File Rename problem

how to achieve the following functions in the shell
to a folder of all files that match the rule according to my rules required to modify the file name
I am using ls and grep access and filter files , and then use sed to modify the original file name , but how to achieve modify the file name ah , I mean how to get to the front of the parameters passed to rename or mv, the key is the corresponding , for example, I put 123.txt modified to abc_123.txt, how do I put in a batch rename 123.txt and abc_123.txt pass or mv?
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for i in `ls`
do
mv $ i `echo $ i | sed 's / ^ / abc_ /'`
done
like this do
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#!/bin/sh
  
  ls | grep ".txt" | while read oldname;
  do
  newname=`echo "$oldname" | sed -e 's/^\([^\.]*\)\.txt$/abc_\1.txt/'`
  echo "mv $oldname $newname"
  done
  

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you in two steps , one step removed original file name , a step of removing the modified file.
then put two parameters , the direct call mv.
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I just want to ask , how to take ?
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good idea , taught , thank you !
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learning , thank you !
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good , learning .