Actually it means nothing and is ignored.

From the [Bash FAQ E10:][1]:

&gt; E10) Why does &#39;cd //&#39; leave $PWD as &#39;//&#39;?
&gt; 
&gt; POSIX.2, in its description of &#39;cd&#39;, says that three or more leading
&gt; slashes may be replaced with a single slash when canonicalizing the
&gt; current working directory.
&gt; 
&gt; This is, I presume, for historical compatibility. Certain versions of
&gt; Unix, and early network file systems, used paths of the form
&gt; //hostname/path to access &#39;path&#39; on server &#39;hostname&#39;.

Also the [Unix standards][2] states:

&gt; A pathname that begins with two successive slashes may be interpreted
&gt; in an implementation-defined manner, although more than two leading
&gt; slashes shall be treated as a single slash.


  [1]: http://tiswww.case.edu/php/chet/bash/FAQ
  [2]: http://pubs.opengroup.org/onlinepubs/000095399/basedefs/xbd_chap04.html#tag_04_11

This [answer][1] on AskUbuntu confirms that they are the same directory. Further, it helpfully quotes from the [POSIX Pathname Resolution specification][2]:

&gt; A pathname consisting of a single slash shall resolve to the root directory of the process. A null pathname shall not be successfully resolved. A pathname that begins with two successive slashes may be interpreted in an implementation-defined manner, although more than two leading slashes shall be treated as a single slash.


  [1]: https://askubuntu.com/a/23809
  [2]: http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap04.html#tag_04_11

&gt; Keep-alives were added to HTTP to basically reduce the significant
&gt; overhead of rapidly creating and closing socket connections for each
&gt; new request. The following is a summary of how it works within HTTP
&gt; 1.0 and 1.1:
&gt; 
&gt;   HTTP 1.0 	The HTTP 1.0 specification does not really delve into how
&gt; Keep-Alive should work. Basically, browsers that support Keep-Alive
&gt; appended an additional header to the request as [edited for clarity] explained below:
&gt; 
&gt;   When the server processes the request and
&gt; generates a response, it also adds a header to the response:
&gt;
&gt;   **Connection: Keep-Alive** 	
&gt;
&gt; When this is done, the socket connection is
&gt; not closed as before, but kept open after sending the response. When
&gt; the client sends another request, it reuses the same connection. The
&gt; connection will continue to be reused until either the client or
&gt; the server decides that the conversation is over, and one of them drops the connection.

The above explanation [comes from here][1]. But I don&#39;t understand one thing 

&gt; When this is done, the socket connection is not closed as before, but
&gt; kept open after sending the response.

As I understand we just send tcp packets to make requests and responses, how this `socket connection` helps and how does it work? We still have to send packets, but how can it somehow establish the persistent connection? It seems so unreal.


  [1]: http://www.jguru.com/faq/view.jsp?EID=704228

Explain http keep-alive mechanism

How can I use Chameleon or Zope Page Templates to easily create CSS zebra striping? I want to add `odd` and `even` classes to each row in a table, but using a condition with `repeat/name/odd` or `repeat/name/even` looks rather verbose even with a conditional expression:

&lt;!-- language: lang-html --&gt;

    &lt;table&gt;
       &lt;tr tal:repeat=&quot;row rows&quot; 
           tal:attributes=&quot;class python:repeat[&#39;row&#39;].odd and &#39;odd&#39; or &#39;even&#39;&quot;&gt;
           &lt;td tal:repeat=&quot;col row&quot; tal:content=&quot;col&quot;&gt;column text text&lt;/td&gt;
       &lt;/tr&gt;
    &lt;/table&gt;

This gets especially tedious if you have multiple classes to calculate.



How to create zebra-stripe CSS with TAL?

I&#39;ve entered a command `cd //` instead of `cd /` by mistake and instead of getting an error as I&#39;d expect the shell (Bash) showed a prompt as if I&#39;m in the `//` directory:-

    &lt;username&gt;@&lt;hostname&gt;://$
    &lt;username&gt;@&lt;hostname&gt;://$ pwd
    //

Listing the content of the directory shows that it is the same as `/`.

So what exactly `cd //` do and what&#39;s the difference from `cd /`?


What does double slash // in `cd //` mean in Linux?

I've entered a command <code>cd //</code> instead of <code>cd /</code> by mistake and instead of getting an error as I'd expect the shell (Bash) showed a prompt as if I'm in the <code>//</code> directory:-
<pre><code class="hljs language-ruby">&#x3C;username>@&#x3C;hostname>://$
&#x3C;username>@&#x3C;hostname>://$ pwd
//
</code></pre>
Listing the content of the directory shows that it is the same as <code>/</code>.
So what exactly <code>cd //</code> do and what's the difference from <code>cd /</code>?

