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
[ -f /usr/sbin/dhcpd ] || exit 0 检验文件是否存在，如果存在就退出
示例：用简化 if 和$1,$2,$3来检测参数，不合理就调用help
[ -z "$1" ] && help 如果第一个参数不存在（-z 字符串长度为0 ）
[ "$1" = "-h" ] && help 如果第一个参数是-h,就显示help
[macg@machome ~]$ man test
[(1) User Commands [(1)
SYNOPSIS
test EXPRESSION
[ EXPRESSION ]
[-n] STRING
the length of STRING is nonzero -n和直接$str都是非0条件
-z STRING
the length of STRING is zero
STRING1 = STRING2
the strings are equal
STRING1 != STRING2
the strings are not equal
INTEGER1 -eq INTEGER2
INTEGER1 is equal to INTEGER2
INTEGER1 -ge INTEGER2
INTEGER1 is greater than or equal to INTEGER2
INTEGER1 -gt INTEGER2
INTEGER1 is greater than INTEGER2
INTEGER1 -le INTEGER2
INTEGER1 is less than or equal to INTEGER2
INTEGER1 -lt INTEGER2
INTEGER1 is less than INTEGER2
INTEGER1 -ne INTEGER2
INTEGER1 is not equal to INTEGER2
FILE1 -nt FILE2
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