Tag Archives: howto

How to set up Docker containers in Travis CI

This post outlines a method for using Docker for testing on Travis CI. It may be useful to you if you are a web application developer who uses GitHub.

I use this setup in my web-based word puzzle generator, so that every change is spun up and tested with a web browser before it is merged.

I got the idea for writing this from a few lines in the docker-compose documentation, which suggested that Docker is an easy way to perform automated testing over a running application:

$ docker-compose up -d
$ ./run_tests
$ docker-compose down

This snippet was missing some setup and an example app, but these three lines do all the heavy lifting.

The Docker setup

In order to focus on the Docker setup, I made a server which simply responds to TCP requests on port 5000 with the text “Hello World”.

This file is called server.sh, and sits in a directory called foo_server:

#!/bin/sh
while true; do
  # Send 'Hello World' to anybody who connects on port 5000
  echo "Hello World" | nc -l 5000
done

Alongside it, I added a Dockerfile to instruct docker to execute this tiny ‘application’ in a container, after installing the dependencies. This machine is built from the Docker-official Debian image:

FROM debian
ADD . /usr/share/test-server
WORKDIR /usr/share/test-server
RUN apt-get update && apt-get install --assume-yes netcat-openbsd
CMD ./server.sh

Lastly, a .dockerignore file is used to avoid loading the Dockerfile to the container:

# Ignore docker files
Dockerfile
.dockerignore

In the directory above, a simple test script, test.sh is used to see that the server is returning the expected output:

#!/bin/sh
set -e
expected="Hello World"
actual=`nc -v localhost 5000`
echo "Expecting: $expected"
echo "Server says: $actual"
if [ "$expected" != "$actual" ]; then
  echo "Test failed"
  exit 1
else
  echo "Test passed"
  exit 0
fi

Alongside the test file, a file called docker-compose.yml instructs Docker to create a container out of the foo_server example, and forward port 127.0.0.1:5000 to it.

version: '2'
services:
  foo:
    build: foo_server
    ports:
     - "5000:5000"
    container_name: foo_1

To try it out for yourself, you need a relatively recent version of Docker and docker-compose. The versions provided in Debian were not new enough to execute the examples, but the Docker project provides repos containing newer builds for Debian & Ubuntu. For my distro, the install was:

curl -sSL "https://get.docker.com/gpg" | sudo -E apt-key add -
echo "deb https://apt.dockerproject.org/repo debian-stretch main" | sudo tee -a /etc/apt/sources.list
sudo apt-get update
sudo apt-get install docker-engine
sudo pip install docker-compose

The versions this got me were docker 1.11.2, and docker-compose 1.7.1. Straight after the install, I could deploy & test an example locally:

$ docker-compose up --build -d
$ ./test.sh
$ docker-compose down

The CI setup

I’ll assume that if you’re reading this, you are familiar with the basics of Travis CI. The large block of code below is the .travis.yml file to set up the test machine, then execute the tests against a container.

---
# Use Ubuntu 'trusty' distribution
sudo: required
dist: trusty

install:
  # Update docker-engine using Ubuntu 'trusty' apt repo
  - >
    curl -sSL "https://get.docker.com/gpg" |
     sudo -E apt-key add -
  - >
    echo "deb https://apt.dockerproject.org/repo ubuntu-trusty main" |
     sudo tee -a /etc/apt/sources.list
  - sudo apt-get update
  - >
    sudo apt-get -o Dpkg::Options::="--force-confdef" \
     -o Dpkg::Options::="--force-confold" --assume-yes install docker-engine
  - docker version
  # Update docker-compose via pip
  - sudo pip install docker-compose
  - docker-compose version

before_script:
  - docker-compose up --build -d

script:
  - ./test.sh

after_script:
  - docker-compose down
...

Note: This uses Travis CI’s trusty distribution, which at the time of writing is the newest stable build platform available on Travis CI. This shipped an outdated version of Docker, which had to be installed over. Because the existing Docker was configured, I had to override a debconf prompt, which is why the apt addon syntax was not used to set up dependencies.

