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？例如-z是什么意思？
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