How to extract substring in Bash

Posted on in Categories , , last updated December 6, 2017

I have a bash shell variable called u="this is a test". How can I extract the “test” string and store into a shell variable?

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
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

Where numbers represents,

  • 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 <space><tab><newline>.

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

Here is a bash code that purge urls from Cloudflare cache along with home page:

#!/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="[email protected]"
 
## 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

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

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