我们都知道使用cd ../是回到系统的上一层目录，但是当你进入到一个很深的目录的时候，你想回到前3层的目录，你就要用cd ../../../来达到目的，如果是目录更深，或许你需要的../就要写的更多。现在通过cd alisa命令来减轻或减少../的输入次数。
假设有如下的目录：
jack@Ubuntu: mkdir -p /tmp/very/long/directory/structure/that/is/too/deep jack@Ubuntu:cd /tmp/very/long/directory/structure/that/is/too/deep jack@Ubuntu:/tmp/very/long/directory/structure/that/is/too/deep$ pwd /tmp/very/long/directory/structure/that/is/too/deep jack@Ubuntu:/tmp/very/long/directory/structure/that/is/too/deep$ cd ../../../../ jack@Ubuntu:/tmp/very/long/directory/structure$ pwd /tmp/very/long/directory/structure
通常的做法是我们要../四次才能回到structure目录
现在提供如下方法：
方法1：定义..n别名来指定到第几层目录。如果要永久生效要把命令写入~/.bash_profile并重启系统。
jack@Ubuntu:/tmp/very/long/directory/structure$ alias ..1="cd ../" jack@Ubuntu:/tmp/very/long/directory/structure$ alias ..2="cd ../../" jack@Ubuntu:/tmp/very/long/directory/structure$ alias ..3="cd ../../../" jack@Ubuntu:/tmp/very/long/directory/structure$ alias ..4="cd ../../../../" jack@Ubuntu:/tmp/very/long/directory/structure/that/is/too/deep$ pwd /tmp/very/long/directory/structure/that/is/too/deep jack@Ubuntu:/tmp/very/long/directory/structure/that/is/too/deep$ ..4 jack@Ubuntu:/tmp/very/long/directory/structure$ pwd /tmp/very/long/directory/structure
比较方便吧。
其他方法跟方法1类似，区别就是对别名的定义如何能更好的让使用者能够快速记住，比如定义为alias cd1="cd ../"等。了解了alias 命令我们就可以自己去定义一些比较常用的命令。
例如：公司通常都不会给普通的开发在prod环境配很高的权限，所以有时候我们就需要sudo 到某个有特定权限的帐号去做一些操作。例如：sudo -u devderiv 切换到devderiv帐号，这个命令经常用，为了节约时间，可以定义alias，alias devderiv=“sudo -u devderiv” 这样就可以直接输入devderiv来切换帐号了。如果命令更长的话，定义别名会更加的方便快捷。
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