`?=` indicates to set the `KDIR` variable only if it&#39;s not set/doesn&#39;t have a value.

For example:

    KDIR ?= &quot;foo&quot;
    KDIR ?= &quot;bar&quot;

    test:
        echo $(KDIR)

Would print &quot;foo&quot;

GNU manual: http://www.gnu.org/software/make/manual/html_node/Setting.html

Thanks to Simon and R.T. for their quick and correct response.

Also, I have found the GNU manual that explains everything in detail:
http://www.gnu.org/software/make/manual/html_node/Setting.html

As the title says, I can&#39;t seem to get migrations working.

The app was originally under 1.6, so I understand that migrations won&#39;t be there initially, and indeed if I run `python manage.py migrate` I get:

    Operations to perform:
      Synchronize unmigrated apps: myapp
      Apply all migrations: admin, contenttypes, auth, sessions
    Synchronizing apps without migrations:
      Creating tables...
      Installing custom SQL...
      Installing indexes...
    Running migrations:
      No migrations to apply.


If I make a change to any models in `myapp`, it still says unmigrated, as expected.

But if I run `python manage.py makemigrations myapp` I get:

    No changes detected in app &#39;myapp&#39;


Doesn&#39;t seem to matter what or how I run the command, it&#39;s never detecting the app as having changes, nor is it adding any migration files to the app.

Is there any way to force an app onto migrations and essentially say &quot;This is my base to work with&quot; or anything? Or am I missing something?

My database is a PostgreSQL one if that helps at all.

Django 1.7 - makemigrations not detecting changes

The &quot;Python Distribute&quot; guide (was at python-distribute.org, but that registration has lapsed) tells me to include `doc/txt` files and `.py` files are excluded in `MANIFEST.in` file

The [sourcedist documentation](https://docs.python.org/2/distutils/sourcedist.html#manifest-template) tells me only sdist uses `MANIFEST.in` and only includes file you specify and to include `.py` files. It also tells me to use: `python setup.py sdist --manifest-only` to generate a `MANIFEST`, but python tells me this doesn&#39;t exist


I appreciate these are from different versions of python and the distribution system is in a
complete mess, but assuming I am using python 3 and `setuptools` (the new one that includes distribute but now called setuptools, not the old setuptools that was deprecated for distribute tools only to be brought back into distribute and distribute renamed to setuptools.....)

and I&#39;m following the &#39;standard&#39; folder structure and `setup.py` file, 

 1. Do I need a `MANIFEST.in` ?
 2. What should be in it ?
 3. When will all these different package systems and methods be made into one single simple process ?

Do python projects need a MANIFEST.in, and what should be in it?

    KDIR ?= $(shell uname -r)

What is the meaning of `?=`?

I have understood the difference between `:=`, `+=` and `=` from another thread available in Stack Overflow, but unable to find the explanation for `?=`.

What is ?= in Makefile

<pre><code class="hljs language-makefile">KDIR ?= $(shell uname -r)
</code></pre>
What is the meaning of <code>?=</code>?
I have understood the difference between <code>:=</code>, <code>+=</code> and <code>=</code> from another thread available in Stack Overflow, but unable to find the explanation for <code>?=</code>.

I have a Linode server running Ubuntu 12.04 LTS and MongoDB instance (service is running and CAN connect locally) that I can&#39;t connect to from an outside source.


I have added these two rules to my IP tables, where &lt; ip address &gt; is the server I want to connect FROM (as outlined in this [MongoDB reference](http://docs.mongodb.org/manual/tutorial/configure-linux-iptables-firewall/)):

&lt;code&gt;iptables -A INPUT -s &lt; ip-address &gt; -p tcp --destination-port 27017 -m state --state NEW,ESTABLISHED -j ACCEPT
&lt;/code&gt;

&lt;code&gt;iptables -A OUTPUT -d &lt; ip-address &gt; -p tcp --source-port 27017 -m state --state ESTABLISHED -j ACCEPT
&lt;/code&gt;

And I see the rule in my IP table allowing connections on 27017 to and from &lt; ip address &gt; however when I try to connect from , &lt; ip address &gt; to my mongo database using a command like this:

&lt;code&gt;mongo databasedomain/databasename -u username -p password&lt;/code&gt;

I get this error:

&lt;code&gt;2014-07-22T23:54:03.093+0000 warning: Failed to connect to databaseserverip:27017, reason: errno:111 Connection refused
2014-07-22T23:54:03.094+0000 Error: couldn&#39;t connect to server &lt; ip address &gt;:27017 (databaseserverip), connection attempt failed at src/mongo/shell/mongo.js:148
exception: connect failed&lt;/code&gt;

Any help is VERY APPRECIATED!!!! Thanks!!!

