一行命令实现url decode

今天看到一个很有趣的 shell 函数，只用一行命令就实现了 url decode 的功能。

urldecode() { : "${*//+/ }"; echo -e "${_//%/\\x}"; }
urldecode https%3A%2F%2Fgoogle.com%2Fsearch%3Fq%3Durldecode%2Bbash

https://google.com/search?q=urldecode+bash

现在我们来分析一下这个函数：

很明显，这个函数其实又两条语句组成：

: "${*//+/ }"

echo -e "${_//%/\\x}"

首先我们看第一条语句， : "${*//+/ }"

其中 : 是一条内置命令，它仅仅对其参数进行扩展，然后什么都不做。

: [arguments]
       No effect; the command does nothing beyond expanding arguments and performing any specified redirections.  The return status is zero.

而 "${*//+/ }" 使用了参数扩展中的模式替换功能来将所有的 + 替换成空格（模式以 / 开头表示全局替换而不仅仅是替换第一个）.

${parameter/pattern/string}
       Pattern substitution.  The pattern is expanded to produce a pattern just as in pathname expansion.  Parameter is  expanded
       and the longest match of pattern against its value is replaced with string.  If pattern begins with /, all matches of pat‐
       tern are replaced with string.  Normally only the first match is replaced.  If pattern begins with #, it must match at the
       beginning of the expanded value of parameter.  If pattern begins with %, it must match at the end of the expanded value of
       parameter.  If string is null, matches of pattern are deleted and the / following pattern may be omitted.  If the  nocase‐
       match  shell  option is enabled, the match is performed without regard to the case of alphabetic characters.  If parameter
       is @ or *, the substitution operation is applied to each positional parameter in turn, and the expansion is the  resultant
       list.   If parameter is an array variable subscripted with @ or *, the substitution operation is applied to each member of
       the array in turn, and the expansion is the resultant list.

那么再让我们分析一下第二条语句 echo -e "${_//%/\\x}"

很明显，这是一条 echo 语句，其中 -e 选项表示对以反斜杠(\) 开头的字符序列进行转义。

echo [-neE] [arg ...]
       Output the args, separated by spaces, followed by a newline.  The return status is 0 unless a write error occurs.   If  -n
       is  specified,  the trailing newline is suppressed.  If the -e option is given, interpretation of the following backslash-
       escaped characters is enabled.  The -E option disables the interpretation of these  escape  characters,  even  on  systems
       where they are interpreted by default.  The xpg_echo shell option may be used to dynamically determine whether or not echo
       expands these escape characters by default.  echo does not interpret -- to mean the end of options.  echo  interprets  the
       following escape sequences:
       \a     alert (bell)
       \b     backspace
       \c     suppress further output
       \e
       \E     an escape character
       \f     form feed
       \n     new line
       \r     carriage return
       \t     horizontal tab
       \v     vertical tab
       \\     backslash
       \0nnn  the eight-bit character whose value is the octal value nnn (zero to three octal digits)
       \xHH   the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
       \uHHHH the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHH (one to four hex digits)
       \UHHHHHHHH
              the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHHHHHH (one to eight hex digits)

这条 echo 命令可以说是本函数的核心命令了，说白了就是将编码过的 URL 中的 %HH 替换成 \xHH 然后借助 echo 命令进行decode

这条 echo 命令中唯一的新内容大概就是 $_, 它会替换成前一条命令的最后参数，也就是所有 + 被替换成空格后的URL

_      At shell startup, set to the absolute pathname used to invoke the shell or shell script being executed as  passed  in  the
             environment  or argument list.  Subsequently, expands to the last argument to the previous command, after expansion.  Also
             set to the full pathname used to invoke each command executed and placed in the  environment  exported  to  that  command.
             When checking mail, this parameter holds the name of the mail file currently being checked.