Shell常用特殊變數

經常會在shell命令中，看到$0, $#, $*, [email protected], $?, $$這樣的取值，這些代表什麼呢？

$* 和 [email protected] 都是將引數一個一個返回
"$*"將所有引數當做一個整體字串返回 , "[email protected]"將引數一個一個返回

常用判斷引數

在shell命令檔案中還經常會看到類似與if [ -z "${SPARK_HOME}" ]; then這樣的判斷語句？是不是也和我一樣很疑惑-z是什麼含義？
下面是幾個常見的引數，供查詢使用：

-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. 

判斷命令

shell中除了有上邊這樣用來判斷檔案是否存在的引數，當然還有判斷兩個數是否相等這樣更常規的命令
例如，if [ $# -gt 0 ]這樣判斷傳入引數個數是否為0