Instant Keyboard Response

The user can set the launch angle and the firing velocity in the manner described.. The first two attributes of the –icanon switch erase and kill enable the special characters that will

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C H A P T E R 1 9

Instant Keyboard Response

There are times you may want to enter some input or perform an action without having

to use the Enter key A while back, I encountered this problem when I needed to view the

numerical output of ballistic equations I wanted to be able to increment or decrement an

input value and recalculate the results by pressing just one key I came up with a script

that would do this, and display both the changed input value and the new results

calcu-lated from it The example script here, while interesting, is just a simple demonstration of

how to process instant response

My script accomplished its task quite well The following script is a simplified version

that calculates and displays the values for a projectile’s trajectory The user can set the

launch angle and the firing velocity in the manner described The script will then display

the projectile’s distance, maximum height, and duration of flight

First we initialize some variables and, because we will alter the terminal settings, we

save the current settings so that we can restore them later

#!/bin/sh

old_stty_settings=$(stty -g) # Save original settings

stty -icanon

theta=0

velocity=0

distance=

height=

time=

To do the keystroke processing, we have to first set up the terminal using stty We then

obtain keystrokes using the head (or dd) command, as described later in this chapter

Here, setting stty with the -icanon switch disables canonical mode for terminal

manipu-lation This changes input data from being read as a whole line to a more fine-grained

byte-based input The first two attributes of the –icanon switch (erase and kill) enable

the special characters that will erase the last character typed and erase the current line,

respectively The last two attributes (werase and rprnt) aren’t very common The werase

attribute enables the special character that will erase the last word typed, whereas rprnt

enables the special character that redraws the current line on the screen For more

infor-mation, review the stty man page

122 C H A P T E R 1 9 ■ I N S T A N T K E Y B O A R D R E S P O N S E

The script consists of a loop that repeatedly computes trajectory values based on the current inputs and then updates the input values based on the user’s keystrokes The following code section is where the calculations are performed for the values we seek Every iteration of the loop recalculates the projectile’s distance, height, and duration.1

while :

do

# convert the angle from degrees to radians

angle=`echo "scale=5;$theta/57.29578" | bc -l`

# gravity is 9.8m/s^2

distance=`echo "scale=5;(($velocity^2)*2*(s($angle))*(c($angle)))/9.8" | bc -l` height=`echo "scale=5;(($velocity*s($angle))^2/(2*9.8))" | bc -l`

time=`echo "scale=5;(2*($velocity*s($angle))/(9.8))" | bc -l`

The value of the angle variable is the radian equivalent of the theta value, which is expressed in degrees The bc utility performs trigonometric functions in radians, so this conversion must be performed before angles can be used The -l switch used with bc

is required to load the standard math library that allows bc to evaluate trigonometric functions, such as the cosine of the angle The scale value that is passed to bc sets the number of decimal places that follow the decimal point More information on shell math functions can be found in Chapter 11

Next the script outputs usage instructions that inform the user how to vary the launch angle and velocity and how to quit the program, plus displays the values just calculated from the current values of launch angle and velocity

clear

echo "j to decrease launch angle - k to increase launch angle"

echo "h to decrease launch velocity - l to increase launch velocity"

echo

echo "x or q to exit."

echo

echo "Launch angle deg.=$theta Velocity M/s=$velocity"

echo

echo "Distance: $distance meters"

echo "Maximum height: $height meters"

echo "Flight Time: $time seconds"

The next command is the central one for handling the input of the script.2 Here each character the user enters is assigned to the Keypress variable

Keypress=$(head -c1)

# Keypress=$(dd bs=1 count=1 2> /dev/null)

1 Trajectory calculations can be found at http://hyperphysics.phy-astr.gsu.edu/hbase/traj.html.

2 Thanks to Stephane Chazelas, who supplied the keypress-detection code found on http://tldp.org/ LDP/abs/html/system.html.

C H A P T E R 1 9 ■ I N S T A N T K E Y B O A R D R E S P O N S E 123

Note that two commands could be used The head command on many UNIX systems

normally displays only the first few lines of a file The GNU version of the head command,

however, has a -c option, as shown here The -c switch specifies how many bytes or

char-acters of data to display The line of code that has been commented out uses the dd

command to do the same thing; this command, with the bs (block size) set to 1 byte and a

count of 1, is functionally the same as the head -c1 command You will need only one of

these lines I’ve shown both here because not all UNIX systems have the GNU version of

head that has the -c switch, and I wanted to include an alternative command that can be

used on both GNU and non-GNU systems

Once the Keypress variable has been assigned, you have to decide how to process it

case $Keypress in

j|J)

if [ $theta -ne 0 ]

then

theta=$(($theta-1))

else

theta=90

fi

;;

k|K)

if [ $theta -ne 90 ]

then

theta=$(($theta+1))

else

theta=0

fi

;;

For each of the valid value-updating keystrokes, the new values are validated In the

case of the launch angle, the valid values lie between 0 and 90 degrees If the angle is going

to become greater than 90 or less than 0, we roll the variable to the opposite end of the

valid range If the current angle theta were 90 degrees and you wanted to add another

degree, the value of theta would be reset to zero The converse happens when

decrement-ing an angle of 0 degrees

The launch velocity should, of course, have only a positive value If the value for some

reason goes below 0, we reset it to 0 There is no upper bound on this value, so you can

increase the initial velocity as much as you like

h|H)

if [ $velocity -ne 0 ]

then

velocity=$(($velocity-1))

else

velocity=0

fi

124 C H A P T E R 1 9 ■ I N S T A N T K E Y B O A R D R E S P O N S E

;;

l|L)

velocity=$(($velocity+1))

;;

Note that the keys used to increment and decrement the input values were chosen to match the keys used for cursor movement within vi, where J is down, K is up, H is left, and

L is right

If one of the keys to quit is pressed, we break out of the loop Otherwise the next itera-tion of the loop displays the recalculated trajectory values and waits for more input

q|Q|x|X)

break

;;

esac

done

Finally, the script resets the terminal settings to the original values and exits cleanly

stty "$old_stty_settings"

exit 0