whatis 用於查詢一個命令執行什麼功能，並將查詢結果列印到終端上
which 檢視可執行檔案的位置
whereis 檢視檔案的位置
man Linux提供了豐富的幫助手冊，當你需要檢視某個命令的引數時不必到處上網查詢，只要man一下即可
locate 配 合數據庫檢視檔案位置
find 實際搜尋硬碟查詢檔名稱
type 用來區分某個命令是否是shell自帶的

1. whatis
   whatis 用於查詢一個命令執行什麼功能，並將查詢結果列印到終端上

whatis whatis
whatis (1)           - display one-line manual page descr..
 
.

whatis which
which (1)            - locate a command

whatis whereis
whereis (1)          - locate the binary, source, and man...

whatis man
man (1)              - an interface to the on-line refere...
man (7)              - macros to format man pages

whatis locate
locate (1)           - find files by name

whatis find
 

find (1)             - search for files in a directory hi...

whatis type
type: nothing appropriate.

whatis who
who (1)              - show who is logged on

whatis top
top (1)              - display Linux processes

2. which
   which命令是查詢這個命令是否存在系統中,以及命令存放的位置在哪兒.

which whatis
/usr/bin/whatis

which
 
 which
/usr/bin/which

which whereis
/usr/bin/whereis

which man
/usr/bin/man

which locate
/usr/bin/locate

which find
/usr/bin/find
which type

which ls
/bin/ls

which cat
/bin/cat

which top
/usr/bin/top

which python
/usr/bin/python

which python3
/usr/bin/python3

which pip
/usr/bin/pip

which mysql
/usr/bin/mysql

which mysqld
/usr/sbin/mysqld

which vim
/usr/bin/vim

which sqlite

2. whereis
   whereis命令只能用於搜尋程式名，而且只搜尋二進位制檔案（引數-b）、man說明檔案（引數-m）和原始碼檔案（引數-s）。如果省略引數，則返回所有資訊。

whereis whatis
whatis: /usr/bin/whatis /usr/share/man/man1/whatis.1.gz

whereis which
which: /usr/bin/which /bin/which /usr/share/man/man1/which.1.gz

whereis whereis
whereis: /usr/bin/whereis /usr/share/man/man1/whereis.1.gz

whereis man
man: /usr/bin/man /usr/local/man /usr/share/man /usr/share/man/man1/man.1.gz /usr/share/man/man7/man.7.gz

whereis locate
locate: /usr/bin/locate /usr/share/man/man1/locate.1.gz

whereis find
find: /usr/bin/find /usr/share/man/man1/find.1.gz /usr/share/info/find.info.gz

whereis type
type:

whereis vim
vim: /usr/bin/vim.basic /usr/bin/vim /usr/bin/vim.tiny /etc/vim /usr/share/vim /usr/share/man/man1/vim.1.gz

whereis mysql
mysql: /usr/bin/mysql /usr/lib/mysql /etc/mysql /usr/include/mysql /usr/share/mysql /usr/share/man/man1/mysql.1.gz

whereis mysqld -b
mysqld: /usr/sbin/mysqld /usr/share/man/man8/mysqld.8.gz

whereis mysqld 
mysqld: /usr/sbin/mysqld /usr/share/man/man8/mysqld.8.gz

whereis python
python: /usr/bin/python /usr/bin/python3.5m /usr/bin/python3.5 /usr/bin/python2.7 /usr/bin/python2.7-config /usr/lib/python3.5 /usr/lib/python2.7 /etc/python /etc/python3.5 /etc/python2.7 /usr/local/lib/python3.5 /usr/local/lib/python2.7 /usr/include/python3.5m /usr/include/python2.7 /usr/share/python /usr/share/man/man1/python.1.gz

whereis python2
python2: /usr/bin/python2 /usr/bin/python2.7 /usr/bin/python2.7-config /usr/lib/python2.7 /etc/python2.7 /usr/local/lib/python2.7 /usr/include/python2.7 /usr/share/man/man1/python2.1.gz

whereis python3
python3: /usr/bin/python3.5m /usr/bin/python3 /usr/bin/python3.5 /usr/lib/python3 /usr/lib/python3.5 /etc/python3 /etc/python3.5 /usr/local/lib/python3.5 /usr/include/python3.5m /usr/share/python3 /usr/share/man/man1/python3.1.gz

