sed: -i may not be used with stdin on Mac OS X

Question

I am using a bison parser in my project. When I run the following command:

sed -i y.tab.c -e "s/  __attribute__ ((__unused__))$/# ifndef __cplusplus\n  __attribute__ ((__unused__));\n# endif/" 

I get this error sed: -i may not be used with stdin

The command works fine in linux machines. I am using Mac OS X 10.9. It throws an error only on mac os x. I am not sure why. Can anyone help?

Thanks

Answer 1

The problem is that Mac OS X uses the BSD version of sed, which treats the -i option slightly differently. The GNU version used in Linux takes an optional argument with -i: if present, sed makes a backup file whose name consists of the input file plus the argument. Without an argument, sed simply modifies the input file without saving a backup of the original.

In BSD sed, the argument to -i is required. To avoid making a backup, you need to provide a zero-length argument, e.g. sed -i '' y.tab.c ....

Your command, which simply edits y.tab.c with no backup in Linux, would attempt to save a backup file using 'y.tab.c' as an extension. But now, with no other file in the command line, sed thinks you want to edit standard input in-place, something that is not allowed.

Answer 2

From the sed manpage:

-i extension Edit files in-place, saving backups with the specified extension. If a zero-length extension is given, no backup will be saved. It is not recommended to give a zero-length extension when in-place editing files, as you risk corruption or partial content in situ- ations where disk space is exhausted, etc.

The solution is to send a zero-length extension like this:

sed -i '' 's/apples/oranges/' file.txt