Connection refused to MongoDB errno 111

when I try to load a web page to terminal it gives `curl: (6) Could not resolve host` error. 

I have internet in my PC and trying from my home internet connection. So as I there is no any proxy involve here. 

    [root@localhost kevin]# curl http://google.com
    curl: (6) Could not resolve host: google.com; Name or service not known


`clean all` and tried again but no lucky. 

But if I use IP instead of the domain name, it works fine. 

`[root@localhost kevin]# curl http://173.194.46.0`

any clue please?


curl: (6) Could not resolve host: google.com; Name or service not known

I use `ssh -p8520 username@remote_host` to login remote server.

**Issue**:

It is always connected and works properly when I am in the work place. Unfortunately, terminal freezes in 10 - 15 minutes after I connected with the remote server from home. 

There&#39;s no error/timeout report on the console but the cursor cannot move any more.

When enter &lt;kbd&gt;w&lt;/kbd&gt; to check the login users, some zombies login users are there, and I have to kill them manually.

This is quite annoying. Can anyone help me? 

Keep SSH session alive

I want to lint a file, and print the stderr (error message), but do not print the stdout (saying the file is OK).

    php -l &quot;foo/bar.php&quot;

- If there are no errors, it prints a &quot;No errors&quot; message to stdout.
- If there are errors, it prints a detailed message to stderr. I want only this one.

I figure it will be some magic with `&gt;&amp;`, but I never understood how those work.

What I want is to consume all stdout, keep stderr.

(Sorry if this is dulicate, but I haven&#39;t found any question exactly about this)


Bash print stderr only, not stdout

We can use docker to pull different images. And these images are different linux distribution.
But no matter which linux distro docker is running on, docker can run these different linux distribution just like in a virtual machine.

I know docker uses [aufs](http://en.wikipedia.org/wiki/Aufs) to control different read-write access level. So it can reuse some file on the host machine. But how can docker run `apt-get` in a container when my host runs `arch linux`? Does the image contain the `apt-get` binary? But different linux distribution have different libs and software version. Even the configuration file are different.How can docker &quot;run&quot; ubuntu in a arch linux?

Why docker has ability to run different linux distribution?

What is the difference between arm-eabi, gnueabi and gnueabi-hf cross compilers?
I am kind of finding it difficult to choose the executable that is correct for my target platform.
Is there a native compiler for arm?

Difference between arm-eabi arm-gnueabi and gnueabi-hf compilers

I&#39;m receiving the following warning from mongodb about THP

    2015-03-06T21:01:15.526-0800 I CONTROL  [initandlisten] ** WARNING: /sys/kernel/mm/transparent_hugepage/defrag is &#39;always&#39;.
    2015-03-06T21:01:15.526-0800 I CONTROL  [initandlisten] **        We suggest setting it to &#39;never&#39;

But I did manage to turned THP off manually

    frederick@UbuntuVirtual:~$ cat /sys/kernel/mm/transparent_hugepage/enabled
    always madvise [never]
    frederick@UbuntuVirtual:~$ cat /sys/kernel/mm/transparent_hugepage/defrag
    always madvise [never]

I did the trick by adding `transparent_hugepage=never` to `GRUB_CMDLINE_LINUX_DEFAULT` in `/etc/default/grub` and adding

    if test -f /sys/kernel/mm/transparent_hugepage/defrag; then
       echo never &gt; /sys/kernel/mm/transparent_hugepage/defrag
    fi

to `/etc/rc.local`

How on earth can I avoid the warning?

How to avoid transparent_hugepage/defrag warning from mongodb?

How can docker run on a Debian host maybe an OpenSUSE in a container? It uses different kernel, with separated modules. Also older Debian versions have used older kernels, so how can run it on a kernel version 3.10+ ? Older kernels have only older built in functions, how can an old distro manage new features?
What is &quot;the trick&quot; in it?

How can Docker run distros with different kernels?

