nix-env writes everything to stderr instead of stdout

[edolstra]

Clearly not true:

$ nix-env -q | wc
     70      70    1064

[knupfer]

True for installing: $ nix-env -i hello | wc replacing old ‘hello-2.10’ installing ‘hello-2.10’ 0 0 0

[edolstra]

Yes, that's correct behaviour.

[goodwillcoding]

@edolstra @rbvermaa

Hi,

Why is something like this correct behaviour. I have the same issue with nix-collect-garbage, where only 1 line gets printed to stdout and the rest goes to stderr. This does not make much sense since there are no errors that happen during execution, the return of the run is 0

The reason I ask is because we run this part of automated processes and having to use tee and return code to cobble to together when there is an actual issue and when there is not one.

For example:

$ nix-collect-garbage --delete-old

stdout:

182 store paths deleted, 1.29 MiB freed

stderr

removing old generations of profile /nix/var/nix/profiles/per-user/user/profile
removing generation 16
removing generation 17
finding garbage collector roots...
deleting garbage...
deleting ‘/nix/store/qbssh84azvgsqah0j0gs5s54q22g0z54-mueval-env.drv’
deleting ‘/nix/store/gzn0cxixa67ffbs3z0l3492c1664j9rd-ghc-7.10.3.drv’
deleting ‘/nix/store/nh923lanvjbvj87v7nlm8v9w887wqj3v-lambdabot-trusted-5.0.2.1.drv’
deleting ‘/nix/store/clzhq069gkr34a6l9iz3md1r0p851ksv-oeis-0.3.6.drv’
...
deleting ‘/nix/store/zna3kkhrp7rjlri7x12sihr8r1gkf8y8-ansi-terminal-0.6.2.3.tar.gz.drv’
deleting ‘/nix/store/59aaxkfnmkqzzdsqj58rg38vlvlp1ya6-nix-prefetch-svn’
deleting ‘/nix/store/trash’
deleting unused links...
note: currently hard linking saves -0.00 MiB

[goodwillcoding]

Looking at the code I would say anything using lvlInfo should probably be going to stdout

[vcunat]

I believe that text showing progress is traditionally output into stderr.

[goodwillcoding]

@vcunat in that case why is that single message going to stdout? And if everything that is lvlInfo goes to stderr, what what should stdout be used for?

[goodwillcoding]

I mean I was taught that traditionally only errors go to stderr

[vcunat]

I don't know if the exact purpose is truly standardized somewhere in such a detail. If you look at git pull, for example, it does the same: stdout only gets Current branch foo is up to date. and the rest goes to sterr (what branches are being fetched, progress, etc.).

[edolstra]

The only stuff that goes to stdout is things with a more-or-less well defined format, i.e., things that can be processed easily by other programs/scripts. For instance, nix-store -r specifies that it prints the resulting store paths on stdout. So it's a bug that nix-collect-garbage prints anything on stdout.

[vcunat]

The current status seems reasonable to me. If anything might be useful to be processed by callers, that single line seems to contain the most useful information. (Perhaps not in very machine-friendly format, but still not too hard to parse.)