Result

The build result for each commit is displayed in Travis CI:

2016-06-ci-result-1

Under this, the output of the passing test script is shown, showing what has been set up:

2016-06-ci-result-2

Using this setup in practice

Moving this from a demo setup to a real setup would be fairly simple:

  1. Replace the installation with a real software stack
  2. Replace the server run with a command to serve the application (such as a Apache HTTP, Tomcat or Node)
  3. Replace the tests with real tests (such as Cucumber or Selenium).

The example in the pre-amble installs a LAMP stack and tests it with Selenium in its CI build.

If your application is a bit larger, your only extra complexity will come from running multiple containers with docker-compose.

Get the code

All of these scripts in a working CI example are available on GitHub:
mike42/minimal-docker-ci

How to edit emulator flags in Android Studio

I’ve recently updated to the new IntelliJ-based Android studio 2.1.

I ran into some issues attempting to launch a “hello world” project in the emulator, which I’m writing up here for the benefit of others.

For context, I run Debian GNU/Linux Jessie on an AMD64 box, with Radeon graphics card, using the free drivers.

The OpenGL error

2016-05-22-android-1

When attempting to launch a simple project with the emulator, the emulator died with the following message:

Cannot launch AVD in emulator.
Output:
libGL error: unable to load driver: radeonsi_dri.so
libGL error: driver pointer missing
libGL error: failed to load driver: radeonsi
libGL error: unable to load driver: swrast_dri.so
libGL error: failed to load driver: swrast
X Error of failed request:  GLXBadContext
  Major opcode of failed request:  155 (GLX)
  Minor opcode of failed request:  6 (X_GLXIsDirect)
  Serial number of failed request:  49
  Current serial number in output stream:  48
libGL error: unable to load driver: radeonsi_dri.so
libGL error: driver pointer missing
libGL error: failed to load driver: radeonsi
libGL error: unable to load driver: swrast_dri.so
libGL error: failed to load driver: swrast
X Error of failed request:  GLXBadContext
  Major opcode of failed request:  155 (GLX)
  Minor opcode of failed request:  6 (X_GLXIsDirect)
  Serial number of failed request:  49
  Current serial number in output stream:  48
libGL error: unable to load driver: radeonsi_dri.so
libGL error: driver pointer missing
libGL error: failed to load driver: radeonsi
libGL error: unable to load driver: swrast_dri.so
libGL error: failed to load driver: swrast
X Error of failed request:  BadValue (integer parameter out of range for operation)
  Major opcode of failed request:  155 (GLX)
  Minor opcode of failed request:  24 (X_GLXCreateNewContext)
  Value in failed request:  0x0
  Serial number of failed request:  33
emulator: WARNING: VM heap size set below hardware specified minimum of 228MB
emulator: WARNING: Setting VM heap size to 384MB
  Current serial number in output stream:  34
QObject::~QObject: Timers cannot be stopped from another thread

The console logs that the command being executed is:

/home/mike/Android/Sdk/tools/emulator -netdelay none -netspeed full -avd Nexus_5X_API_23

Fixing on the command-line

Quick list of things that didn’t work:

  • Installing more libraries via apt-get
    apt-get install libstdc++6 xserver-xorg-video-radeon
  • Pre-loading libraries
    LD_PRELOAD='/usr/lib64/libstdc++.so.6' /home/mike/Android/Sdk/tools/emulator -netdelay none -netspeed full -avd Nexus_5X_API_23
  • Attempting to understand LibGL’s debug output
    LIBGL_DEBUG=verbose /home/mike/Android/Sdk/tools/emulator -netdelay none -netspeed full -avd Nexus_5X_API_23

But the solution turned out to be this very specific flag:

$ ./emulator -help | grep libstdc++
    -use-system-libs               Use system libstdc++ instead of bundled one

So launching the emulator from the CLI worked for me, using this command:

/home/mike/Android/Sdk/tools/emulator -netdelay none -netspeed full -avd Nexus_5X_API_23

Fixing in the IDE

So this was fantastic progress, but without being able to launch this emulator from Android Studio, the development environment wasn’t really integrated yet.

