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Arcane Linux Commands: dc

Post date: December 31, 2007, 01:12 Category: Miscellaneous Views: 4168 Comments
Tutorial quote: If anybody within earshot of you is struggling to learn sed and ever remarks "This is like learning Klingon! Could they make it any more cryptic?", you can always point them to dc.

dc is the command-line Unix "Reverse-Polish Notation"(RPN) calculator. The name stands for "desktop calculator". OK, so what is a Polish calculator and why would you want to reverse one? The math that you do in school uses infix notation, with the operator between the numbers (3 + 5). Prefix notation puts the operator first (+ 3 5) and is what the Lisp language uses. The prefix notation is known as "Polish notation" after the nationality of Jan Łukasiewicz who invented it. Postfix notation, then, has the operator at the rear (3 5 +), and so is also the reverse of Polish notation.

What's the difference? In computer programming, you have to specify what order you want a multi-part calculation to be in. Everybody is familiar with the old problem of A*B-C. For A=5, B=3, C=2, multiplying first and subtracting second gives you 13, while subtracting first and multiplying second gives you 5. To specify which operations you want performed first, you have to either memorize the complex orders of operations (which change from one language to the next) or use a lot of parenthesis ((A*B)-C) or (A*(B-C)). Hey, that's what everybody complains about in all those languages that use parenthesis!

Database Server With postgresql and pgadmin3 in Ubuntu

Post date: April 20, 2008, 18:04 Category: Software Views: 4191 Comments
Tutorial quote: PostgreSQL is a powerful, open source relational database system. It has more than 15 years of active development and a proven architecture that has earned it a strong reputation for reliability, data integrity, and correctness. It runs on all major operating systems, including Linux, UNIX (AIX, BSD,HP-UX, SGI IRIX, Mac OS X, Solaris, Tru64), and Windows. It is fully ACID compliant, has full support for foreign keys, joins, views, triggers, and stored procedures (in multiple languages). It includes
most SQL92 and SQL99 data types, including INTEGER, NUMERIC, BOOLEAN, CHAR, VARCHAR, DATE, INTERVAL, and
TIMESTAMP. It also supports storage of binary large objects, including pictures, sounds, or video. It has native programming interfaces for C/C++, Java, .Net, Perl, Python, Ruby, Tcl, ODBC, among others.


Getting started with RHEL4's built-in LVM tools

Post date: June 3, 2005, 16:06 Category: System Views: 6697 Comments
Tutorial quote: Many Unix administrators I know (you know who you are), always used to smirk when I talked about Linux. They could always point to the fact that regardless of whatever I could say, they had journaling file systems, which they could manage using various Logical Volume Management (LVM) tools, and I couldn't touch that.

Well, not any more! Not only does Red Hat offer ext3 as their default file system, but they offer great management tools to boot. As we know, ext2 had a great lifespan, but it was not an enterprise-ready file system that could handle large disk partitions, fast recovery from systems crashes, or large amounts of files. Journaling file systems give you the ability to recover almost instantly from a crash, as you do not need to run fsck after a restart. Similar to how databases recover from crashes, a journaling file system tracks changes to file system metadata and pretty much guarantees that either all or no updates have completed. Of course, these file systems also need elaborate tools to help better configure and manage them accordingly.

Using Software RAID-1 with FreeBSD

Post date: November 29, 2005, 03:11 Category: System Views: 4361 Comments
Tutorial quote: Have you ever needed a software RAID solution for a low-end server install? Perhaps you've wanted your workstation to take advantage of the redundancy provided by a disk mirror without investing in a hardware RAID controller. Has a prior painful configuration experience turned you off software RAID altogether on Unix systems?

Since 5.3-Release, FreeBSD comes with gmirror(8), which allows you to easily configure a software RAID 1 solution. While tutorials on gmirror exist, I found them to require either manual calculations of partition sizes with bsdlabel or the use of a fix-it floppy on an existing system.

It made more sense to me to configure RAID during the install of the operating system. I also wanted a procedure that was easy to follow and didn't introduce human error in the form of a math miscalculation. After cobbling together the available documentation and experimenting my way through various configurations, I came across a procedure that has worked well for me on several different systems. I also received valuable feedback from Pawel Jakub Dawidek, the author of gmirror, who gave some insight into some of the not yet documented features of gmirror.

Easy Automated Snapshot-Style Backups with Linux and Rsync

Post date: February 1, 2006, 00:02 Category: Software Views: 3694 Comments
Tutorial quote: This document describes a method for generating automatic rotating "snapshot"-style backups on a Unix-based system, with specific examples drawn from the author's GNU/Linux experience. Snapshot backups are a feature of some high-end industrial file servers; they create the illusion of multiple, full backups per day without the space or processing overhead. All of the snapshots are read-only, and are accessible directly by users as special system directories. It is often possible to store several hours, days, and even weeks' worth of snapshots with slightly more than 2x storage. This method, while not as space-efficient as some of the proprietary technologies (which, using special copy-on-write filesystems, can operate on slightly more than 1x storage), makes use of only standard file utilities and the common rsync program, which is installed by default on most Linux distributions. Properly configured, the method can also protect against hard disk failure, root compromises, or even back up a network of heterogeneous desktops automatically.

The PartImage Handbook

Post date: May 21, 2005, 15:05 Category: Software Views: 3084 Comments
Tutorial quote: - Partition Image is a Linux/UNIX partition imaging utility: it saves partitions formatted using the Ext2FS (the linux standard), ReiserFS (a new journaled and powerful file system), JFS IBM journaled file systems from AIX, NTFS (Windows NT File System), FAT16/32 (DOS & Windows file systems), or HPFS (OS/2 file system) file system formats to an image file. Only used blocks are copied. The image file can be compressed in the GZIP/BZIP2 formats to save disk space, and split into multiple files to be copied on removable media (ZIP for example), or burned on a CD-R ...

- This allows the user to save a full Linux/Windows system, with a single operation. When problems occur (viruses, crash, error, ...), you just have to restore, and after several minutes, all your system is restored (boot, files, ...), and fully working.

- This is very useful when installing the same software on many machines: just install one of them, create an image, and then restore the image on all other machines. After the first one, each subsequent installation can be made automaticaly, and only requires a few minutes.

Linux Web Filtering in 7 steps

Post date: April 12, 2005, 18:04 Category: Network Views: 4657 Comments
Tutorial quote: How to install a transparent Squid proxy server with real-time HTTP virus scanning on Mandrake 10.0 using DansGuardian and ClamAV?

People quickly and easily access volumes of research on the Internet and correspond with a mouse click. For more and more companies, content filtering is part of the large battle to combat all kinds of online threats, including hackers, worms and viruses. Linux content filtering allows administrators to configure and manage Internet access across the entire network and to block unwanted Web content like pornography, shopping Web sites, games and gambling.

This guide contains all the necessary information for installing and understanding the architectural layout of the implementation. It was written with the assumption that you understand how to install programs and have a basic understanding of Linux Mandrake. This includes installing Linux Mandrake and RPM packages, editing files, making directories, compiling software and understanding general UNIX commands. This guide doesn’t explain how to use or configure Squid, DansGuardian and ClamAV but information on where to obtain this information can be found in the “Additional information” section.
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