Syllabus 20091027

Some examples: "jpegloop" and "foodphotos"

Some exercises to round off our BASH experiences:

Pimp your prompt
Custom commands with shell scripts
Return to "Command-line After Effects" aka Parameterized Movie

Customizing your shell prompt

(Handout: Prompt string customizations)

Making your own custom scripts

1. Save the file in your "bin" folder (or somewhere in your $PATH). You don't need to use the ".sh" extension (if you want a "cleaner" command name). For example, you might call it "myscript".
2. Add the "sh-bang" as the FIRST line in the file:

#!/bin/bash

3. From the terminal:

chmod +x myscript

Positional parameters

Exercise: Cut out a circular shape from an image There isn't a single command to do this in imagemagick (the built in crop command only works for rectangles) but it's possible to combine multiple steps to produce a circular cut-out.

Command Substitution

Command substitution allows you to have the bash do something, then paste the result into your script to do something else with.

$( command here )

A useful example is using imagemagick's identify command to get the width or height of an image.

identify -format %w myimage.png

width=$(identify -format %w myimage.png)
echo Your image is $width pixels wide.

Example: Make an image labeller, using image magick

#!/bin/bash

in=$1
out=$2
text=$3
width=$(identify -format %w $in)

convert -background '#0008' -fill white -gravity SouthWest -size ${width}x36 \
        -font /home/murtaugh/.fonts/NotCourier-sans-Bold.ttf -pointsize 36 \
        caption:"$text" +size $in +swap -gravity south -composite $out

Looping

Counting to 10

for ((i=1;i<=10;i++))
do
echo i say $i
done

Now with play...

for ((i=1;i<=10;i++))
do
play -c1 -r8000 -n synth 0.25 sine 220 vol 0.7
done

Sequential filenames, using command-substitution and the printf command:

f=$(printf %04d.wav $i)

mkdir out

for ((i=1;i<=10;i++))
do
f=out/$(printf %04d $i)
sox -c1 -r44100 -n $f.wav synth 0.25 sine %$i vol 0.7
done

# combine all the individual files to a single file
sox out/*.wav audio.wav

Example with imagemagick:

mkdir out

for (( i=1; i<=100; i++ ))
do
imagefile=$(printf %04d $i)
echo i is $i, imagefile is $imagefile
convert frame.png -blur ${i}x${i} out/$imagefile.png
done

ffmpeg -i out/%04d.png -y -sameq blurry.mpeg

Combining audio & video into a single movie.

ffmpeg -i audio.wav -i video.mpeg final.mpeg

Or using OGG as the video format:

ffmpeg -i audio.wav -i video.ogv final.ogv

On to Python

Central themes: How does the computer deal with different "types" of data. What are "ints", "floats", and "strings" really? What is an encoding? How do numbers, text, image, sound all end up as "one's and zero's" of binary?

Reading for next week: ThinkPython Chapter 2 'plus' additional exercises

Values & Types, Variables, Keywords, Statements, Operators, Expressions, Order of operation, Comments

Linear Mapping: evt. examples in JavaScript (to match their AJAX experiments?)