which： show the full path of (shell) commands，it prints to stdout the full path of the executables,it does this by searching for an executable or script in the directories listed in the environment variable PATH。
查找命令的完整文件路径（查找可执行文件的完整文件路径，在变量PATH配置的目录中寻找）
-a(--all)，print all matching executables in PATH,not just the first.
whereis：locate the binary,source,and manual page files for a command
-b:search only for binaries
-m:search only for manual sections
-s:search only for sources
-u:search for unusual entries. 不在b(binary)，m(manual)，s(source)下的特殊文件
locate
find：search for files in a directory hierarchy
root用户操作
which ifconfig
/sbin/ifconfig
su myuser
which ifconfig
which:no ifconfig in (/root/bin:/usr/local/bin:/usr/bin:/bin:/usr/bin/X11:/usr/X11R6/bin:/usr/games:/usr/lib/mit/bin:/usr/lib/mit/sbin)
which touch
/usr/bin/touch
which -a touch
/usr/bin/touch
/bin/touch
/usr/bin/X11/touch
which touch/which -a touch：列出第一个/列出所有同名的
root用户操作
whereis ifconfig
ifconfig: /sbin/ifconfig /usr/share/man/man8/ifconfig.8.gz
su myuser
whereis ifconfig
ifconfig:/sbin/ifconfig /usr/share/man/man8/ifocnfig.8.gz
which找不到的，whereis找到了
whereis passwd
passwd:/usr/bin/passwd /etc/passwd.old /etc/passwd /usr/bin/X11/passwd /usr/share/man/man1/passwd.1.gz /usr/share/man/man1/passwd.lssl.gz /usr/share/man/man5/passwd.5.gz
whereis -m passwd
passwd:/usr/share/man/man1/passwd.1.gz /usr/share/man/man1/passwd.lssl.gz /usr/share/man/man5/passwd.5.gz
whereis -b passwd
passwd:/usr/bin/passwd /etc/passwd.old /etc/passwd /usr/bin/X11/passwd
which/whereis/locate 搜索数据库文件，find搜索硬盘，所以
which/whereis/locate 搜索速度块，find搜索速度慢。