tee: Replicate Standard Output

Now and then I come across a situation when I need to run a script or a Unix command and would like to not only see the output of it on the screen, but also save this output to some log file. Redirecting the standard output using standard Unix stream redirection isn't always useful because your output will either be shown to you, or sent to the file – but not both at the same time

tee command

That's where the tee command becomes really useful. You pipe your output to this command, and let it take care of the rest.

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How To Find Out RedHat Version And More

If you're a really curious mind, you won't be satisfied with simply knowing the current release of your RedHat Linux, that's why there's a few more commands you could use to satisfy your interest.

RedHat release

If you simply want to confirm whether you're using a RHEL4, RHEL5 or any of the previous RedHat Linux releases, this is the first place to look:

bash-3.1$ cat /etc/redhat-release
Red Hat Enterprise Linux Client release 5 (Tikanga)

RedHat kernel version and type

Next step is to find out the exact Linux kernel version on your system, and also confirm whether it's 64-bit or not:

bash-3.1$ uname -a
Linux rhserver123 2.6.18-8.el5 #1 SMP Fri Jan 26 14:15:14 EST 2007 x86_64 x86_64 x86_64 GNU/Linux

RedHat kernel build

For the most curious ones, here's the last command. Use it to confirm who and when compiled the RedHat kernel you're using, and what gcc compiler was used in the build process.

bash-3.1$ cat /proc/version
Linux version 2.6.18-8.el5 (brewbuilder@ls20-bc1-14.build.redhat.com) (gcc version 4.1.1 20070105 (Red Hat 4.1.1-52)) #1 SMP Fri Jan 26 14:15:14 EST 2007

See also:

How To Take A Screenshot in Unix (xwd)

Quite often there's a need for you to take a screenshot of your Unix desktop, and as always there's a number of ways to do it. Today I'm going to cover the command line approach to taking screenshots.

Taking a Screenshot with xwd

Most modern Unix desktop systems come with Gnome desktop environment by default, and use Xorg as their default X11 server. This means you are likely to have the xwd tool in your OS, which allows you to take screenshots.

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visudo tutorial

visudo is a tool for safely updating the /etc/sudoers file, found in most Linux systems (Ubuntu for example).

Here's what the Ubuntu man page says about it, I think it's a great summary:

visudo edits the sudoers file in a safe fashion, analogous to vipw(8). visudo locks the sudoers file against multiple simultaneous edits, provides basic sanity checks, and checks for parse errors. If the sudoers file is currently being edited you will receive a message to try again later.

Attention: due to the sensitive content of the /etc/sudoers file, you can only run visudo as root.

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How To Mount An ISO image

Mounting an ISO image of a CD/DVD before burning it is one of the basic steps to verifying you're going to get exactly the desired result. It's also a neat trick to access files from a CD/DVD image when you only need a file or two and not a whole CD. Why burn it at all when you can access files much quicker and easier by simply mounting the ISO image?

Every Unix OS has a way to access ISO filesystem, and today I'll only give you examples for Linux and Solaris. In both cases, the two things you need for the example to work are the ISO image itself and an available mount point (basically, an empty directory) on your filesystem to mount it under.

Here's how to mount an ISO in Linux:

# mount -o loop /net/server/linux-bootcd.iso /mnt

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script: Save Your Session Log

If you ever need to save the history of your Unix shell session, one of the easiest ways to do it is to use the script command, found in most Unix systems.

Simply provide the file name for your log as a command line parameter:

$ script /tmp/unix-session.log

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Perl: Searching Through Directory Trees

I had a need to scan a huge directory tree today, identifying the users and Unix groups owning all the files. The problem I faced was too long usernames and group names which meant the

find /directory -ls

command which I normally use for such tasks wasn't terribly useful because there was no space delimiter between a username and a group. Results of such scan of the directory tree will have to later be parsed by other tools, and that's why proper splitting of the output into separate fields is so important.

 

This issue was motivational enough to refresh my Perl skills and sketch the following script (based entirely on this Never Run Unix Find Again article).

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Unix File Types

In Unix systems, there are 6 file types. Below I will give a very short description of each.

How to find out the type of file in Unix

The first and most obvious way to confirm the type of a particular file is to use the long-format output of ls command, invoked by the -l option:

$ ls -l * 
 -rw-r--r-- 1 greys greys       1024 Mar 29 06:31 text

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How To: Use apt-get behind proxy

If you run your Ubuntu system behind a firewall and have to use proxy server for http and ftp access, then your apt-get on a newly installed Ubuntu system will probably not work.

To make it use proxy, simply set the http_proxy environment variable. Once you get it working (try something like apt-get update), you'll probably want to add it to your .bashrc file.

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URL file-access is disabled in the server configuration

I've recently upgraded Apache and PHP on my VPS, and one of the unpleasant surprises was that some scripts which tried including pages from remote sites (I know, not the most secure approach, but there were reasons for that) got broken.

allow_url_fopen

Traditionally, all the websites Google finds suggest that you double-check that your php.ini config has the allow_url_fopen enabled:

allow_url_fopen = On

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