Apparently there used to be a feature for adding command-line flags to the emulator, but this is now gone.

So, adapting a post here, I jumped in and replaced the emulator binary into a wrapper to inject some flags.

 $ cd ~/Android/Sdk/tools/
$ mv emulator emulator.0
$ touch emulator
$ chmod +x emulator

I then opened up the emulator in a text editor and punched in this:

#!/bin/sh
set -ex
$0.0 $@ -use-system-libs

Result

Success!

2016-05-22-android-2

Quick guide: Running stock Debian on the Raspberry Pi 2

At the time of writing, the ‘Raspbian’ port of Debian is often used on the Raspberry Pi. It was created to match the CPU architecture, for better performance. These reasons don’t apply to the newer Raspberry Pi 2, so if you’re a Debian desktop or server user, you can do away with the fork and just run Debian Jessie armhf.

The info from Debian is: https://wiki.debian.org/RaspberryPi2

A bit more background about why this only applies to the Raspberry Pi 2-

  • The Raspberry Pi 1 uses ARMv6 chipset with hard floats
    • The Debian armhf port requires ARMv7
    • The Debian armel port doesn’t use hard floats, so is unnecessarily slow on the Pi.
    • So Raspbian was created for the Raspberry Pi 1’s ARMv6 w/ hard-floats, and gets the most juice out of the CPU on the Raspberry Pi 1.
  • The Raspberry Pi 2 uses ARMv7 with hard floats, so Debian armhf port is fine.

Install the image

Image is linked to from this page:

I will assume that your machine has an SD card slot. To find the device name, list out disks and look for one of the correct size, which appears when you plug in the card:

df

Download a copy of the image, extract it out, and dd the file on to the card:

wget -c https://images.collabora.co.uk/rpi2/jessie-rpi2-20150705.img.gz
gunzip jessie-rpi2-20150705.img.gz 
sudo dd if=jessie-rpi2-20150705.img of=/dev/sdX bs=4M
sudo sync
umount /media/$USER/*

Plug in the Raspberry pi, and then log in. If you are using SSH, then arp-scan is a good tool to pick up devices on the network:

sudo apt-get install arp-scan
sudo arp-scan -l
ssh root@x.y.z.w

Configure pi- Things like screen resolution and HDMI go here:

cd /boot/firmware/
nano config.txt

Perform a software upgrade:

nano /etc/apt/sources.list
apt-get update
apt-get dist-upgrade

Start fixing security defaults. Remember that this is not a clean install, so start by setting your own passwords:

passwd

Check that there are no other accounts with passwords set:

cat /etc/shadow

Regenerate all SSH Server keys (commands from here):

ssh-keygen -f /etc/ssh/ssh_host_ecdsa_key -N '' -t ecdsa -b 521
ssh-keygen -f /etc/ssh/ssh_host_dsa_key -N '' -t dsa
ssh-keygen -f /etc/ssh/ssh_host_rsa_key -N '' -t rsa

Lastly, generate some locales:

sudo locale-gen en_US en_US.UTF-8 en_GB en_GB.UTF-8

Convert a PC to a HTPC with Debian and Kodi

I recently converted an old workstation to run as a home-theatre PC (HTPC). I’ve noted down the setup here for others who are making an installation like this. Some steps depend on using a radeon chipset, and will need to be adjusted for your computer.

Hardware

First up, Desktop ‘towers’ are not a good form-factor for sitting in TV cabinets. If your PC is this sort of size, then source a small form-factor case and power supply, and load the computer’s components into it:

2016-02-htpc

I also used a Logitech k400r keyboard and mouse for wireless input.