I&#39;ve CSV file (around 10,000 rows ; each row having 300 columns) stored on LINUX server. 
I want to break this CSV file into 500 CSV files of 20 records each. (Each having same CSV header as present in original CSV) 

Is there any linux command to help this conversion? 

How to split CSV files as per number of rows specified?

I would like to know whether there is any simple shell command to change the user home directory in Linux/Unix (one similar to **chsh** which changes the default login shell of an existing valid user) without touching the `/etc/passwd` file. Thanks

Command to change the default home directory of a user

I had Screen running inside an SSH session.  Terminal froze.  After restarting Terminal, that Screen session still thinks it&#39;s attached.  Maybe it is.  Perhaps I don&#39;t really know what that means.

I&#39;d like to attach to that Screen session from a fresh SSH login.  I do not want to kill that Screen session as important things are happening over there.  : )

The options I think I have ( none of which I know how to solve ):

- How do I detach that Screen session?
- How do I kill the SSH session that that Screen session is attached to?



How do I force detach Screen from another SSH session?

I&#39;ve noticed with docker that I need to understand what&#39;s happening inside a container or what files exist in there. One example is downloading images from the docker index - you don&#39;t have a clue what the image contains so it&#39;s impossible to start the application.

What would be ideal is to be able to ssh into them or equivalent. Is there a tool to do this, or is my conceptualisation of docker wrong in thinking I should be able to do this.

Exploring Docker container&#39;s file system

I am new to pthreads, and I am trying to understand it. I saw some examples like the following. 

I could see that the `main()` is blocked by the API `pthread_exit()`, and I have seen examples where the main function is blocked by the API `pthread_join()`. I am not able to understand when to use what? 

I am referring to the following site - [https://computing.llnl.gov/tutorials/pthreads/][1]. I am not able to get the concept of when to use `pthread_join()` and when to use `pthread_exit()`. 

Can somebody please explain? Also, a good tutorial link for pthreads will be appreciated.

    #include &lt;pthread.h&gt;
    #include &lt;stdio.h&gt;
    #define NUM_THREADS     5
    
    void *PrintHello(void *threadid)
    {
       long tid;
       tid = (long)threadid;
       printf(&quot;Hello World! It&#39;s me, thread #%ld!\n&quot;, tid);
       pthread_exit(NULL);
    }
    
    int main (int argc, char *argv[])
    {
       pthread_t threads[NUM_THREADS];
       int rc;
       long t;
       for(t=0; t&lt;NUM_THREADS; t++){
          printf(&quot;In main: creating thread %ld\n&quot;, t);
          rc = pthread_create(&amp;threads[t], NULL, PrintHello, (void *)t);
          if (rc){
             printf(&quot;ERROR; return code from pthread_create() is %d\n&quot;, rc);
             exit(-1);
          }
       }
    
       /* Last thing that main() should do */
       pthread_exit(NULL);


  [1]: https://computing.llnl.gov/tutorials/pthreads/


Realized one more thing i.e.  

    pthread_cancel(thread);
    pthread_join(thread, NULL);

Sometimes, you want to cancel the thread while it is executing.
You could do this using pthread_cancel(thread);.
However, remember that you need to enable pthread cancel support.
Also, a clean up code upon cancellation.

    thread_cleanup_push(my_thread_cleanup_handler, resources);
    pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, 0);

    static void my_thread_cleanup_handler(void *arg)
    {
      // free
      // close, fclose
    }



When to use pthread_exit() and when to use pthread_join() in Linux?

When I tried running the following command on MySQL from within Terminal:

    mysql -u $user -p$password -e &quot;statement&quot;

The execution works as expected, but it always issues a warning:

&gt; *Warning: Using a password on the command line interface can be insecure.* 

However, I have to conduct the statement above using an environment variable (`$password`) that stores my password, because I want to run the command iteratively in bash script from within Terminal, and I definitely don&#39;t like the idea of waiting a prompt showing up and forcing me to input my password 50 or 100 times in a single script. So here&#39;s my question:

* Is it feasible to suppress the warning? The command works properly as I stated, but the window becomes pretty messy when I loop over and run the command 50 or 100 times.

* Should I obey the warning message and do NOT write my password in my script? If that&#39;s the case, then do I have to type in my password every time the prompt forces me to do so?

Running `man mysql` doesn&#39;t help, saying only 

&gt; `--show-warnings`  
Cause warnings to be shown after each statement if there are any. This option applies to interactive and batch mode.

and mentions nothing about how to turn off the functionality, if I&#39;m not missing something.

I&#39;m on OS X 10.9.1 Mavericks and use MySQL 5.6 from homebrew.

