FILE1 is newer (modification date) than FILE2
FILE1 -ot FILE2
FILE1 is older than FILE2
-b FILE
FILE exists and is block special
-c FILE
FILE exists and is character special
-d FILE
FILE exists and is a directory
-e FILE
FILE exists 文件存在
-f FILE
FILE exists and is a regular file 文件存在且是普通文件
-h FILE
FILE exists and is a symbolic link (same as -L)
-L FILE
FILE exists and is a symbolic link (same as -h)
-G FILE
FILE exists and is owned by the effective group ID
-O FILE
FILE exists and is owned by the effective user ID
-p FILE
FILE exists and is a named pipe
-s FILE
FILE exists and has a size greater than zero
-S FILE
FILE exists and is a socket
-w FILE
FILE exists and is writable
-x FILE
FILE exists and is executable
最常用的简化if语句
&& 如果是“前面”，则“后面”
[ -f /var/run/dhcpd.pid ] && rm /var/run/dhcpd.pid 检查 文件是否存在，如果存在就删掉
|| 如果不是“前面”，则后面
