9

我有一部分 bash 脚本正在获取不带扩展名的文件名，但我试图了解这里到底发生了什么。“%%”是干什么用的？有人可以详细说明 bash 在幕后做什么吗？如何在一般基础上使用这种技术？

#!/bin/bash

for src in *.tif
    do
    txt=${src%%.*}
    tesseract ${src} ${txt}
    done
4

5 回答 5

15

它摆脱了文件扩展名（这里: .tif），示例：

$ for A in test.py test.sh test.xml test.xsl; do echo "$A: ${A%%.*}"; done
test.py: test
test.sh: test
test.xml: test
test.xsl: test

来自 bash 手册：

   ${parameter%%word}
          The word is expanded to produce a pattern just as in pathname expansion.  If the
          pattern matches a trailing portion of the expanded value of parameter, then  the
          result  of  the  expansion  is the expanded value of parameter with the shortest
          matching pattern (the ``%'' case) or the longest matching  pattern  (the  ``%%''
          case) deleted.  If parameter is @ or *, the pattern removal 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 pattern removal
          operation is applied to each member of the array in turn, and the  expansion  is
          the resultant list.
于 2009-02-11T20:08:37.070 回答
4

这是 bash 手册页的输出

 ${parameter%%word}
          The word is expanded to produce a pattern just  as  in  pathname
          expansion.   If  the  pattern  matches a trailing portion of the
          expanded value of parameter, then the result of the expansion is
          the  expanded value of parameter with the shortest matching pat-
          tern (the ``%'' case)  or  the  longest  matching  pattern  (the
          ``%%''  case)  deleted.   If  parameter  is  @ or *, the pattern
          removal 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  pattern  removal
          operation  is  applied  to each member of the array in turn, and
          the expansion is the resultant list.
于 2009-02-11T20:10:07.330 回答
3

显然 bash 有几个“参数扩展”工具，其中包括：

只需替换值...

${parameter}

扩展为子字符串...

${parameter:offset}
${parameter:offset:length}

代入参数值的长度...

${#parameter}

扩展参数开头的匹配...

${parameter#word}
${parameter##word}

扩展参数末尾的匹配...

${parameter%word}
${parameter%%word}

扩展参数以查找和替换字符串...

${parameter/pattern/string}

这些是我对我认为从手册页的这一部分中理解的部分的解释。如果我错过了重要的事情，请告诉我。

于 2009-02-11T20:50:17.657 回答
1

查看 bash 手册页中的“参数扩展”。该语法扩展了 $src 变量，从中删除了与 .* 模式匹配的内容。

于 2009-02-11T20:06:27.250 回答
1

这是一个字符串删除操作，格式如下：${str%%substr}

其中 str 是您正在操作的字符串，而 substr 是要匹配的模式。它在 str 中查找 substr 的最长匹配，并从那时起删除所有内容。

于 2009-02-11T20:10:41.853 回答