关于双引号的困惑

long1225

脚本A

#!/bin/bash
if [ "${x+X}" = X ]  #if x is set
then
        if [ -n $x ] #if x is not empty
        then
        printf " \$x = %s\n" "$x"
        else
        printf " \$x is set but empty \n"
        fi
else
printf " \$x is not set.\n"
fi


脚本B

#!/bin/bash
if [ "${x+X}" = X ]  #if x is set
then
        if [ -n "$x" ] #if x is not empty
        then
        printf " \$x = %s\n" "$x"
        else
        printf " \$x is set but empty \n"
        fi
else
printf " \$x is not set.\n"
fi


红色部分的不同，造成了脚本的运行错误，不加引号的话，即使变量x为空，打印出来却是$x=     但是加上引号后，当x为空时，可以识别出来为空



但是我想不明白在这里$x 和"$x" 有什么区别，求教大侠！！！

netocool

试一下这样:

'"$x"'

long1225

'"$x"' 的的结果和$x一样，无法识别空变量

blackold

因为word splitting

long1225

The shell scans the results of parameter expansion, command substitution, and arithmetic expansion that did not occur within double quotes for word splitting.

The shell treats each character of $IFS as a delimiter, and splits the results of the other expansions into words on these characters. If IFS is unset, or its value is exactly "'<space><tab><newline>'", the default, then any sequence of IFS characters serves to delimit words. If IFS has a value other than the default, then sequences of the whitespace characters "space" and "Tab" are ignored at the beginning and end of the word, as long as the whitespace character is in the value of IFS (an IFS whitespace character). Any character in IFS that is not IFS whitespace, along with any adjacent IF whitespace characters, delimits a field. A sequence of IFS whitespace characters is also treated as a delimiter. If the value of IFS is null, no word splitting occurs.

Explicit null arguments ("""" or "''") are retained. Unquoted implicit null arguments, resulting from the expansion of parameters that have no values, are removed. If a parameter with no value is expanded within double quotes, a null argument results and is retained.


Expansion and word splitting
  If no expansion occurs, no splitting is performed.