Install Debian and apps

Write the latest copy of Debian Stable to a CD or flash drive (this is version 8.3 at time of writing), and install it on the computer. Check “Debian Desktop environment” / GNOME during setup.

After installation, open a terminal, and type “su” to get root privileges.

su

Edit the software sources to include ‘contrib’ and ‘non-free’, as well as ‘jessie-backports’.

nano /etc/apt/sources.list
deb http://ftp.us.debian.org/debian/ jessie main contrib non-free
deb-src http://ftp.us.debian.org/debian/ jessie main contrib non-free

deb http://security.debian.org/ jessie/updates main contrib non-free
deb-src http://security.debian.org/ jessie/updates main contrib non-free

# jessie-updates, previously known as 'volatile'
deb http://ftp.us.debian.org/debian/ jessie-updates main contrib non-free
deb-src http://ftp.us.debian.org/debian/ jessie-updates main contrib non-free

# jessie-backports
deb http://ftp.us.debian.org/debian/ jessie-backports main contrib non-free
deb-src http://ftp.us.debian.org/debian/ jessie-backports main contrib non-free

Update sources and install Kodi:

apt-get install --install-suggests kodi

Also install the firmware packages that you may need.

apt-get install firmware-linux-free firmware-amd-graphics

Tweaks

Sudo

sudo allows you to run commands as root from your regular user account. Install the package and add yourself to the sudo group:

apt-get install sudo
usermod -a -G sudo mike

To apply the change, log out and back in again. The rest of this guide assumes you are logged in as yourself, and will use sudo where necessary.

Auto-start Kodi

Open the tweak tool, and locate the list of startup programs.

gnome-tweak-tool

Add Kodi to the list, log out, log in, and Kodi will launch automatically.

Auto-login

For a PC attached to a TV, user permissions are not so importnat, so set the user to log in automatically.

sudo nano /etc/gdm3/daemon.conf

Un-comment this block and enter your username:


# Enabling automatic login
#  AutomaticLoginEnable = true
#  AutomaticLogin = user1

Plymouth start-up screen

Install plymouth and configure grub to change the Debian boot sequence (a menu with timeout, followed by lots of text) into a graphical splash screen. This takes a bit of configuration.

sudo apt-get install plymouth

Set it up according to these instructions:

sudo nano /etc/initramfs-tools/modules

Set drm correctly for your chipset:

# KMS
drm
radeon modeset=1

Configure grub:

sudo nano /etc/default/grub
...
GRUB_TIMEOUT=0
...
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
...
GRUB_GFXMODE=1920x1080
...

Update grub, set the theme in Plymouth:

sudo update-grub2
sudo /usr/sbin/plymouth-set-default-theme --list
sudo /usr/sbin/plymouth-set-default-theme joy

Run update-initramfs to apply the changes

sudo update-initramfs -u

Samba

Samba will let you share folders over your network. A basic folder with guest read/write is simple to set up:

sudo apt-get install nautilus-share samba libpam-smbpass winbind
sudo usermod -a -G sambashare mike

Log out, and back in to apply the group change, and then share the Public folder over the network by right-clicking on it and opening the “Sharing Options”:

2016-02-samba-share

Gnome will warn that the folder as shared if you open it:

2016-02-samba-shared

Test the setup by typing smb://localhost into the address bar:

2016-02-samba-test

Overscan correction

In my case, I was able to set the TV to treat the input as a “PC” input. If that doesn’t work for you, then use xrandr in a login script:

Find the name of your input:

xrandr --query

Set underscan (get the horizontal and vertical values by trial and error):

xrandr --output HDMI-0 --set underscan on
xrandr --output HDMI-0 --set "underscan hborder" 32 --set "underscan vborder" 16

Kodi plugins

Add these as needed. The Australian catchup TV plugins repository from GitHub worked well.

Kodi RSS

The RSS feed shows Kodi updates by default, and is part of your user profile.

