本节的目的：
-
How to use a list of paths
-
Search for executables, libraries, and so on
常见的两个环境变量如下：
$ echo $PATH
/usr/bin:/bin
$ echo $LD_LIBRARY_PATH
/usr/lib:/lib
将/opt/myapp/bin添加到环境变量中：
$ export PATH=/opt/myapp/bin:$PATH
$ export LD_LIBRARY_PATH=/opt/myapp/lib;$LD_LIBRARY_PATH
bash: /opt/myapp/lib: No such file or directory
现在PATH和LD_LIBRARY_PATH变为：
$ echo $PATH
/opt/myapp/bin:/usr/bin:/bin
$ echo $LD_LIBRARY_PATH
/opt/myapp/lib
使用自定义的函数完成这一任务：
$ prepend() { [ -d "$2" ] && eval $1="$2':'$$1" && export $1; }
$ prepend PATH /opt/myapp/bin
$ prepend LD_LIBRARY_PATH /opt/myapp/lib
prepend()的修改版，修正了环境变量为空时产生的bug：
$ prepend() { [ -d "$2" ] && eval $1="$2${$1:+':'$$1}" && export $1 ; }
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