Why does "echo 123 > a.txt b.txt" only create one file?

Question

When I execute echo 123 > a.txt b.txt I expect to see 123 written to two files, a.txt and b.txt. But the actual result is that only one file is created. The content 123 b.txt is written in the file called a.txt. Why does this happen?

Answer 1

Two things are needed to make sense of this:

1. The redirection operator > only applies to a single file name. Anything after that file name is not part of the redirection; it's an additional argument to the echo command.
2. A lesser known fact is that redirections can be written anywhere in a command and have the same effect.

Putting these facts together, it means these commands are all equivalent:

echo 123 b.txt >a.txt   # traditional order
echo 123 >a.txt b.txt   # your command
echo >a.txt 123 b.txt
>a.txt echo 123 b.txt

All of them run the command echo 123 b.txt with output redirected to a.txt.

(I've condensed > a.txt to >a.txt to make it easier to see what > is binding to.)