sed in-place editing (-i) difference on Mac OS X: “undefined label” errors

I ran into this problem about a month ago with a bash script that I was writing on my MacBook Pro.  For some reason, sed in place editing with ‘sed -i‘ just wouldn’t work, no matter what I did, and I couldn’t find any solution online.

When running something like

sed -i 's/before/after/' test.txt

you get the error message sed: 1: "test.txt": undefined label 'est.txt'.

Eventually, it turned out to be because sed’s ‘-i‘ option takes a parameter to indicate what extension to add to the file name when making a backup.  For example,

sed -i '.bak' 's/before/after/' test.txt

leaves you with two files, ‘test.txt‘ and ‘test.txt.bak‘, where ‘test.txt.bak‘ is the original version.  Unlike Ubuntu and other linux versions, on OS X, this extension parameter is required.  If you really know what you’re doing and don’t want a backup made, you need to provide an empty string ”.

The correct way to run this command on OSX is

sed -i '' 's/before/after/' test.txt