find and replace string in a file

Question

I'm trying to find and replace a string in a folder of files.

Could someone possibly help me?

My script is as follows:

#!/bin/bash
OLD="This is a"
NEW="I am a"
DPATH="/home/user/test/*.txt"
BPATH="/home/user/test/backup/foo"
[ ! -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"
   else
    echo "Error: Cannot read $f"
  fi
done

Now this seems to find the string 'This is a' and replaces with 'I am a', but this only prints to screen.

I need it to replace in the files themselves.

Thanks

Answer 1

Use the -i option of sed to make the changes in place:

sed -i "s/$OLD/$NEW/g" "$f"
    ^^

Answer 2

The output goes to screen (stdout) because of the following:

sed "s/$OLD/$NEW/g" "$f"

Try redirecting to a file (the following redirects to a new files and then renames it to overwrite the original file):

sed "s/$OLD/$NEW/g" "$f" > "$f.new" && mv "$f.new" "$f"