2016-02-feed-file

Edit the configuration file, and adjust the paths to your news sources of choice.

2016-02-feed

Boot speed

Readahead is the tool of choice for boot speed optimisation. Install it, and reboot.

sudo apt-get install readahead
sudo touch  /.readahead_collect
sudo reboot

Desktop Apps

If you quit Kodi, you are dropped back to the GNOME desktop. These apps are simply to improve the desktop user experience.

Google Chrome

Download the .deb file for Chrome from Google, install with dpkg, and then clean up dependencies:

dpkg -i google-chrome-stable_current_amd64.deb 
apt-get -f install

Firefox

Download and extract the Firefox for Linux tarball from Mozilla.

Move it to /usr/share, and change the owner to match other applications there.

mv firefox /usr/share/
cd /usr/share/
ls -Ahl
chown root:root firefox
chown -R root:root firefox

Find the main menu editor, and add Firefox to the menu.

2016-02-menu

Name
Firefox Web Browser
Command
/usr/share/firefox/firefox-bin
Icon
/usr/share/firefox/browser/icons/mozicon128.png

2016-02-firefox-icon

2016-02-firefox-menu

Test the new icon by searching:

2016-02-ff

Auto-clear browser profiles

Because you don’t need a password to log in to the user account, you can add this as a bit of insurance so that your box wont remember any passwords or sessions.

crontab -e

This job removes the Firefox and Chrome user profiles each boot.

@reboot rm --preserve-root -Rf --~/.config/google-chrome ~/.cache/google-chrome ~/.mozilla/firefox ~/.cache/mozilla/firefox

VLC

For file format support, best to have another media player:

sudo apt-get install vlc

Result

You should now have a PC which boots into Kodi for media and TV, and lets you quit into a desktop to browse the web or run regular desktop apps.

2016-02-kodi

2016-02-gnome

On the 1GB RAM/ dual core workstation, the shortened the boot to around 45 seconds from BIOS handing over control, to Kodi being ready.

How to print red/black on an impact receipt printer

I recently deployed an Epson TM-U220 impact receipt printer. These printers work by striking a ribbon onto the paper, like a type-writer. One of the up-sides to using these intead of a thermal printer is the ability to install a red/black ribbon in place of the default (black) one:

2015-10-colour-receipt-printing-1

2015-10-colour-receipt-printing-2

I connected up my printer using a USB-parallel cable, so my previous posts (Linux, Windows) apply for the connector setup.

Using the escpos-php driver on GitHub, a line of red text is printed like this:

<?php
/*
 * Example of two-color printing, tested on an epson TM-U220 with two-color ribbon installed.
 */
require __DIR__ . '/autoload.php';
use Mike42\Escpos\Printer;
use Mike42\Escpos\PrintConnectors\FilePrintConnector;
$connector = new FilePrintConnector("php://stdout");
$printer = new Printer($connector);

try {
    $printer = new Escpos($connector);
    $printer -> text("Hello World!\n");
    $printer -> setColor(Printer::COLOR_2);
    $printer -> text("Red?!\n");
    $printer -> setColor(Printer::COLOR_1);
    $printer -> text("Default color again?!\n");
    $printer -> cut();
} finally {
    /* Always close the printer! */
    $printer -> close();
}

With this result:

2015-10-colour-receipt-printing-3

How to connect a USB receipt printer up on Mac OS X

This post will show you how to set up a USB receipt printer on Max OS X. These steps were written on Yosemite, but should work on 10.6 onwards (ie, also Snow Leopard through to El Capitan).