`ip link` shows this:

    [root@0-25-90-68-79-4a log]# ip link
    1: lo: &lt;LOOPBACK,UP,LOWER_UP&gt; mtu 65536 qdisc noqueue state UNKNOWN 
        link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    2: eth0: &lt;BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP&gt; mtu 1500 qdisc mq master bond0 state UP qlen 1000
        link/ether 00:25:90:68:79:4a brd ff:ff:ff:ff:ff:ff
    5: bond0: &lt;BROADCAST,MULTICAST,MASTER,UP,LOWER_UP&gt; mtu 1500 qdisc noqueue state UP 
        link/ether 00:25:90:68:79:4a brd ff:ff:ff:ff:ff:ff
    8: gre0: &lt;NOARP&gt; mtu 1476 qdisc noop state DOWN 
        link/gre 0.0.0.0 brd 0.0.0.0
    10: 35978f0: &lt;BROADCAST,MULTICAST,UP,LOWER_UP&gt; mtu 1500 qdisc pfifo_fast master br35978f0 state UP qlen 32
        link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
    13: br35978f0: &lt;BROADCAST,MULTICAST,UP,LOWER_UP&gt; mtu 1500 qdisc noqueue state UP 
        link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff

Here, the interface **35978f0** is the virtual interface which is tied to the bridge **br 35978f0**. I&#39;ve already done `ifconfig 35978f0 down` and `brctl delif br35978f0 35978f0`.

But how do I delete interface **35978f0**?


How do I delete virtual interface in Linux?

If I am running a container in privileged mode, does it have all the Kernel capabilities or do I need to add them separately?

Privileged containers and capabilities

Is there any differences in invoking variables with syntax `${var}` and `$(var)`? For instance, in the way the variable will be expanded or anything?


What&#39;s the difference between parenthesis $() and curly bracket ${} syntax in Makefile?

I code in C/C++ and use a (GNU) Makefile to compile the code. I can do the same with CMake and get a Makefile. However, what is the difference between using a Makefile and CMake to compile the code?



What is the difference between using a Makefile and CMake to compile the code?

When inserting a shell script inside a Makefile we have (?) to use a double dollar sign ($$) to make reference to variables. Why is that so?

    for number in 1 2 3 4 ; do \
        echo $$number ; \
    done

What is the meaning of a double dollar sign in bash/Makefile?

I&#39;m currently learning how to use the `autoconf`/`automake` toolchain.  I seem to have a general understanding of the workflow here - basically you have a `configure.ac` script which generates an executable `configure` file.  The generated `configure` script is then executed by the end user to generate `Makefile`s, so the program can be built/installed.

So the installation for a typical end-user is basically:

    ./configure
    make
    make install
    make clean

Okay, now here&#39;s where I&#39;m confused:

As a developer, I&#39;ve noticed that the auto-generated configure script sometimes won&#39;t run, and will error with:

    config.status: error: cannot find input file: `somedir/Makefile.in&#39; 

This confuses me, because I thought the configure script is supposed to *generate* the `Makefile.in`.  So Googling around for some answers, I&#39;ve discovered that this can be fixed with an `autogen.sh` script, which basically &quot;resets&quot; the state of the `autoconf` environment.  A typical `autogen.sh` script would be something like:

    aclocal \
    &amp;&amp; automake --add-missing \
    &amp;&amp; autoconf

Okay fine.  But as an end-user who&#39;s downloaded countless tarballs throughout my life, I&#39;ve *never* had to use an `autogen.sh` script.  All I did was uncompress the tarball, and do the usual configure/make/make install/make clean routine.

But as a developer who&#39;s now *using* `autoconf`, it seems that `configure` doesn&#39;t actually run unless you run `autogen.sh` first.  So I find this very confusing, because I thought the end-user shouldn&#39;t have to run `autogen.sh`. 

So why do I have to run `autogen.sh` first - in order for the configure script to find `Makefile.in`?  Why doesn&#39;t the configure script simply generate it?


Confused about configure script and Makefile.in

Is there a way to configure CLion to use a local makefile to compile code, rather than CMake? I can&#39;t seem to find the way to do it from the build options. 

Content Type	Original Author	Original Content on Stackoverflow
Question	codedoc	View Question on Stackoverflow
Solution 1 - Linux	Simon	View Answer on Stackoverflow
Solution 2 - Linux	codedoc	View Answer on Stackoverflow

What is ?= in Makefile

Linux Problem Overview

Linux Solutions

Solution 1 - Linux

Solution 2 - Linux

Do python projects need a MANIFEST.in, and what should be in it?

Django 1.7 - makemigrations not detecting changes

Attributions