选题: How to extract substring in Bash

sources/tech/20171206 How to extract substring in Bash.md

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+translating by lujun9972
+How to extract substring in Bash
+======
+A substring is nothing but a string is a string that occurs “in”. For example “3382” is a substring of “this is a 3382 test”. One can extract the digits or given string using various methods.
+
+ [![How to Extract substring in Bash Shell on Linux or Unix](https://www.cyberciti.biz/media/new/faq/2017/12/How-to-Extract-substring-in-Bash-Shell-on-Linux-or-Unix.jpg)][2] 
+
+This quick tutorial shows how to obtain or finds substring when using bash shell.
+
+### Extract substring in Bash
+
+The syntax is: ## syntax ## ${parameter:offset:length} The substring expansion is a bash feature. It expands to up to length characters of the value of parameter starting at the character specified by offset. For example, $u defined as follows:
+
+| 
+```
+## define var named u ##
+u="this is a test"
+```
+ |
+
+The following substring parameter expansion performs substring extraction:
+
+| 
+```
+var="${u:10:4}"
+echo "${var}"
+```
+ |
+
+Sample outputs:
+
+```
+test
+```
+
+*   10 : The offset
+
+*   4 : The length
+
+### Using IFS
+
+From the bash man page:
+
+> The Internal Field Separator that is used for word splitting after expansion and to split lines into words with the read builtin command. The default value is .
+
+Another POSIX ready solution is as follows:
+
+| 
+```
+u="this is a test"
+set -- $u
+echo "$1"
+echo "$2"
+echo "$3"
+echo "$4"
+```
+ |
+
+Sample outputs:
+
+```
+this
+is
+a
+test
+```
+
+| 
+```
+#!/bin/bash
+####################################################
+## Author - Vivek Gite {https://www.cyberciti.biz/}
+## Purpose - Purge CF cache
+## License - Under GPL ver 3.x+
+####################################################
+## set me first ##
+zone_id="YOUR_ZONE_ID_HERE"
+api_key="YOUR_API_KEY_HERE"
+email_id="YOUR_EMAIL_ID_HERE"
+
+## hold data ##
+home_url=""
+amp_url=""
+urls="$@"
+
+## Show usage 
+[ "$urls" == "" ] && { echo "Usage: $0 url1 url2 url3"; exit 1; }
+
+## Get home page url as we have various sub dirs on domain
+## /tips/
+## /faq/
+
+get_home_url(){
+	local u="$1"
+	IFS='/'
+	set -- $u
+	echo "${1}${IFS}${IFS}${3}${IFS}${4}${IFS}"
+}
+
+echo
+echo "Purging cache from Cloudflare..."
+echo
+for u in $urls
+do
+     home_url="$(get_home_url $u)"
+     amp_url="${u}amp/"
+     curl -X DELETE "https://api.cloudflare.com/client/v4/zones/${zone_id}/purge_cache" \
+          -H "X-Auth-Email: ${email_id}" \
+          -H "X-Auth-Key: ${api_key}" \
+          -H "Content-Type: application/json" \
+          --data "{\"files\":[\"${u}\",\"${amp_url}\",\"${home_url}\"]}"
+     echo
+done
+echo
+```
+ |
+
+I can run it as follows: ~/bin/cf.clear.cache https://www.cyberciti.biz/faq/bash-for-loop/ https://www.cyberciti.biz/tips/linux-security.html
+
+### Say hello to cut command
+
+One can remove sections from each line of file or variable using the cut command. The syntax is:
+
+| 
+```
+u="this is a test"
+echo "$u" | cut -d' ' -f 4
+echo "$u" | cut --delimiter=' ' --fields=4
+##########################################
+## WHERE
+##   -d' ' : Use a whitespace as delimiter
+##   -f 4  : Select only 4th field
+##########################################
+var="$(cut -d' ' -f 4 <<< $u)"
+echo "${var}"
+```
+ |
+
+For more info read bash man page: man bash man cut See also: [Bash String Comparison: Find Out IF a Variable Contains a Substring][1]
+
+--------------------------------------------------------------------------------
+
+via: https://www.cyberciti.biz/faq/how-to-extract-substring-in-bash/
+
+作者：[Vivek Gite][a]
+译者：[lujun9972](https://github.com/lujun9972)
+校对：[校对者ID](https://github.com/校对者ID)
+
+本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
+
+[a]:https://www.cyberciti.biz
+[1]:https://www.cyberciti.biz/faq/bash-find-out-if-variable-contains-substring/
+[2]:https://www.cyberciti.biz/media/new/faq/2017/12/How-to-Extract-substring-in-Bash-Shell-on-Linux-or-Unix.jpg