A.sed command is designed for this kind of work i.e. replace strings.

sed replace word / string syntax

sed -i 's/old-word/new-word/g' *.txt

GNU sed command can edit files in place (makes backup if extension supplied) using -i option. If you are using old or UNIX sed command try the following syntax:

sed 's/old/new/g' input.txt > output.txt

I recommend using old syntax along with for loop:

#!/bin/bash
OLD=”xyz”
NEW=”abc”
DPATH=”/home/you/foo/*.txt”
BPATH=”/home/you/bakup/foo”
TFILE=”/tmp/out.tmp.$$”
[ ! -d $BPATH ] && mkdir -p $BPATH || :
for f in $DPATH
do
if [ -f $f -a -r $f ]; then
/bin/cp -f $f $BPATH
sed “s/$OLD/$NEW/g” “$f” > $TFILE && mv $TFILE “$f”
else
echo “Error: Cannot read $f”
fi
done
/bin/rm $TFILE

perl -pie trick

perl -pie 's/old-word/new-word/g' input > output

A. GNU sed and other version does support a case-insensitive search using I flag after /regex/.

UNIX / Linux: sed Case Insensitive Search

To perform a case-insensitive search, enter:

cat file.txt | sed -e ‘s/find-word/replace-word/gI’
cat file.txt | sed -e ‘s/find-word/replace-word/gI’ > output.txt
sed ‘s/find-word/replace-word/gI’ input.txt > output.txt

If you are using older sed version try,

sed ‘s/[wW][oO][rR][dD]/replace-word/g’ input.txt > output.txt

It is easy to match first few characters, for example match both Linux and linux word:

sed ‘s/[Ll]inux/Unix/g’ input.txt > output.txt