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How to install Slackware from a USB flash drive ?

tl;dr : use the usbboot.img file from the iso file to boot, then use the content from the iso (extracted to another pen drive or to a partition) to install.

We're not in 2007 anymore, and I was unable to find a spare DVD-R to burn the Slackware ISO. I read online you can install Slackware from a USB flash drive, but there is no IMG file to be found ! Well, there are a few tutorials online about it, but I found them bit lacking - or let's say that they felt clearer once I had done the install myself. So here is a break-down on the installation, that I hope will help newcomers who wonder about unetbootin or the lack of a "all-in-one" USB medium that does everything the same way as other Linux distribution.
So, first things first :
- You don't need unetbootin,
- the ISO file is not a usb bootable image. Don't try, I did and felt stupid afterward.

It is actually pretty easy. I installed Slackware 14.2 this way and was pleasantly surprised by the simplicity and efficiency of the installer (despite its tedious loading time).

You will need :
- the Slackware installation ISO file, that is a 3Go download, either from a torrent (http://www.slackware.com/getslack/torrents.php) or a mirror (https://mirrors.slackware.com/)
- any program to extract the content of an ISO file to your hard drive (Winimage or Izarc on Windows for example),
- one USB flash drive with 1Go memory, that will be erased,
- if you are going to install Slackware on another computer, some additional storage is required to hold the installation medium. A 4Go USB drive is perfect, and you don't even have to format it. Just drop the files extracted from the ISO there.
- any program to write a IMG file to your USB drive (WIN32diskimager for example)
- some basic knowledge of linux command line. While I found Slackware surprisingly easy to set-up and use, I would not recommend it as a first dip in the Linux world (except maybe if you come from BSD systems, where you may find yourself more at home on this sensible OS compared to the frantic and automated world of the most popular distributions)
- to know how to partition your hard drive.

What to do next ?

The quick explanation :
Extract the ISO file. Use the "usb-and-pxe-installers/usbboot.img" image to boot the installer, and point it toward the extracted content of the ISO file (either on a hard drive partition or a mobile device) to install Slackware.

The detailed explanation :
1 - Extract the content of the Slackware ISO file somewhere on your hard drive. You should get a bunch of folders and text files, that from now on I will call "ISO extract". Now is a good time to have a look at the "Slackware-HOWTO" file. If you like well-written documentation, you are going to love Slackware.
2 - Find the "usbboot.img" file in the "usb-and-pxe-installers" folder, and with your IMG writer program write it on your USB drive (remember that it will wipe its content)
3 - (optional) Copy all the ISO extract to your second USB drive
4 - Now is the time to install : boot on the bootable USB drive and follow the instructions (probably pressing "enter" and waiting). Don't forget to connect the other drive if also needed.
5 - Format the hard drive. I recommend using the excellent cfdisk program. Remember to create at least a LINUX SWAP partition, and your data partition. write down the name of the partition holding your ISO extract (usually /dev/sdaX for a hard drive or /dev/sdcX for a flash drive),
6 - mount this partition. A simple "mkdir mntinstall" then "mount /dev/sdxX /mntinstall" will do the job,
7 - Type "setup" to launch the installation program. Feel free to read the documentation (first option) where you can learn useful things, such as using Alt+function keys to swap TTYs if you have some more commands to launch while the setup is opened,
8 - Enter the setup commands (ADDSWAP, TARGET, etc.). Once in the SOURCE screen, write the path to the ISO extract.
9 - Carry on and enjoy the ride.

Note that the bootable USB drive makes a decent recovery system, should you need some way to get a root access to your system or to reinstall LILO, or whatever else.
Oh, and if you plan to use wifi, remember to install wicd from the "extra" folder on the ISO extract (installation details in extra/README.txt). It makes wireless connexions in a breeze.
If you need a thorough guide on system administration, look for the Slackbook (ISO extract, "slackbook" folder).