH ($PATH) and the new environment variable value (/usr/local/arm/3.4.1/bin ) are side by side and separated by symbols. Because arm-Linux-gcc only exists under /usr/local/arm/3.4.1/bin, the path of arm-linux-gcc can be separated by symbols during execution as /usr/local/arm/3.4 .1/bin Question 2: Why is it set in the /root/bashrc file? Not setting it in /etc/bashrc or some other file?
回答：linux给大家提供了自动设置环境变量的方法，那就是修改 .bashrc 文件。一般说来，有两个文件可以提供这种“进入系统时自动设置”的功能，一个是 /etc/bashrc，另一个是 ~/.bashrc。其中/etc/bashrc是被每个用户执行的，而~/.bashrc只被当前用户执行。所以/etc/bashrc只有root用户能更改，而~/.bashrc是各个用户私有的文件。 "~"指的是用户的home目录。我自己用root帐号登录的，所以对我来所~/bashrc就是/root/bashrc。
这样，各个用户有自己独立的".bashrc"文件，可以根据需要自行更改，而不会互相干扰。 /etc/bashrc文件则是共享的，整个Linux中只有一份。一般来说，用户的"~/.bashrc"开头都会调用/etc/bashrc，因此更改/etc/bashrc会影响到所有用户，这个文件一般是root用户管理的。
提问3：Linux系统中/etc/profile与/etc/bashfile文件作用的区别？
（1）/etc/profile：此文件为系统的每个用户设置环境信息,当用户第一次登录时,该文件被执行. 并从/etc/profile.d目录的配置文件中搜集shell的设置。他是为用户定制系统定制环境变量。
（2）/etc/bashrc: 是为每一个bash shell的用户定制环境变量，当bash shell被打开时,该文件被读取。~/bashrc是为某个bash shell用户定制环境变量
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环境变量就是系统或软件设置的一些参数，用户环境变量就是用户登录系统后，都有自已专用的运行环境。在Windows系统中用户环境变量保存在用户家目录，Linux也是同样的。本文主要是讲解Linux常用的环境变量和环境变量的设置。
一、环境变量相关操作（只应用于当前，要永久保存需要写入到相关文件。）
#echo $PATH
显示PATH设置。
#env
显示当前用户变量。
#set
显示当前Shell变量。
#export
显示当前导出成用户变量的shell变量。
#a=abc
定义一个Shell变量。
#export a=abc
定义一个Shell变量，并导出成用户变量。
#unset a
清除环境变量
#readonly a
设置只读环境变量
二、常见的环境变量
PATH 决定了shell将到哪些目录中寻找命令或程序
HOME 当前用户主目录
HISTSIZE 历史记录数
LOGNAME 当前用户的登录名
HOSTNAME 指主机的名称
SHELL 前用户Shell类型
LANGUGE 语言相关的环境变量，多语言可以修改此环境变量
MAIL 当前用户的邮件存放目录
PS1 基本提示符，对于root用户是#，对于普通用户是$
PS2 附属提示符，默认是“>”
三、Linux环境变量设置文件
/etc/profile
全局用户，应用于所有的Shell。
/$HOME/.profile
当前用户，应用于所有的Shell。
/etc/bash_bashrc
全局用户，应用于Bash Shell。
~/.bashrc
局部当前，应用于Bash Sell。
四、其它
在用户目录下用ls -a，能看见很多“.”开头的隐藏文件。如果不是用户建立的，那么这些文件都是环境设置文件。有其它Shell的设置文件，也有软件设置的文件。比 如：.lftp，它就是lftp软件的设置文件。如果安装了x-window的话，菜单和桌面设置都在.local里面。这些大家用久了自然就知道了。
export set env区别baidu一下就知道了。
补充：开启启动Xwindow，有些Linux发行版不会加载SHELL环境变量，因为Xwindow有自已的会话设置。例如gnome，有~/.gnomerc 或 /etc/X11/Xsession.d/55gnome-session_gnomerc。
http://blog.sina.com.cn/s/blog_5420e00001017z66.html