Suppress warning messages using mysql from within Terminal, but password written in bash script

In his [answer][1] @Grundlefleck explains how to check whether a directory exists or not. I tried some to use this inside a `makefile` as follow:

    foo.bak: foo.bar
    	echo &quot;foo&quot;
    	if [ -d &quot;~/Dropbox&quot; ]; then
    		echo &quot;Dir exists&quot;
    	fi

Running `make foo.bak` (given that `foo.bar` exists) yields the following error:

    echo &quot;foo&quot;
    foo
    if [ -d &quot;~/Dropbox&quot; ]; then
    /bin/sh: -c: line 1: syntax error: unexpected end of file
    make: *** [foo.bak] Error 2

The workaround I made was to have a standalone bash script where the test is implemented and I called the script from the `makefile`. This, however, sounds very cumbersome. Is there a nicer way to check whether a directory exists from within a `makefile`?

  [1]: https://stackoverflow.com/a/59839/671013

Test whether a directory exists inside a makefile

I need to write a script that starts my program with different arguments, but I&#39;m new to Bash. I start my program with:

`./MyProgram.exe Data/data1.txt [Logs/data1_Log.txt]`.

Here is the pseudocode for what I want to do:

    for each filename in /Data do
      for int i = 0, i = 3, i++
        ./MyProgram.exe Data/filename.txt Logs/filename_Log{i}.txt
      end for
    end for

So I&#39;m really puzzled how to create second argument from the first one, so it looks like dataABCD_Log1.txt and start my program. 

How to loop over files in directory and change path and add suffix to filename

I&#39;m about to write a shell script to detect if several homebrew packages are installed in the system. Is there a way to use a brew command to achieve that?

I tried using the exit code of `brew install &lt;formula&gt; --dry-run`. But this builds the package if it is missing.

Detect if homebrew package is installed

I followed the following tutorial: http://davidtsadler.com/archives/2012/06/03/how-to-install-magento-on-ubuntu/

At some point it told me to execute the following command:

    sudo bash -c &quot;cat &gt;&gt; /etc/apache2/sites-available/magento-store.com &lt;&lt;EOF
    &lt;VirtualHost *:80&gt;

      ServerName  localhost.magento-store.com
      ServerAlias www.localhost.magento-store.com

      DocumentRoot /home/dev/public_html/magento-store.com/public

      LogLevel warn
      ErrorLog  /home/dev/public_html/magento-store.com/log/error.log
      CustomLog /home/dev/public_html/magento-store.com/log/access.log combined

    &lt;/VirtualHost&gt;
    EOF&quot;

What did this command do, and how I can cancel that?

I restarted the computer, and it seems that it is still running. I looked in `.bashrc` and `.profile`, but I did not find it inside.



What does &#39;bash -c&#39; do?

I would like to mark a Jenkins build to fail on one scenario for example: 

    if [ -f &quot;$file&quot; ]
    then
        echo &quot;$file found.&quot;
    else
        echo &quot;$file not found.&quot;
        #Do Jenkins Build Fail
    fi

Is it possible via Shell Script?


**Answer:** If we exit with integer 1, Jenkins build will be marked as failed.  So I replaced the comment with `exit 1` to resolve this.

Do manual build fail in Jenkins using shell script

I&#39;m trying to execute this command from the application emulator terminal (you can find it in google play) in this app i write `su` and press &lt;kbd&gt;enter&lt;/kbd&gt;, so write:

`screenrecord --time-limit 10 /sdcard/MyVideo.mp4` 

and press again &lt;kbd&gt;enter&lt;/kbd&gt; and start the recording of the screen using the new function of android kitkat.

so, i try to execute the same code from java using this:

    Process su = Runtime.getRuntime().exec(&quot;su&quot;);
    Process execute = Runtime.getRuntime().exec(&quot;screenrecord --time-limit 10 /sdcard/MyVideo.mp4&quot;);

But don&#39;t work because the file is not created. obviously i&#39;m running on a rooted device with android kitkat installed. where is the problem? how can i solve? because from terminal emulator works and in Java not?

execute shell command from android

So this doesn&#39;t seem like a terribly complicated question I have, but it&#39;s one I can&#39;t find the answer to.  I&#39;m confused about what the `-p` option does in Unix.  I used it for a lab assignment while creating a subdirectory and then another subdirectory within that one.  It looked like this:

    mkdir -p cmps012m/lab1

This is in a private directory with normal rights (`rlidwka`). Oh, and would someone mind giving a little explanation of what `rlidwka` means?  I&#39;m not a total noob to Unix, but I&#39;m not really familiar with what this means.  Hopefully that&#39;s not too vague of a question.

mkdir&#39;s &quot;-p&quot; option

I know there is actually no single right way. However I&#39;ve found that it&#39;s hard to create a directory structure that works well and remain clean for every developer and administrator. There is some standard structure in most projects on github. But it does not show a way to organize another files and all projects on pc.

