It's equivalent of `export FOO='bar'; ./program` , which is not same as `FOO='bar' ./program`, which, in turn, is equivalent to `OLD_FOO="$FOO"; export FOO='bar

It's equivalent of `export FOO='bar'; ./program` , which is not same as `FOO='bar' ./program`, which, in turn, is equivalent to `OLD_FOO="$FOO"; export FOO='bar'; ./program; export FOO="$OLD_FOO";` .