Unix/Linux tutorials for beginners
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.
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.
visudo is a tool for safely updating the /etc/sudoers file, found in most Linux systems (Ubuntu for example). This is the file that is required for allowing regular users run commands with superuser privileges – using sudo command.
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.
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.
# mount -o loop /net/server/linux-bootcd.iso /mnt
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
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).
In Unix systems, there are 6 file types. Below I will give a very short description of each.
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
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.
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.
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
[Read more…] about URL file-access is disabled in the server configuration
If you have used your fresh Ubuntu install for longer than half an hour, chances are that you've discovered the sudo command already.
sudo allows certain users to execute a command under another user's privileges. Most commonly, using sudo implies running a command as a superuser, but the approach works equally well for allowing you to inherit a user ID (uid) and group ID (gid) of any user on the system.
To gain access, a password is asked, and by default it is your password, and not the password of a user you're trying to run a command as. This allows for the system' s administrator to effectively manage user privileges without having any user share their password.
[Read more…] about Ubuntu: Using Sudo to Grant User Privileges
Starting with RHEL4, the system only accepts local (socket-based) X11 server connections. This means that if you go to another Unix server and try forwarding X11 output by using DISPLAY variable to point to your RHEL4 box, it will no longer work.
