current community

your communities

more stack exchange communities

Stack Exchange
tour help
Take the 2-minute tour ×
Unix & Linux Stack Exchange is a question and answer site for users of Linux, FreeBSD and other Un*x-like operating systems.. It's 100% free, no registration required.
up vote 9 down vote favorite
3

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.

share|improve this question
    
# apt-cache policy vim 2>&1 |grep . vim: Installed: 2:7.4.712-2 Candidate: 2:7.4.712-2 Version table: *** 2:7.4.712-2 0 500 http://ftp.debian.org/debian/ sid/main amd64 Packages 100 /var/lib/dpkg/status –  Mohsen Pahlevanzadeh yesterday
1  
@MohsenPahlevanzadeh that's right, now try the exact call (package name) I provided :-) –  zerkms yesterday
3  
@MohsenPahlevanzadeh so? I'm sorry, but are you sure you have read the question (and the title)? –  zerkms yesterday
2  
@MohsenPahlevanzadeh not it's not equal (not even close) –  zerkms yesterday
1  
I run strace apt-cache policy foo 2>&1 and there is a system call ioctl(1, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0 I think because of this call 1(stdout) has problems. I mean it is not written to tty anymore –  Esref 23 hours ago

4 Answers 4

up vote 6 down vote accepted

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
share|improve this answer
up vote 10 down vote

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 .
share|improve this answer
up vote 6 down vote

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");
share|improve this answer
    
Okay. Any way to capture them both? –  zerkms 22 hours ago
    
I could not find any way other than like @Mr_Mig's answer. (Mine was apt-cache policy foo 1>&2 2>&1 | grep .) But I find that in the source code apt apt-cache :) // Deal with stdout not being a tty if (!isatty(STDOUT_FILENO) && _config->FindI("quiet", -1) == -1) _config->Set("quiet","1"); –  Esref 20 hours ago
    
Btw, I also was pointed by someone to the same point in sources just few minutes ago :-) And a potentially better solution script -c "sudo apt-cache policy foo" | grep Unable which requires installing a script though. As advised - I will put +50 here in 2 days (SE does not let to do it earlier) –  zerkms 20 hours ago
1  
@Esref Your comment about "I find that in the source code apt apt-cache..." should be in the answer, so please add it there. +1. : –  Faheem Mitha 12 hours ago
up vote 3 down vote

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.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.