How to extract substring in Bash – nixCraft

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}"
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="$@"

## 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

 

Source: How to extract substring in Bash – nixCraft

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