What is the most convenient way to organize all these directories on development machine? How do you name them, and how do you connect and deploy this to server?

 - projects (all projects that your are working on)
 - source files (the application itself)
 - working copy of repository (I use git)
 - virtual environment (I prefer to place this near the project)
 - static root (for compiled static files)
 - media root (for uploaded media files)
 - README
 - LICENSE
 - documents
 - sketches
 - examples (an example project that uses the application provided by this project)
 - database (in case sqlite is used)
 - anything else that you usually need for successful work on project

The problems that I want to solve:

 - Good names of directories so that their purpose is clear.
 - Keeping all project files (including virtualenv) in one place, so I can easily copy, move, archive, remove whole project or estimate disk space usage.
 - Creating multiple copies of some selected file sets such as entire application, repository or virtualenv, while keeping single copy of another files that I don&#39;t want to clone.
 - Deploying right set of files to the server simply by rsyncing selected one dir.

Best practice for Django project working directory structure

Android Studio is giving me a Gradle build error that looks like this:

    Error:(3, 22) compileSdkVersion android-22 requires compiling with JDK 7

Now it gives me these clickable prompts:

    Download JDK 7
    Select a JDK from the File System
    Open build.gradle File

And I have already downloaded and installed JDK 7. The problem is when I go to select it in the &quot;File System&quot; i can only find a directory named 1.6.0 JDK. Furthermore, the installation of JDK 7 skipped the bullet point where I would&#39;ve selected the install directory, so I&#39;m really not sure where it is. My java control panel says I have &quot;Java 7 Update 79&quot; so I&#39;m pretty sure I&#39;m close, I just need to tell android studio where it is. I also ran the `java -version` command in the terminal and it says my version is &quot;1.7.0_79&quot;.

Any help would be appreciated!



How to specify the JDK version in Android Studio?

I&#39;ve been struggling with imports in my package for the last hour.

I&#39;ve got a directory structure like so:

    main_package
     |
     | __init__.py
     | folder_1
     |  | __init__.py
     |  | folder_2
     |  |  | __init__.py
     |  |  | script_a.py
     |  |  | script_b.py
     |
     | folder_3
     |  | __init__.py
     |  | script_c.py

I want to access code in `script_b.py` as well as code from `script_c.py` from `script_a.py`. How can I do this?

If I put a simple `import script_b` inside `script_a.py`, when I run

    from main_package.folder_1.folder_2 import script_b

I am met with an

    ImportError: no module named &quot;script_b&quot;

For accessing `script_c.py`, I have no clue. I wasn&#39;t able to find any information about accessing files two levels up, but I know I can import files one level up with

    from .. import some_module

How can I access both these files from `script_a.py`?

Python relative-import script two levels up

(*Probably a noob question, but I didn&#39;t find a solution after googling for 20 minutes.*)

I created a new *pure Python* project with PyCharm which yielded the following folder structure

    myproject
    └── venv
        ├── bin
        │&#160;&#160; ├── activate
        │&#160;&#160; ├── activate.csh
        │&#160;&#160; ├── activate.fish
        │&#160;&#160; ├── easy_install
        │&#160;&#160; ├── easy_install-3.5
        │&#160;&#160; ├── pip
        │&#160;&#160; ├── pip3
        │&#160;&#160; ├── pip3.5
        │&#160;&#160; ├── python
        │&#160;&#160; ├── python3
        │&#160;&#160; └── python3.5
        ├── include
        ├── lib
        │&#160;&#160; └── python3.5
        ├── lib64 -&gt; lib
        └── pyvenv.cfg

Where do I put `myproject.py` or the `myproject` folder now?

- Inside or outside of `venv`?
- In the `venv/bin`folder?
- Just inside `venv`, i.e. `myproject/venv/myproject.py`?

Where do I put my python files in the venv folder?

I used this command:  
`docker exec compassionate_mclean cd /root/python`  
 The error returned is 
 

&gt; docker-exec: failed to exec: exec: &quot;cd&quot;: executable file not found in
&gt; $PATH

Kindly help me out

Content Type	Original Author	Original Content on Stackoverflow
Question	vitaut	View Question on Stackoverflow
Solution 1 - Linux	Rahul Tripathi	View Answer on Stackoverflow
Solution 2 - Linux	Serp C	View Answer on Stackoverflow

What does double slash // in `cd //` mean in Linux?

Linux Problem Overview

Linux Solutions

Solution 1 - Linux

Solution 2 - Linux

How to create zebra-stripe CSS with TAL?

Explain http keep-alive mechanism

Attributions