Grep SED

The grep -il statement finds all instances of Hello in files ending in txt (i = ignore case, , l = only list the filename). The file names are passed to sed, which runs a regular expression to change all instances of Hello to Goodbye. Since sed doesn't overwrite a file, I redirected the output to a temp file and then renamed it back to the original file name.

#!/bin/sh

for file in $(grep -il "Hello" *.txt)
do
sed -e "s/Hello/Goodbye/ig" $file > /tmp/tempfile.tmp
mv /tmp/tempfile.tmp $file
done