Yuliang's Blog

Play only audio of movies with mplayer

Yuliang Jin — Sun, 16 Sep 2012 11:54:36 +0000

I have a piece of very good music in an MP4 video file and I’d like to listen over and over to it. With mplayer, I can single cycle this file with option -loop 0. Moreover, I want just play the audio but not the video, so I search the man page of mplayer and find this:

mplayer -loop 0 -novideo filename.mp4

It works very well.

PS. I also find this post very useful if you want to extract the audio from an mp4 file to mp3 file.

© 2012, Yuliang Jin. All rights reserved.
This post is under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

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How to scp a file with colon in its name

Yuliang Jin — Sun, 05 Feb 2012 08:20:29 +0000

We know that scp is a command for secure copy. It copies file between hosts on a network. Normally, we follow the below two examples to copy a file from/to localhost to/from a remote machine:

scp /path/to/filename username@domian-name-or-ip-address:
scp username@domain-name-or-ip-address:/path/to/filename /path/to/store/this/file

These commands and their deformation usually work fine. But when I tried to scp a file to my VPS this afternoon. I encountered a problem:

scp file\:name yuliang@gnailuy.com:
ssh: Could not resolve hostname example: Name or service not known

I Googled this problem for quite a while and finally found out that the culprit is the colon in the name of my file. The solution to this problem turns out to be very simple:

scp ./file\:name yuliang@gnailuy.com:
scp /path/to/file\:name yuliang@gnailuy.com:

© 2012, Yuliang Jin. All rights reserved.
This post is under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

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Install CentOS via network without CD-ROM and without USB

Yuliang Jin — Sun, 25 Dec 2011 15:03:40 +0000

There is an extremely old machine in our lab and today I want to use it as a node in our Hadoop cluster. There was a Fedora 8 on it. But I’d like to change it to a new CentOS 5.7. The problem I met, was that our machine was too old, and that neither the CD-ROM nor the USB ports work well.

Seems troublesome. But I still find a way to install my new CentOS on this machine, without CD-ROM, without USB and without any external device. It’s net-installation! You might argue that even net-installation need an CD or USB disk too, which contains the net-install ISO image, to boot up our machine. But we can take the GRUB on our old operating system in use, to boot the net-install kernel. Here’s how.

First, we download the net-install image in the old OS, and check its MD5 hash:

wget http://mirrors.163.com/centos/5.7/isos/i386/CentOS-5.7-i386-netinstall.iso http://mirrors.163.com/centos/5.7/isos/i386/md5sum.txt
diff <(grep "netinstall" md5sum.txt) <(md5sum CentOS-5.7-i386-netinstall.iso) && echo OK

Then we extract the only directory in the ISO file, isolinux, to the /boot directory. We put isolinux in the /boot directory just for convenience. Because after we boot into the GRUB command line, here is the default root of GRUB.

mkdir tmp
mount -o loop CentOS-5.7-i386-netinstall.iso tmp/
cp -Rv tmp/isolinux/ /boot/

After that, we reboot and press c on the GRUB menu to enter the GRUB Command Line Interface. Type the below commands to boot the kernel in the isolinux directory.

kernel /isolinux/vmlinuz
initrd /isolinux/initrd.img
boot

If you put the isolinux in other partitions, you may need to specify the root path of GRUB using the root command.

The system will boot up correctly just like we insert the net-install disk into the CD-ROM. On the Installation Method screen, we choose HTTP or FTP and then configure the network. Next, we choose an exist mirror site or our own server (will be faster) which host CentOS files, and the remain steps are very familiar to us.

（本文中文版链接）

© 2011, Yuliang Jin. All rights reserved.
This post is under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

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无光驱、无USB，网络安装CentOS

Yuliang Jin — Sun, 25 Dec 2011 15:02:33 +0000

实验室有台非常老的机器，今天被我翻出来打算当作我们Hadoop集群中的一个节点。这台机器上原有一个Fedora 8可以用，不过我还是想给它安装一个全新的CentOS 5.7。不过有个问题，就是这台机器太老了，光驱和USB接口竟然全都坏掉了。

看起来问题似乎很棘手，不过最后还是找到了一个办法，不借助光驱或者U盘等任何一种外部设备，通过网络将新的CentOS安装到这台机器上。也许你会疑惑，网络安装不也需要一张CD或者U盘之类的启动介质吗？确实，通常的网络安装也需要在某种介质中写入网络安装的镜像文件，使用它们启动来安装系统。但是我的机器中原本安装了一个Fedora 8，我们可以利用这个旧系统中的GRUB，来启动网络安装的内核。下面是详细步骤：

首先，启动旧的操作系统，下载CentOS网络安装的ISO镜像并校验它的MD5值：

wget http://mirrors.163.com/centos/5.7/isos/i386/CentOS-5.7-i386-netinstall.iso http://mirrors.163.com/centos/5.7/isos/i386/md5sum.txt
diff <(grep "netinstall" md5sum.txt) <(md5sum CentOS-5.7-i386-netinstall.iso) && echo OK

