对shell中常见参数及判断命令介绍

Shell常用特殊变量

经常会在shell命令中，看到$0, $#, $*, $@, $?, $$这样的取值，这些代表什么呢？

变量	含义
$0	当前脚本的文件名
$n	传递给脚本或函数的参数。n 是一个数字，表示第几个参数。例如，第一个参数是$1，第二个参数是$2
$#	传递给脚本或函数的参数个数
$*	传递给脚本或函数的所有参数
$@	传递给脚本或函数的所有参数。被双引号(” “)包含时，与 $* 稍有不同
$?	上个命令的退出状态，或函数的返回值。成功返回0，失败返回1
$$	当前Shell进程ID。对于 Shell 脚本，就是这些脚本所在的进程ID

$* 和 $@ 都是将参数一个一个返回

"$*"将所有参数当做一个整体字符串返回 , "$@"将参数一个一个返回

常用判断参数

在shell命令文件中还经常会看到类似与if [ -z "${SPARK_HOME}" ]; then这样的判断语句？是不是也和我一样很疑惑-z是什么含义？

下面是几个常见的参数，供查询使用：

-a file exists. 
-b file exists and is a block special file. 
-c file exists and is a character special file. 
-d file exists and is a directory. 
-e file exists (just the same as -a). 
-f file exists and is a regular file. 
-g file exists and has its setgid(2) bit set. 
-G file exists and has the same group ID as this process. 
-k file exists and has its sticky bit set. 
-L file exists and is a symbolic link. 
-n string length is not zero. 
-o Named option is set on. 
-O file exists and is owned by the user ID of this process. 
-p file exists and is a first in, first out (FIFO) special file or named pipe. 
-r file exists and is readable by the current process. 
-s file exists and has a size greater than zero. 
-S file exists and is a socket. 
-t file descriptor number fildes is open and associated with a terminal device. 
-u file exists and has its setuid(2) bit set. 
-w file exists and is writable by the current process. 
-x file exists and is executable by the current process. 
-z string length is zero.

判断命令

shell中除了有上边这样用来判断文件是否存在的参数，当然还有判断两个数是否相等这样更常规的命令

例如，if [ $# -gt 0 ]这样判断传入参数个数是否为0

命令	含义
-eq	等于
-ne	不等于
-gt	大于
-lt	小于
ge	大于等于
le	小于等于

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