转自：http://codingstandards.iteye.com/blog/831504
在脚本中type可用于检查命令或函数是否存在，存在返回0，表示成功；不存在返回正值，表示不成功。
$ type foo >/dev/null 2>&1 || { echo >&2 "I require foo but it's not installed. Aborting."; exit 1; }
用途说明
type命令用来显示指定命令的类型。一个命令的类型可以是如下之一
- alias 别名
- keyword 关键字，Shell保留字
- function 函数，Shell函数
- builtin 内建命令，Shell内建命令
- file 文件，磁盘文件，外部命令
- unfound 没有找到
它是Linux系统的一种自省机制，知道了是那种类型，我们就可以针对性的获取帮助。比如内建命令可以用help命令来获取帮助，外部命令用man或者info来获取帮助。
常用参数
type命令的基本使用方式就是直接跟上命令名字。
type -a可以显示所有可能的类型，比如有些命令如pwd是shell内建命令，也可以是外部命令。
type -p只返回外部命令的信息，相当于which命令。
type -f只返回shell函数的信息。
type -t 只返回指定类型的信息。
使用示例
示例一 type自己是什么类型的命令
[root@new55 ~]# type -a type
type is a shell builtin
[root@new55 ~]# help type
type: type [-afptP] name [name ...]
For each NAME, indicate how it would be interpreted if used as a
command name.
If the -t option is used, `type' outputs a single word which is one of
`alias', `keyword', `function', `builtin', `file' or `', if NAME is an
alias, shell reserved word, shell function, shell builtin, disk file,
or unfound, respectively.
If the -p flag is used, `type' either returns the name of the disk
file that would be executed, or nothing if `type -t NAME' would not
return `file'.
If the -a flag is used, `type' displays all of the places that contain
an executable named `file'. This includes aliases, builtins, and
functions, if and only if the -p flag is not also used.
The -f flag suppresses shell function lookup.
The -P flag forces a PATH search for each NAME, even if it is an alias,
builtin, or function, and returns the name of the disk file that would
be executed.
typeset: typeset [-afFirtx] [-p] name[=value] ...
Obsolete. See `declare'.
[root@new55 ~]#
示例二 常见命令的类型
[root@new55 ~]# type -a cd
cd is a shell builtin
[root@new55 ~]# type -a pwd
pwd is a shell builtin
pwd is /bin/pwd
[root@new55 ~]# type -a time
time is a shell keyword
time is /usr/bin/time
[root@new55 ~]# type -a date
date is /bin/date
[root@new55 ~]# type -a which
which is aliased to `alias | /usr/bin/which --tty-only --read-alias --show-dot --show-tilde'
which is /usr/bin/which
[root@new55 ~]# type -a whereis
whereis is /usr/bin/whereis
[root@new55 ~]# type -a whatis
whatis is /usr/bin/whatis
[root@new55 ~]# type -a function
function is a shell keyword
[root@new55 ~]# type -a ls
ls is aliased to `ls --color=tty'
ls is /bin/ls
[root@new55 ~]# type -a ll
ll is aliased to `ls -l --color=tty'
[root@new55 ~]# type -a echo
echo is a shell builtin
echo is /bin/echo
[root@new55 ~]# type -a bulitin
-bash: type: bulitin: not found
[root@new55 ~]# type -a builtin
builtin is a shell builtin
[root@new55 ~]# type -a keyword
-bash: type: keyword: not found
[root@new55 ~]# type -a command
command is a shell builtin
[root@new55 ~]# type -a alias
alias is a shell builtin
[root@new55 ~]# type -a grep
grep is /bin/grep