Question

I am using sed in a script to do a replace and I want to have the replaced file overwrite the file. Normally I think that you would use this:

% sed -i 's/cat/dog/' manipulate
sed: illegal option -- i

However as you can see my sed does not have that command.

I tried this:

% sed 's/cat/dog/' manipulate > manipulate

But this just turns manipulate into an empty file (makes sense).

This works:

% sed 's/cat/dog/' manipulate > tmp; mv tmp manipulate

But I was wondering if there was a standard way to redirect output into the same file that input was taken from.

Answer 1

I commonly use the 3rd way, but with an important change:

sed 's/cat/dog/' manipulate > tmp && mv tmp manipulate

I.e. change ; to && so the move only happens if sed is successful; otherwise you’ll lose your original file as soon as you make a typo in your sed syntax.

Note! For those reading the title and missing the OP’s constraint "my sed doesn’t support -i": For most people, sed will support -i, so the best way to do this is:

sed -i 's/cat/dog/' manipulate