UNIX中的$ @和$ *有什么区别？

Question

UNIX中的$@和$*什么区别？ 当在脚本中回显时，它们似乎都产生相同的输出。

Answer 1

请参阅特殊参数下的bash手册页。

   Special Parameters
       The shell treats several parameters specially.   These  parameters  may
       only be referenced; assignment to them is not allowed.
       *      Expands  to  the positional parameters, starting from one.  When
              the expansion occurs within double quotes, it expands to a  sin‐
              gle word with the value of each parameter separated by the first
              character of the IFS special variable.  That is, "$*" is equiva‐
              lent to "$1c$2c...", where c is the first character of the value
              of the IFS variable.  If IFS is unset, the parameters are  sepa‐
              rated  by  spaces.   If  IFS  is null, the parameters are joined
              without intervening separators.
       @      Expands to the positional parameters, starting from  one.   When
              the  expansion  occurs  within  double  quotes,  each  parameter
              expands to a separate word.  That is, "$@" is equivalent to "$1"
              "$2"  ...   If the double-quoted expansion occurs within a word,
              the expansion of the first parameter is joined with  the  begin‐
              ning  part  of  the original word, and the expansion of the last
              parameter is joined with the last part  of  the  original  word.
              When  there  are no positional parameters, "$@" and $@ expand to
              nothing (i.e., they are removed).

Answer 2

一个区别在于它们如何处理输出上的IFS变量。

#!/bin/sh
echo "unquoted asterisk " $*
echo "quoted asterisk $*"
echo "unquoted at " $@
echo "quoted at $@"
IFS="X"
echo "IFS is now $IFS"
echo "unquoted asterisk " $*
echo "quoted asterisk $*"
echo "unquoted at " $@
echo "quoted at $@"

如果你这样运行： ./demo abc def ghi ，你得到这个输出：

unquoted asterisk abc def ghi
quoted asterisk abc def ghi
unquoted at abc def ghi
quoted at abc def ghi
IFS is now X
unquoted asterisk abc def ghi
quoted asterisk abcXdefXghi
unquoted at abc def ghi
quoted at abc def ghi

请注意，（仅）“引用的星号”行在IFS更改为“X”后显示每个“单词”之间的X. 如果IFS的值包含多个字符，则只有第一个字符用于此目的。

此功能也可用于其他阵列：

$ array=(123 456 789)
$ saveIFS=$IFS; IFS="|"
$ echo "${array[*]}"
123|456|789
$ IFS=$saveIFS

Answer 3

使用"$@"而不是$*更安全。 当您使用多字符串作为shell脚本的参数时，只有"$@"将每个引用的参数解释为单独的参数。

如上面的输出所示，如果使用$* ，则shell会对参数进行错误计数。

Answer 4

for i in "$@"
do
    echo $i  # loop $# times
done

for i in "$*"
do
    echo $i  # loop 1 times
done