然后我们把这个ISO镜像中唯一的目录，isolinux目录，释放到/boot目录下。这里我们将isolinux放在/boot目录下，只是为了后面的步骤方便。因为旧系统的GRUB启动后，这里是默认的根目录。

mkdir tmp
mount -o loop CentOS-5.7-i386-netinstall.iso tmp/
cp -Rv tmp/isolinux/ /boot/

再然后，重启操作系统。在GRUB菜单出现后按c进入GRUB命令行。敲下面的命令启动isolinux目录中的迷你系统。

kernel /isolinux/vmlinuz
initrd /isolinux/initrd.img
boot

如果isolinux被放在了其他的分区里，还需要使用GRUB的root命令来指定isolinux目录的位置。

这样网络安装镜像中的系统就可以正确的启动了，和我们使用光盘或者U盘的效果一样。接下来只需在Installation Method这一屏上选择HTTP或者FTP方式，然后配置好网络，选择一个镜像站点的地址或者自己搭建一个本地CentOS服务器(会更快)，就可以按照常规的步骤安装CentOS了。

(This post is also available in English: link)

© 2011, Yuliang Jin. All rights reserved.
This post is under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

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Spoofing MAC address in Archlinux

Yuliang Jin — Tue, 18 Oct 2011 23:51:40 +0000

The first issue I met, after I logged in to my newly installed Arch Linux, is that I have to change the MAC address to gain access to our campus network. I wrote a short essay about how to change the MAC address in Linux before (see this). In that article, I introduced how to change the MAC address temporarily using ifconfig and permanently editing the network configure file in Red Hat/CentOS/Fedora or Debian/ubuntu. But I found no ifconfig available in my new Arch Linux (I must had missed some essential package that contains ifconfig during the installation.). So I have to find another method, and fortunately, there is an “Arch Way”.

(本文中文版链接)

According to the Arch Wiki, we can use macchanger or ip to make a temporary change of MAC address:

macchanger --mac=XX:XX:XX:XX:XX:XX

ip link set dev eth0 down
ip link set dev eth0 address XX:XX:XX:XX:XX:XX
ip link set dev eth0 up

where eth0 is the name of my wired network device. I have not macchanger installed on my system, so I use the second method above. After we can access the Internet. You may want to install macchanger using pacman. It can give a random MAC address to a device with the r parameter, very interesting.

With the above two methods, the MAC address will return to its initial default value after reboot. In order to spoof MAC address on boot, Arch Wiki gives an Arch Way. We can create the file /etc/rc.d/functions.d/macspoof with the following content:

spoof_mac() {
	ip link set dev eth0 address XX:XX:XX:XX:XX:XX
}
 
add_hook sysinit_end spoof_mac

This file add a hook at the end of system initial process with a function which use the ip command to change the MAC address on system boot.

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Archlinux中伪装MAC地址

Yuliang Jin — Tue, 18 Oct 2011 23:51:22 +0000

装好Archlinux以后，我遇到的第一个麻烦就是如何修改MAC地址来接入校园网。之前我写过一篇短文，里面介绍Linux下如何修改MAC地址(在这里)。那篇文章介绍了如何使用ifconfig程序临时修改MAC地址，以及在Red Hat/CentOS/Fedora或Debian/ubuntu中如何修改网络配置文件来修改MAC地址。但是我发现我新装的Archlinux没有ifconfig程序(我安装系统时选择的包很少)。所以我得找个其他的方法，幸好，Arch有自己的Arch Way。

(This post is also available in English: link)

在Arch Wiki中提到了两种临时改变MAC地址的方法，使用macchanger或者使用ip命令：

macchanger --mac=XX:XX:XX:XX:XX:XX

或者

ip link set dev eth0 down
ip link set dev eth0 address XX:XX:XX:XX:XX:XX
ip link set dev eth0 up

其中eth0是有线网卡的设备名字。我新装的系统里还没有macchanger，所以我使用了第二种方法(以后应该熟悉一下ip这个程序了，它是用来代替陈旧的ifconfig程序的)。连上互联网以后，你可能会对macchanger这个工具比较有兴趣，它甚至可以为你的设备增加一个随机的MAC地址，非常有趣。

使用上述两个方法修改MAC地址都是临时的，重启之后修改就会失效。Arch Wiki给出了一种非常符合Arch Way的方法，可以在启动时就修改MAC地址。只需要创建文件/etc/rc.d/functions.d/macspoof ，文件内容如下：

spoof_mac() {
	ip link set dev eth0 address XX:XX:XX:XX:XX:XX
}
 
add_hook sysinit_end spoof_mac

这个文件在系统初始化过程最后增加了一个钩子函数，这个函数调用ip这个程序修改MAC地址，从而在每次启动时都使用修改的MAC地址。

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中秋快乐！Happy Mid-autumn Festival!

Yuliang Jin — Mon, 12 Sep 2011 02:39:21 +0000

开学快半个月了，一直都没有更新文章。前阵子好多事啊，排版论文，抱布贸丝，又买了个吉他晚上练习，生活充实多了。

不过博客还是不能荒废的，过了中秋节我就继续开博，保持一周两到三篇文章。我要加油哈！

还有，祝大家中秋快乐！Happy Mid-autumn Festival!

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