This is another post in a series, which has so far covered direct USB printing on Windows and Linux. The printer tested here is this Epson TM-T20:

2015-03-printer-back
2015-03-printer-top

CUPS is the printing system that’s used on Mac, but most users would be more familiar with the system print dialog:

2015-11-mac-usb-epson-12

In our case, we need to set up the printer via the CUPS web interface. This is accessed via a web browser at this address:

http://localhost:631

At first, you will get knocked back:

2015-11-mac-usb-epson-0

To fix this up, open up Applications → Utilities → Terminal and type in:

cupsctl WebInterface=yes

You can then reload the browser and click through to Administration:

2015-11-mac-usb-epson-1

Click Add Printer and log in:

2015-11-mac-usb-epson-2

2015-11-mac-usb-epson-3

Select the USB printer from the list, and optionally share it:

2015-11-mac-usb-epson-4

2015-11-mac-usb-epson-5

Click Select Another Make/Manufacturer, and select Raw → Raw Queue:

2015-11-mac-usb-epson-6

2015-11-mac-usb-epson-7

2015-11-mac-usb-epson-8

Use the defaults for the other options:

2015-11-mac-usb-epson-9

Test print

Type some junk into a file called foo.txt and attempt to print it, using the CUPS printer name:

nano foo.txt
lpr -o raw -H localhost -P EPSON_TM-T20 foo.txt

The prints will be delayed for a few moments, as CUPS spools the jobs.

Disable CUPS web

Once you’re done, for security reasons you should reset this option from before, to disable the web interface to CUPS:

cupsctl WebInterface=no

Fix merge conflicts in git with Meld

When you’re writing code collaboratively, there’s plenty of situations when you need to combine two sets of changes.

This could happen, for example, if Bob and yourself both fix different bugs by making edits to the same file.

foo

This post assumes that your source code is tracked in git.

First up, install meld. The Meld homepage has instructions for other platforms, but on Debian/Ubuntu, it’s just:

sudo apt-get install meld

Now tell git to use it as a tool:

git config --global merge.tool meld

Once you have a merge conflict, you can then fire up Meld like this:

git mergetool

For each file, you will get a 3-way diff. Click the arrows on the sides to move the code you want into the middle:

2015-10-meld

Once you’ve saved the file and closed Meld, you will be prompted on the command-line. You just tell it whether you’ve successfully merged the file, until it stops giving you new files to merge.

After this, commit the changes:

git commit

Done!

Simpler usage

If you don’t use git, you can simply call Meld from the command-line as well. This shows you differences between files in a similar window, and lets you move blocks of code around as well:

meld foo.c bar.c

It’s time to migrate away from Outlook Express

Outlook Express is obsolete, so you need to migrate if you’re still using it. This post is a quick guide to saving your local data so that you can jump ship.

If you are keen on desktop-based email, then there are only two real contenders for a replacement mail client:

  • Mozilla Thunderbird (suggested).
  • Windows Mail.

So where is all my data?

The script below is a Windows bat script for backing up an Outlook Express setup. Just fill in the Identity variable and run it from anywhere to produce a folder containing the Outlook saved emails and contacts.

You can find your identity string as a folder name in your profile path, under Local Settings\Application Data\Identities\. The profile path for a user called bob would usually be in a folder like C:\Documents and Settings\bob or C:\Users\bob.

These files can be read by both of the suggested replacements, so a good transition might be:

  1. Make a backup.
  2. Install an alternative & set it up.
  3. Try to import as much as you can.
  4. Once you’re happy, uninstall Outlook Express, or at least delete the shortcuts to it.
@echo off
ECHO -------------------------------------------------------------------------------
SET BACKUPDIR="%username% - Outlook Backup"
SET IDENTITY={AAAAAAAA-AAAA-AAAA-AAAA-AAAAAAAAAAAA}
ECHO Backing up outlook files to: %backupdir%
ECHO --------------------------------------------------------------------------------

ECHO - Clearing backup location... (1 of 3) 
DEL /F /S /Q %backupdir% > NUL 2>&1
MKDIR %backupdir% 2> NUL
ECHO - - Done

