According to the runsv man page:

If the directory service/log exists, runsv creates a pipe, redirects
service/run's and service/finish's standard output to the pipe,
switches to the directory service/log and starts ./run script. The
standard input of the log service is redirected to read from the pipe.

Obviously, runsv only redirects service's standard output, but not standard
error output, to svlogd standard input. My question is: why? Of course I want
to log my unit's standard error output; why do I have to pay extra attention
to adding exec 2>&1 to the beginning of each unit file?

Cheers!

Actually, you don't.

runsv takes this behaviour from svscan in the original Bernstein daemontools, which did the same. Almost everyone else has copied it. Bruce Guenter's svscan from daemontools-encore, Laurent Bercot's s6-svscan from s6, and Wayne Marshall's perpd from perp all do likewise.

Even Adam Sampson's svscan from freedt connects only standard output, despite calling the file descriptor err in the code. ☺

Noticing how extensively exec 2>&1 and fdmove -c 2 1 had become the norm, and observing that some programming languages explicitly define a standard log stream that ends up being file descriptor 2 (e.g. std::clog in C++), I made service-manager in the nosh toolset connect both standard output and standard error to the pipe when plumbing services together.

Further reading

• Jonathan de Boyne Pollard (2014). A side-by-side look at run scripts and service units.. Frequently Given Answers.


• Jonathan de Boyne Pollard (2015). "Logging". The daemontools family. Frequently Given Answers.


• https://unix.stackexchange.com/a/294206/5132