Normally we use SED command to replace one word to another.

The following example will replace the word old with the word new in the test.txt file

$ echo “old is old” > test.txt

$ cat test.txt
old is old

$ sed -i ‘s/old/new/’ test.txt

$ cat test.txt
new is old

Note : The above sed command only replaced the first occurance. If you want to replace all the occurance in the line. you have use ‘g’

$ sed -i ‘s/old/new/g’ test.txt

Now, we see how to replace in all the files from the current directory.

$ find . -type f -exec sed -i ‘s/old/new/g’ {} \;

Here is the sample script to take backup of the original file before you do the changes with SED command.

#!/bin/bash
for i in *; do
cp $i $i.bak
sed -i ‘s/old/new/g’ $i
done

SED tutorial: http://www.grymoire.com/Unix/Sed.html