Why isn't apt-cache policy output piped?

Question

Cannot get why

$ apt-cache policy foo
N: Unable to locate package foo

but

$ apt-cache policy foo 2>&1 | grep .

is empty.

Where in the latter call am I doing the wrong assumption?

The original task: I need to process the apt-cache policy output presumably :-)

UPD:

foo used in my example may be substituted with any package name that does not exist in your apt-get index.

UPD 2:

there is an answer with a workaround. Additional +50 bounty will be awarded to anyone who explains why the 2>&1 solution does not work.

Answer 1

If stdout is not a tty (i.e. it's a regular file or a pipe) and if no --quiet option has been specified, apt-cache acts as if you had passed it --quiet=1. A workaround is to pass it a --quiet=0 option.

$ apt-cache --quiet=0 policy foo 2>&1 | grep .
N: Unable to locate package foo

Answer 2

There seems to be some cheaty behavior for redirections in apt-cache. But we can cheat a cheater by swapping stdout and stderr!

Try this one, it should work:

apt-cache policy foo 3>&1 1>&2 2>&3 3>&- | grep .

Answer 3

If you run strace apt-cache policy foo 2>&1 command, you can see the line ioctl(1, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0

Because that command manipulates the 1(stdout), 1 is not written to stdout anymore. And if you redirect 2 to 1, you lost both of them.

Edit: Here is a some code sample from apt-cache source code:

// Deal with stdout not being a tty
   if (!isatty(STDOUT_FILENO) && _config->FindI("quiet", -1) == -1)
      _config->Set("quiet","1");

Answer 4

A "better" solution would be to use a script utility:

script -c "apt-cache policy foo" /dev/null | grep .

That way it intercepts all the output and forwards it to the stdout.

The only drawback is that you need to install the script if you don't have it yet. In ubunty it's provided by bsdutils package.