ECHO - Copying emails... (2 of 3) 
MKDIR %backupdir%\Emails 2> NUL
XCOPY /E /H /C /R /Y "%userprofile%\Local Settings\Application Data\Identities\%IDENTITY%\Microsoft\Outlook Express" %backupdir%\Emails > NUL
ECHO - - Done

ECHO - Copying address book... (3 of 3)
MKDIR %backupdir%\AddressBook 2> NUL
XCOPY /E /H /C /R /Y "%appdata%\Microsoft\Address Book" %backupdir%\AddressBook > NUL
ECHO - - Done

ECHO - Completed 3 of 3 tasks.
ECHO -------------------------------------------------------------------------------
ECHO The backup is complete.
ECHO Please copy %BACKUPDIR% to external storage.
ECHO --------------------------------------------------------------------------------
PAUSE

How to create an animated GIF from a series of images

Sometimes, you end up with a folder full of images, which you want to animate. With the open source ImageMagick tool, this is easy on the command line:

animate *.png

This will show you all of the PNG files in the folder in quick succession, like a flip book.

ImageMagick works on just about any OS. For Linux users, the package is generally imagemagick or ImageMagick:

sudo apt-get install imagemagick
yum install ImageMagick

But this blog post is about animated GIFs, so lets make one of those. This is a compact way to combine images (here and here for examples in context), gives you a re-usable at-a-glance illustration of something that changes over time.

Example from an older post:

2015-04-tetris

The steps to make a good conversion command are:

  1. Check that alphabetically, your images are in order. If not, rename them:
    echo *
  2. Convert them to a GIF a few times, and find the delay that suits you (hundredths of a second between frames)
    convert -delay 80 *.png animated.gif
  3. Choose an output size (width x height):
    convert -resize 415x -delay 80 *.png animated.gif
  4. Compress with -Layers Optimize for a smaller file:
    convert -resize 415x -delay 80 *.png -layers Optimize animated.gif

Notes

  • Generated thumbnails usually take the first frame only, which is why we ask Imagemagick to resize it (WordPress users: Choose “Full Size”).
  • To pause at the start of the loop for a moment, just copy the first image a few times.

How to use a Raspberry Pi as a print server

This post is designed for people who want to share a simple USB printer, such as this receipt printer, over the network.

Usually, you just connect up the printer to the computer like this:

2015-04-rpi-printer1

But if you are sending the print jobs from a central server, you would instead follow these steps, and hook up a Raspberry Pi near the printer to pass on the print-outs for you:

2015-04-rpi-printer2

This post will show you a very fuss-free way to do this. Because of its simplicity, if you have multiple computers printing (read: you need a server that can spool), or need two-way communication with the printer, then this setup will not be sufficient for your use case.

One-off setup

If your printer is /dev/usb/lp0, then the command to run is:

nohup nc -klp 9100 > /dev/usb/lp0 2> /dev/null&

There is quite a lot going on in this command, so I’m going to break it down into parts and explain what each one does.

nohup
Lets the command keep running after you log-out.
nc -klp 9100
Listens on port 9100 (-lp), and returns to listening after each connection (-k)
> /dev/usb/lp0
Redirects any incoming data to the printer device
2> /dev/null
Suppresses errors by sending them to /dev/null
&
Runs the command in the background so that you can keep using the terminal.

Run every boot

Simply schedule the command in cron as a @reboot task.

crontab -e

And add the line:

@reboot nohup nc -klp 9100 > /dev/usb/lp0 2> /dev/null&

Note that if you reboot the printer, you will also need to reboot the raspberry pi to get it to reconnect without logging in!

Send some tests

From a computer somewhere else on the network, send a test print-out:

echo "Hello world" | nc 10.x.x.x 9100

If the target printer is a thermal receipt printer, then you could also use escpos-php to send it more elaborate commands:

<?php
$fp = fsockopen("10.x.x.x", 9100);
/* Print a "Hello world" receipt" */
$printer = new Escpos($fp);
$printer -> text("Hello World!\n");
$printer -> cut();
fclose($fp);