[9fans] echo -n and zero-length write(2)

[cherry <lunaria21@gmail.com>]

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Hello Fans,

When echo(1) is invoked with -n and no or empty arguments, it calls
write(2) with zero length. When writing to a pipe, the other end of the
pipe read()'s zero byte, which causes it thinks it's got to the end and
close the pipe, and commands after the "echo -n" would fail to write. I
start thinking whether this is the desired behavior. On one hand, it is an
easy way to do a zero-length write() through echo -n, also it is consistent
for echo(1): to do a write(2). On the other hand, it makes rc script
behaves differently if sending to a pipe, and probably some people would
expect empty echo -n is just a no-op. Which should be better?

The context is, as posted on the werc list, it's template.awk generates rc
script with echo -n, and the output of the script is sent to a pipe. When
the argument is empty it fails. My last post was wrapping echo as

fn echo { builtin echo $* | cat }

But invoking cat for every echo hurts the performance. Perhaps the most
efficient way is to write an ad hoc echo that does not do zero-length
write(2)? Any suggestion?

Thanks,
- cherry

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