The man page for sed says the following:
sed [-Ealnru] command [-I extension] [-i extension] [file ...]
Based on this, I believe the following should work:
sed -E '/^A/d' -i '' tmp.txt
where cat tmp.txt gives:
A
B
But the output is:
sed: -i: No such file or directory
sed: : No such file or directory
B
And tmp.txt has remained unchanged.
For the above command to work, I should instead do the following:
sed -E -i '' '/^A/d' tmp.txt
My question is, why? What I tried first should have worked according to the man page. The order of options was just as described there. Yet it did not work.
I'm on a Mac running macOS 12, which is based on FreeBSD.
POSIX utility syntax guidelines specify that options -- including options with option-arguments -- should come before positional arguments. While a GNUish command-line parser will recognize options in later positions so long as there isn't an end-of-options sigil preceding them, this behavior (as you've observed) does not reliably extend to non-GNU implementations.
This means that the documentation showing -i coming after the expression to run was faulty. Reorder your arguments as follows:
sed -E -i '' '/^A/d' tmp.txt