3. man
   man 是 Linux提供了豐富的幫助手冊，當你需要檢視某個命令的引數時不必到處上

man whatis
output a lot
man which
output a lot
man whereis
output a lot
man man
output a lot
man locate
output a lot
man find
output a lot
man type
No manual entry for type

4. locate
   　　locate命令不是實時查詢，所以查詢的結果不精確，但查詢速度很快。因為它查詢的不是目錄，而是一個數據庫（/var/lib/locatedb），這個資料庫中含有本地所有檔案資訊。Linux系統自動建立這個資料庫，並且每天自動更新一次，所以使用locate命令查不到最新變動過的檔案。為了避免這種情況，可以在使用locate之前，先使用updatedb命令，手動更新資料庫。

sudo updatedb

locate whatis
output a lot

locate which
output a lot

locate whereis
/usr/bin/whereis
/usr/share/bash-completion/completions/whereis
/usr/share/man/man1/whereis.1.gz

locate man
output a lot

locate locate
output a lot

locate find
output a lot

locate type
output a lot

locate passwd
will output a lot

locate -n 5 passwd
/etc/passwd
/etc/passwd-
/etc/cron.daily/passwd
/etc/init/passwd.conf
/etc/pam.d/chpasswd

locate vim
output a lot

5. find
   　　find是最常用和最強大的查詢命令。它能做到實時查詢，精確查詢，但速度慢。
   　　find的使用格式如下：
   　　#find [指定目錄] [指定條件] [指定動作]
   　　指定目錄：是指所要搜尋的目錄和其子目錄。如果不指定，則預設為當前目錄。
   　　指定條件：是指所要搜尋的檔案的特點。
   　　指定動作：是指對搜尋的結果如何處理。
   　　常見選項:
   　　-name 根據檔名稱查詢（嚴格區分大小寫，如果檔名中間有空格，則必須把檔名用雙引號引起來）
   　　-iname 根據檔名查詢（不區分大小寫）
   　　-regex “PATTERN” 正則表示式匹配
   　　-user 根據檔案屬主進行查詢
   　　-group 根據檔案屬組進行查詢
   　　-uid 根據檔案的uid查詢
   　　-gid 根據使用者的gid查詢
   　　-nouser 檢視所有的沒有屬主的檔案
   　　-nogroup 檢視所有的沒有屬組的檔案

find whatis
find: ‘whatis’: No such file or directory
find which
find: ‘which’: No such file or directory
find whereis
find: ‘whereis’: No such file or directory
find man
find: ‘man’: No such file or directory
find locate
find: ‘locate’: No such file or directory
find find
find: ‘find’: No such file or directory
find type
find: ‘type’: No such file or directory

6. type
   　　type命令用來區分某個命令到底是由shell自帶的，還是由shell外部的獨立二進位制檔案提供的。如果一個命令是外部命令，那麼使用-p引數，會顯示該命令的路徑，相當於which命令。

type whatis
whatis is hashed (/usr/bin/whatis)

type which
which is hashed (/usr/bin/which)

type whereis
whereis is hashed (/usr/bin/whereis)

type man
man is hashed (/usr/bin/man)

type locate
locate is hashed (/usr/bin/locate)

type find
find is hashed (/usr/bin/find)

type type
type is a shell builtin

type cd
cd is a shell builtin

type grep
grep is aliased to `grep --color=auto'

type cat
cat is /bin/cat

type vim
vim is hashed (/usr/bin/vim)

type -p vim
/usr/bin/vim

type python
python is /usr/bin/python