Read/write the brightness on my Ubuntu 14.04 LTS installation on my ThinkPad 440s.

bright.c

/*
  The build of this file should have the user id bit set and the owner should be root (and this must be on an unencrypted file system which honours the user id bit) because the system file it must modify is writable only by the root user.

  Usage:
    bright -- read current level
    bright <number> -- set bright to <number>
    bright +<number> -- increment bright by <number>
    bright -<number> -- decrement bright by <number>
*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define FILENAME "/sys/class/backlight/intel_backlight/brightness"
#define BRIGHTMAX 150
#define BRIGHTMIN 1

FILE * out;

int newValue; // value to be set

// Read current value from file
int readCurrentValue ();

// Apply user input to current value arithmetically, return sum/difference
int inferNewValueFromModifier (char * argv[], int multiplier);


int inferNewValueFromModifier (char * argv[], int multiplier)
{
	int modifier;
	sscanf(&argv[1][1], "%d", &modifier);
	return (modifier * multiplier) + readCurrentValue();
}



// Main
int main (int argc, char * argv[])
{
	int res;

	if (argc <= 1) {
		printf("Usage: $0 <brightness>\nCurrent brightness: ");
		printf("%d\n", readCurrentValue());
	}
	else {
		int argIntPart;
		sscanf(argv[1], "%d", &argIntPart);
		if (argv[1][0] == '-')
			if (strlen(&argv[1][1]))
				newValue = readCurrentValue() + argIntPart; // argIntPart will be negative if there was a minus on the command line
			else
				newValue = readCurrentValue() / 2;
		else if (argv[1][0] == '+')
			if (strlen(&argv[1][1]))
				newValue = readCurrentValue() + argIntPart;
			else
				newValue = readCurrentValue() * 2;
		else
			newValue = argIntPart;
		/* Require that newValue for bright never exceeds BRIGHTMAX and never exceeds BRIGHTMIN */
		if (newValue > BRIGHTMAX)
			newValue = BRIGHTMAX;
		else if (newValue < BRIGHTMIN)
			newValue = BRIGHTMIN;
		/* Write newValue for bright to file. The OS will observe the change and make the screen adjustment accordingly. */
		out = fopen(FILENAME, "w");
		if (!out) {
			fputs("Can't open output file " FILENAME "\n", stderr);
			exit(1);
		}
		res = fprintf(out, "%d", newValue);
		if (res < 1) {
			fputs("Failed to write to output file " FILENAME "\n", stderr);
			exit(2);
		}
		fclose(out);
		fprintf(stdout, "Set brigtness to %d\n", newValue);
	}
}

int readCurrentValue ()
{
	int curValue, res;
	FILE * in = fopen(FILENAME, "r");
	if (!in)
	{
		fputs("Can't open input file " FILENAME "\n", stderr);
		exit(3);
	}
	res = fscanf(in, "%d", &curValue);
	if (res < 1)
	{
		fputs("Failed to read current value from input file " FILENAME "\n", stderr);
		exit(4);
	}
	fclose(in);
	return curValue;

}

bright.desktop

[Desktop Entry]
Name=Adjust Bright
Exec=/usr/bin/env python /usr/bin/local/brightener.py
Icon=/home/markham/Pictures/Clipart/crops/ghost-blade-128x128.jpg
Type=Application
Terminal=false
StartupNotify=true
Categories=GTK;GNOME;Utility;

Brighten.md

Convenient CLI and GUI for adjusting the brightness on my Ubuntu installation in my Thinkpad T440

brightener.py

# Provides a GUI of buttons for quick, repeated clicks to adjust monitor brightness.

from Tkinter import *
from subprocess import Popen, PIPE
from threading import Timer
import ImageTk
import re

def getImg(basename):
	img = ImageTk.Image.open("/home/markham/Pictures/Clipart/"+basename)
	img.resize((150,150), ImageTk.Image.ANTIALIAS)
	pi = ImageTk.PhotoImage(img)
	return pi

class CP(Frame):
	def __init__(self, parent):
		self.parent = parent
		Frame.__init__(self, parent)
		self.initUI()
		self.parent.bind("<Control-q>", self.close)
		self.parent.bind("<Control-w>", self.close)
		self.set_and_get(None)

	def initUI(self):
		self.pack(fill=BOTH, expand=True)

		self.imgBelmont = getImg("crops/belmont-red-128x128.jpg")
		self.imgBelmon2 = getImg("crops/belmont-cross-128x128.jpg")
		self.imgGhost = getImg("crops/ghost-blade-128x128.jpg")
		self.imgGrave = getImg("grave-angel.png")

		imgFrame = Frame(self)
		imgFrame.pack(side=TOP)
		entryFrame = Frame(self)
		entryFrame.pack(side=BOTTOM)

		frame1 = Frame(imgFrame)
		frame1.pack(side=LEFT)
		up5Btn = Button(frame1, text="+5", image=self.imgBelmont, command=(lambda : self.set_and_get('+5')))
		up5Btn.pack(side=TOP, padx=5)
		dn5Btn = Button(frame1, text="-5", image=self.imgBelmon2, command=(lambda : self.set_and_get('-5')))
		dn5Btn.pack(side=BOTTOM, padx=5)

		frame2 = Frame(imgFrame)
		frame2.pack(side=RIGHT)

		up2Btn = Button(frame2, text="x2", image=self.imgGhost, command=(lambda : self.set_and_get('+')))
		up2Btn.pack(side=TOP, padx=5)
		dn2Btn = Button(frame2, text="/2", image=self.imgGrave, command=(lambda : self.set_and_get('-')))
		dn2Btn.pack(side=BOTTOM, padx=5)

		self.entry = Entry(entryFrame, width=4)
		self.entry.pack(side=LEFT, pady=8)
		entryBtn = Button(entryFrame, text="Change", command=(lambda : self.set_and_get(self.entry.get())))
		entryBtn.pack(side=RIGHT, pady=8)

	def set_and_get(self, arg):
		if hasattr(self, 'timer') and self.timer:
			self.timer.cancel()
		self.timer = Timer(7, self.close, (self,))
		self.timer.start()
		params = ['bright', arg] if arg else ['bright']
		p = Popen(params, stdout=PIPE)
		out, err = p.communicate()
		m = re.search('(\d+)\s*\Z', out)
		if m != None:
			self.entry.delete(0, END)
			self.entry.insert(0, m.group(1))

	def close(self, evt):
		self.parent.destroy()


if __name__ == '__main__':
	root = Tk()
	root.geometry("276x310")
	root.title("Brighten Your Day")
	app = CP(root)
	root.mainloop()

Makefile

build: bright.c
	gcc -o bright bright.c

install:
	chown root bright
	chmod u+s bright
	mv bright /usr/local/bin/