Shell常用特殊變量
經(jīng)常會在shell命令中，看到$0, $#, $*, $@, $?, $$這樣的取值，這些代表什么呢？
| 變量 | 含義 |
|---|---|
| $0 | 當(dāng)前腳本的文件名 |
| $n | 傳遞給腳本或函數(shù)的參數(shù)。n 是一個(gè)數(shù)字，表示第幾個(gè)參數(shù)。例如，第一個(gè)參數(shù)是$1，第二個(gè)參數(shù)是$2 |
| $# | 傳遞給腳本或函數(shù)的參數(shù)個(gè)數(shù) |
| $* | 傳遞給腳本或函數(shù)的所有參數(shù) |
| $@ | 傳遞給腳本或函數(shù)的所有參數(shù)。被雙引號(” “)包含時(shí)，與 $* 稍有不同 |
| $? | 上個(gè)命令的退出狀態(tài)，或函數(shù)的返回值。成功返回0，失敗返回1 |
| $$ | 當(dāng)前Shell進(jìn)程ID。對于 Shell 腳本，就是這些腳本所在的進(jìn)程ID |
$* 和 $@ 都是將參數(shù)一個(gè)一個(gè)返回
"$*"將所有參數(shù)當(dāng)做一個(gè)整體字符串返回 , "$@"將參數(shù)一個(gè)一個(gè)返回
常用判斷參數(shù)
在shell命令文件中還經(jīng)常會看到類似與if [ -z "${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(gè)常見的參數(shù)，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(shù)，當(dāng)然還有判斷兩個(gè)數(shù)是否相等這樣更常規(guī)的命令
例如，if [ $# -gt 0 ]這樣判斷傳入?yún)?shù)個(gè)數(shù)是否為0
| 命令 | 含義 |
|---|---|
| -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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