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