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Phoshell

  1. Bash script — the script language which programmers love to hate.

It is indispensable, yet impossible to remember.

  1. ffmpeg — perhaps the most powerful and yet useful tool especially in today's world dominated by every growing social media; another tool that programmers love to hate, infamous for its complicated and yet necessary parameters for editing audio and video.

  2. Phoshell — a very thin Bash wrapper based on Phoscript, a simplified metaprogramming script derived from the Forth programming language, based on Reverse Polish Notation.

The core of Phoshell script sm (abbreviation for "stack machine", chosen as we want a two letter short name and for being generic), and Phoscript implementations in other programming languages, consists of the following steps:

  • i. split a space delimited string into tokens or "words" in Forth or Phoscript terminology.
  • ii. loop through the list of words (tokens):
    • execute (evaluate) the current word if it is a function word
    • push the current word on to the stack if it is a non-function word

The full source code of sm is given here:

In fact, the above is the generic design of any stack machine and can be found in the interpreter in any programming languages.

This mechanism is so simple that it is unfortunate that it has not been more widely publicized. It so therefore one of the aims of this project to promote it so that Phoscript, or any Reverse Polish Notation based programming language, can become a universal metaprogramming script to interface to any host programming language, thereby solving the problem of diminishing returns due to growing complexities of programming langauges.

args=("$@")
S=()

# TODO: change i to longer var name to avoid global var conflict ??
for ((i=0; i < $#; i++))
{
    isFunction ${args[$i]}
    if [ $? -eq 0 ]; then
        # echo 'Function exists!'
        # E=eval "${args[$i]}"
        eval "${args[$i]}"
        # `eval ${args[$i]}`
        # echo $E
        # $($E)
    elif alias ${args[$i]} 2>/dev/null >/dev/null; then 
        echo ${args[$i]} is alias
        eval "${args[$i]}"
    elif [ "${args[$i]}" = "+" ]; then
        # echo "is plus"
        add:
    else
        # echo 'Function does not exist...'
        S+=("${args[$i]}")    
    fi
}

A. Examples

  1. Addition and Stack

> ./sm 55 22 + f_s
1: 77

./sm: sm is the abbreviation for stack machine. We choose this name because it is short and generic.

55 22 +: 55 + 22 in Reverse Polish Notation. The result is pushed on to the stack.

f_s: displays the stack.

1: 77: 1: the length of the stack. 77 is the result.

  • The simplest task in making a presentation video consists of the following:
    • a. Create a slide. Make a screenshot. Convert the screenshot into a video clip with voice over. The ffmpeg commands to accomplish this are:
    # get length of audio file
ffprobe -i $a -show_entries format=duration -v quiet -of csv="p=0"   

# create a silent mp4 from png
ffmpeg -loop 1 -i $i -c:v libx264 -vf "pad=ceil(iw/2)*2:ceil(ih/2)*2 -t $t -pix_fmt yuv420p $o
    • b. Record the audio using mobile phone or other deivces.
    • c. Merge the audio and video stream into one mp4 file.
    ffmpeg -i $i1 -i $i2 -c:v copy -c:a aac $o
    • d. Reencode the mp4 to the resolution required to eliminate inconsistencies create in the previous steps.
    ffmpeg -i $i1 -c:a aac -vcodec libx264 -s 1920x1080 -r 60 -strict experimental $o

As you can see the complexities of the ffmpeg parameters involved would turn off the most seasoned programmers. With Phoshell, we have turned the above to something like the following:

  • a. Get audio duration and make mp4 from png:
> ./sm t.m4a f_duration dup: GEISHA/img/GEISHA.png o_1.mp4 f_p2m
  • c. Merge the audio and video stream into one mp4 file.
> ./sm t.m4a o_1.mp4 o_1x.mp4 f_mav
  • d. Reencode the mp4 to the resolution required to eliminate inconsistencies created in the previous steps.
> ./sm o_1x.mp4 o_1r.mp4 f_renc

Explanations

Phoshell simplifies both bash and ffmpeg commands and parameters via the following mechanism:

  1. hiding details

  2. renaming or alias of functions with something easier to remember. This can be done arbitrarily according to the choice of programmers as well as using native langauge (national language) with Unicode. e.g. more than one aliases can point to the same function.

To illustrate, for the step to obtain the duratoin of the audio file, the original command is:

  # get length of audio file
  ffprobe -i $a -show_entries format=duration -v quiet -of csv="p=0"

The corresponding Phoscript command is:

> ./sm t.m4a f_duration

The Phoscript function f_duration is defined as (hiding details):

function f_duration
{
    a="`f_end`"; f_pop;
    echo $a
    b="ffprobe -i $a -show_entries format=duration -v quiet -of csv="
    c='"p=0"'
    echo $b$c
    # echo `eval $b$c`
    S+=(`eval $b$c`)
}

Further, f_duration can be aliased as dur:

alias dur:=f_duration
  • a. Get audio duration and make mp4 from png:
> ./sm t.m4a f_duration dup: GEISHA/img/GEISHA.png o_1.mp4 f_p2m

t.m4a: input audio file f_duration: function to determine duration of input audio

function f_duration
{
    a="`f_end`"; f_pop;
    echo $a
    b="ffprobe -i $a -show_entries format=duration -v quiet -of csv="
    c='"p=0"'
    echo $b$c
    # echo `eval $b$c`
    S+=(`eval $b$c`)
}

a="``f_end``" reads the "top of stack" (TOS) item. f_pop pops TOS.

String a is then inserted into string b along with the command ffprobe and its parameters.

eval $b$c executes the string $b$c.

S+=(...) pushes the result (duration of audio file) to the stack S.

dup: duplicates audio length (now at TOS) and pushes it on to the stack.

GEISHA/img/GEISHA.png: input file slide screenshot

o_1.mp4: output file silent mp4

f_p2m: converts input png to silent mp4

We adopt 2 naming conventions for Phoscript functions: f_* prefix or : suffix, e.g. dur:. This is to improve readability, i.e. to distinguish function words (tokens) from non-function words. These conventions do not exist in traditional Forth.

Step (c) (merge audio and video) is performed by f_mav — we have chosen a function name that is easy to remember

Step (d) (reencode video file) is performed by f_renc.

The definitions of these functions are listed in sm:

https://github.com/udexon/Phoshell/blob/master/Phoshell/sm

Programmers just need to keep the main loop of sm at the end. Additional functions and aliases can be defined independently to extend the functionalities of sm.

  • Sanwich API