HelloWorld.sh

 # Demonstrate 2-tag system can be implemented with `find` and `mkdir` only,
# using an example from Wikipedia.
# en.wikipedia.org/wiki/Tag_system#Example:_A_simple_2-tag_illustration

# _'s are used as a separtor between states.
mkdir _

# Production rules for an M-tag system given as constants.
M=2
PROD_A="c/c/b/a/H"
PROD_B="c/c/a"
PROD_C="c/c"

# Initial state (surrounded by _'s).
mkdir -p _/b/a/a/_

# Alphabets (constant size)
S="(/[abcH])"

# Keep appending the next state to the end of the state until seeing the halting symbol. e.g.
# _/b/a/a/_ -> _/b/a/a/_/a/c/c/a/_ -> _/b/a/a/_/a/c/c/a/_/c/a/c/c/b/a/H/_ -> ... -> .../_/H/c/c/c/c/c/c/a/_ (halt)
#
# Line 2:    Halting condition
# Line 3-6:  Copy the previous state removing M characters from the start, utilizing back-references.
# Line 7-29: Apply the production rule for a, b, c.
find _ -regextype posix-extended -empty \( \
    -regex ".*_/H.*_|.*_(/[^/]?){,$M}_" -prune -o \
    -regex ".*_$S{$M}($S*)/a$S*/_\2" \( -execdir mkdir a/a b/a c/a H/a _/a \; -o -true \) -o \
    -regex ".*_$S{$M}($S*)/b$S*/_\2" \( -execdir mkdir a/b b/b c/b H/b _/b \; -o -true \) -o \
    -regex ".*_$S{$M}($S*)/c$S*/_\2" \( -execdir mkdir a/c b/c c/c H/c _/c \; -o -true \) -o \
    -regex ".*_$S{$M}($S*)/H$S*/_\2" \( -execdir mkdir a/H b/H c/H H/H _/H \; -o -true \) -o \
    \( \
        -regex ".*_/a$S*/_$S*" \( \
            -execdir find a \; -execdir mkdir -p a/$PROD_A/_ \; -o \
            -execdir find b \; -execdir mkdir -p b/$PROD_A/_ \; -o \
            -execdir find c \; -execdir mkdir -p c/$PROD_A/_ \; -o \
            -execdir find H \; -execdir mkdir -p H/$PROD_A/_ \; -o \
            -execdir find _ \; -execdir mkdir -p _/$PROD_A/_ \; \
        \) -o \
        -regex ".*_/b$S*/_$S*" \( \
            -execdir find a \; -execdir mkdir -p a/$PROD_B/_ \; -o \
            -execdir find b \; -execdir mkdir -p b/$PROD_B/_ \; -o \
            -execdir find c \; -execdir mkdir -p c/$PROD_B/_ \; -o \
            -execdir find H \; -execdir mkdir -p H/$PROD_B/_ \; -o \
            -execdir find _ \; -execdir mkdir -p _/$PROD_B/_ \; \
        \) -o \
        -regex ".*_/c$S*/_$S*" \( \
            -execdir find a \; -execdir mkdir -p a/$PROD_C/_ \; -o \
            -execdir find b \; -execdir mkdir -p b/$PROD_C/_ \; -o \
            -execdir find c \; -execdir mkdir -p c/$PROD_C/_ \; -o \
            -execdir find H \; -execdir mkdir -p H/$PROD_C/_ \; -o \
            -execdir find _ \; -execdir mkdir -p _/$PROD_C/_ \; \
        \) \
    \) \
\) &> /dev/null

# Output the result. (_/H/c/c/c/c/c/c/a/_)
find _ -depth ! -empty -name _ -execdir find _ -empty \; -quit

created 4 months ago

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About Bash

Bash (Bourne Again Shell) is a shell program written by Brian Fox and is an upgraded version of Bourne Shell program 'sh'.

Features

Open source GNU project
Read and execute the commands from a Shell Script
Can be invoked by either single-character command line options (-a, -b, -c, -i, -l, -r, etc. ) or by multi-character command line options also like --help, --debugger,--login, etc.
Consists of Key bindings
Available in restricted mode for the environment security
Contains one-dimensional arrays to manipulate the lists of data.

Syntax help

Variables

name="Foo"
echo $name
echo "$name"
echo "${name}"

Conditional Statements

If

if [ conditional-expression ]  
then  
statements  
fi

If-else

if [ conditional-expression ]  
then  
   statements  
else  
  statements
fi

Else-If

if [ conditional-expression ]  
then  
   statements  
elif [ conditional-expression ]  
then  
 statements  
else  
  statements
fi