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166 lines
4.0 KiB
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166 lines
4.0 KiB
Markdown
如何在 Bash 中抽取子字符串
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======
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所谓“子字符串”就是出现在其它字符串内的字符串。 比如 “3382” 就是 “this is a 3382 test” 的子字符串。 我们有多种方法可以从中把数字或指定部分字符串抽取出来。
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[][2]
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本文会向你展示在 bash shell 中如何获取或者说查找出子字符串。
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### 在 Bash 中抽取子字符串
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其语法为：
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```shell
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## 格式 ##
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${parameter:offset:length}
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```
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子字符串扩展是 bash 的一项功能。它会扩展成 `parameter` 值中以 `offset` 为开始，长为 `length` 个字符的字符串。 假设， `$u` 定义如下:
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```shell
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## 定义变量 u ##
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u="this is a test"
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```
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那么下面参数的子字符串扩展会抽取出子字符串:
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```shell
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var="${u:10:4}"
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echo "${var}"
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```
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结果为:
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```
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test
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```
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其中这些参数分别表示：
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+ 10 : 偏移位置
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+ 4 : 长度
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### 使用 IFS
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根据 bash 的 man 页说明:
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> [IFS （内部字段分隔符）][3]用于在扩展后进行单词分割，并用内建的 read 命令将行分割为词。默认值是<space><tab><newline>。
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另一种 <ruby>POSIX 就绪<rt>POSIX ready</rt></ruby>的方案如下：
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```shell
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u="this is a test"
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set -- $u
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echo "$1"
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echo "$2"
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echo "$3"
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echo "$4"
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```
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输出为：
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```shell
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this
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is
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a
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test
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```
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下面是一段 bash 代码，用来从 Cloudflare cache 中去除带主页的 url。
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```shell
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#！/bin/bash
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####################################################
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## Author - Vivek Gite {https://www.cyberciti.biz/}
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## Purpose - Purge CF cache
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## License - Under GPL ver 3.x+
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####################################################
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## set me first ##
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zone_id="YOUR_ZONE_ID_HERE"
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api_key="YOUR_API_KEY_HERE"
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email_id="YOUR_EMAIL_ID_HERE"
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## hold data ##
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home_url=""
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amp_url=""
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urls="$@"
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## Show usage
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[ "$urls" == "" ] && { echo "Usage: $0 url1 url2 url3"; exit 1; }
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## Get home page url as we have various sub dirs on domain
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## /tips/
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## /faq/
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get_home_url(){
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local u="$1"
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IFS='/'
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set -- $u
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echo "${1}${IFS}${IFS}${3}${IFS}${4}${IFS}"
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}
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echo
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echo "Purging cache from Cloudflare。.。"
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echo
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for u in $urls
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do
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home_url="$(get_home_url $u)"
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amp_url="${u}amp/"
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curl -X DELETE "https://api.cloudflare.com/client/v4/zones/${zone_id}/purge_cache" \
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-H "X-Auth-Email: ${email_id}" \
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-H "X-Auth-Key: ${api_key}" \
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-H "Content-Type: application/json" \
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--data "{\"files\":[\"${u}\"，\"${amp_url}\"，\"${home_url}\"]}"
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echo
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done
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echo
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```
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它的使用方法为：
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```shell
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~/bin/cf.clear.cache https://www.cyberciti.biz/faq/bash-for-loop/ https://www.cyberciti.biz/tips/linux-security.html
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```
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### 借助 cut 命令
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可以使用 `cut` 命令来将文件中每一行或者变量中的一部分删掉。它的语法为：
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```shell
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u="this is a test"
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echo "$u" | cut -d' ' -f 4
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echo "$u" | cut --delimiter=' ' --fields=4
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##########################################
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## WHERE
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## -d' ' : Use a whitespace as delimiter
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## -f 4 : Select only 4th field
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##########################################
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var="$(cut -d' ' -f 4 <<< $u)"
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echo "${var}"
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```
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想了解更多请阅读 bash 的 man 页：
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```shell
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man bash
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man cut
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```
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另请参见： [Bash String Comparison: Find Out IF a Variable Contains a Substring][1]
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--------------------------------------------------------------------------------
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via: https://www.cyberciti.biz/faq/how-to-extract-substring-in-bash/
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作者：[Vivek Gite][a]
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译者：[lujun9972](https://github.com/lujun9972)
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校对：[wxy](https://github.com/wxy)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]:https://www.cyberciti.biz
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[1]:https://www.cyberciti.biz/faq/bash-find-out-if-variable-contains-substring/
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[2]:https://www.cyberciti.biz/media/new/faq/2017/12/How-to-Extract-substring-in-Bash-Shell-on-Linux-or-Unix.jpg
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[3]:https://bash.cyberciti.biz/guide/$IFS
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