sed in bash not working

Question

I am trying to use the stream editor sed in bash to replace a string in a file to a new string that I entered, I have this thus far:

echo "name to change"
read updatenaamold
echo "enter new name:"
read updatenaamnew
sed -i 's/$updatenaamold/$updatenaamnew/g' $naam_dir
echo "name changed"

This should work! But it doesn't somehow, in the file that I want to edit there is a name called "bryan" I want to edit it to "bryan name". I've put the old name in a variable that i need to enter obviously, and the new name in a variable aswell. Can someone point out to me why it doesn't work?

Answer 1

Just use double quote instead of single quote. (Single quotes don't allow variable expansion).

echo "name to change"
read updatenaamold
echo "enter new name:"
read updatenaamnew
sed -i "s/$updatenaamold/$updatenaamnew/g" $naam_dir
echo "name changed"

Note 1: If $naam_dir is a directory it won't work either. Afaik sed works on a file-by-file basis.

Note 2: If user enters special characters (like: / ^ $ [] ) things won't work as expected, because it will confuse sed. Workaround (not sure if this would be the best way):

updatenaamold=$( POSIXLY_CORRECT=1 /bin/echo "$updatenaamold" | sed 's/[^a-zA-Z0-9]//g' )
updatenaamnew=$( POSIXLY_CORRECT=1 /bin/echo "$updatenaamnew" | sed 's/[^a-zA-Z0-9]//g' )

Add it before sed. It will remove characters that are other than letters and numbers (no spaces, no international chars, etc). Another approach would be aborting when found weird chars (and prevent accidental errors).

Answer 2

Since you're using bash I'd suggest using checking out bash string manipulations.