對shell中常見參數及判斷命令介紹

2020-07-27 18:46:56

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Shell常用特殊變量

經常會在shell命令中，看到$0, $#, $*, $@, $?, $$這樣的取值，這些代表什么呢？

變量	含義
$0	當前腳本的文件名
$n	傳遞給腳本或函數的參數。n 是一個數字，表示第幾個參數。例如，第一個參數是$1，第二個參數是$2
$#	傳遞給腳本或函數的參數個數
$*	傳遞給腳本或函數的所有參數
$@	傳遞給腳本或函數的所有參數。被雙引號(” “)包含時，與 $* 稍有不同
$?	上個命令的退出狀態，或函數的返回值。成功返回0，失敗返回1
$$	當前Shell進程ID。對于 Shell 腳本，就是這些腳本所在的進程ID

$* 和 $@ 都是將參數一個一個返回

"$*"將所有參數當做一個整體字符串返回 , "$@"將參數一個一個返回

常用判斷參數

在shell命令文件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是什么含義？

下面是幾個常見的參數，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文件是否存在的參數，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 ]這樣判斷傳入參數個數是否為0