语法
cd [-L|[-P [-e]]] [dir]
改变shell当前工作目录。仅对执行命令所在的shell有效。
参数
-L 按符号链接进入目录。
-P 按物理链接进入目录
-e 如果指定-P,且路径不存在，则返回-e后指定的退出码。
说明
cd =change directory
进入主目录。不带参数dir，就默认进入当前用户的主目录。等价命令还有 cd ~ 或 cd $HOME。
进入上一次切换前的工作目录。cd - 或 cd $OLDPWD。如果反复执行，则会在最后两次工作目录间来回切换。
cd的默认行为，等于cd <=> cd -L。如果路径中包含符号连接，则跟着符号链接走。
CDPATH环境变量
CDPATH环境变量为cd 提供一组备选的当前目录。也就是说，如果你输入的不是/开头的绝对路径，则进入的目录的绝对路径是CDPATH中的目录+你的目录。举例来说，
架设你当前目录是/usr，myhome在你的/home下。
export CDPATH=/root:/home/:/etc
cd myhome
执行以上命令后，bash会依次将CDPATH中的目录+myhome来进入，最终进入到/home/myhome目录下。【CDPATH中多个路径下都有dir怎么办？】
进入你刚创建的目录
mkdir <dir> && cd $_
不论你创建的目录是否你当前工作目录下的，这个命令都能用。
创建只有一个下划线名字的目录
mkdir ./_
help cd
1 cd: cd [-L|[-P [-e]]] [dir] 2 Change the shell working directory. 3 4 Change the current directory to DIR. The default DIR is the value of the 5 HOME shell variable. 6 7 The variable CDPATH defines the search path for the directory containing 8 DIR. Alternative directory names in CDPATH are separated by a colon (:). 9 A null directory name is the same as the current directory. If DIR begins 10 with a slash (/), then CDPATH is not used. 11 12 If the directory is not found, and the shell option `cdable_vars' is set, 13 the word is assumed to be a variable name. If that variable has a value, 14 its value is used for DIR. 15 16 Options: 17 -L force symbolic links to be followed 18 -P use the physical directory structure without following symbolic 19 links 20 -e if the -P option is supplied, and the current working directory 21 cannot be determined successfully, exit with a non-zero status 22 23 The default is to follow symbolic links, as if `-L' were specified. 24 25 Exit Status: 26 Returns 0 if the directory is changed, and if $PWD is set successfully when 27 -P is used; non-zero otherwise.