一般Linux发行版自带的bash版本都比较老，如果要体验一些新特征就无法实现，比如关联数组，需要bash 4.0以上，所以需要升级，方法很简单只需编译安装即可。

了解系统当前bash的版本

[root@mail ~]# /bin/bash -version
GNU bash, version 3.2.25(1)-release (x86_64-redhat-linux-gnu)
Copyright (C) 2005 Free Software Foundation, Inc.

bash下载地址：http://ftp.gnu.org/gnu/bash/

安装升级bash

wget http://ftp.gnu.org/gnu/bash/bash-4.2.tar.gz
tar zxvf bash-4.2.tar.gz
cd bash-4.2
./configure
make
make install

因为bash默认是安装在/usr/local/bin/目录下，所以需要创建一个链接到 /bin/目录下，安装完毕后需要重启生效！

mv /bin/bash /bin/bash.bak; ln -s /usr/local/bin/bash /bin/bash

[root@mail ~]# /bin/bash -version
GNU bash, version 4.2.0(1)-release (x86_64-unknown-linux-gnu)
Copyright (C) 2011 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.