Bash

Implementation of programming language Unix shell

Bash (acronym for “Bourne-Again SHell”) is a Unix shell implementation, written by Brian Fox in C in 1987 for the GNU Project.

Bourne-again shell command syntax is a superset of the Bourne shell command syntax. All of the Bourne shell builtin commands are available in Bash and the vast majority of Bourne shell scripts can be executed by Bash without modification.

Bash command syntax includes ideas drawn from the Korn shell (ksh) and the C shell (csh) such as command line editing, command history, the directory stack, the $RANDOM and $PPID variables, and POSIX command substitution syntax $(…).

Links:

Examples:

Factorial:

Example for versions Bash 3.0, Bash 4.0.35

This example uses recursive factorial definition. 

factorial ()
{
    local num=$1;
    if [ $num = 0 ]; then
        echo 1
        return ;
    fi;
    echo $(( $num * $(factorial $(( $num - 1 )) ) ))
}

for ((n = 0; n <= 16; n++))
do
    echo "$n! = " $(factorial $(($n)))
done

Hello, World!:

Example for versions Bash 3.0

This example demonstrates script declaration line, comment, variables and difference between single and double quotes. This is likely enough for Hello. As it should be in bash, message can be configured through parameters. 

# Prints "Hello world" message in Bash, Unix shell.

MESSAGE='hello'
TARGET='world'

echo "$MESSAGE $TARGET"

Quadratic equation:

Example for versions Bash 3.0, Bash 4.0.35

Bash itself can’t process floating-point numbers, so to calculate roots we have to use bc. 

read A;
if [ $A = 0 ]; then
    echo "Not a quadratic equation.";
    exit 0;
fi
read B;
read C;
D=$(( ($B)*($B)-4*($A)*($C) ));
#integer math only!
if [ $D = 0 ]; then
    echo -n "x = "
    echo -e "scale=3\n-0.5*($B)/($A)" | bc
    exit 0;
fi
echo $D
if [ $D -gt 0 ]; then
    echo -n "x1 = "
    echo -e "scale=3\n0.5*(-($B)+sqrt($D))/($A)" | bc
    echo -n "x2 = "
    echo -e "scale=3\n0.5*(-($B)-sqrt($D))/($A)" | bc
else
    echo -n "x1 = ("
    echo -e "scale=3\n-0.5*($B)/($A)" | bc
    echo -n ", "
    echo -e "scale=3\n0.5*sqrt(-($D))/($A)" | bc
    echo ")"
    echo -n "x2 = ("
    echo -e "scale=3\n-0.5*($B)/($A)" | bc
    echo -n ", "
    echo -e "scale=3\n-0.5*sqrt(-($D))/($A)" | bc
    echo ")"
fi

Hello, World!:

Example for versions Bash 4.0.35

This example uses dc (Desktop Calculator), which is a popular non-standard tool that allows processing arbitrary-precision numbers. | dc means that dc has to be applied to the command.

P command (the last character before | dc) outputs the last element of the stack. The huge number before P when written in hexadecimal looks as 0x48656C6C6F2C20576F726C64210A; pairs of adjacent digits form ASCII-codes of characters of “Hello, World!”: 0x48 = H, 0x65 = e, 0x6c = l etc. Thus, when printed, this number is processed as a string. 

echo 1468369091346906859060166438166794P | dc

Fibonacci numbers:

Example for versions Bash 4.0.35 
a=0
b=1
 
for (( n=1; $n<=16; n=$n+1 ));
do
  a=$(($a + $b))
  echo -n "$a, "
  b=$(($a - $b))
done
echo "..."

Factorial:

Example for versions Bash 4.0.35

This example uses iterative factorial definition. 

f=1

for (( n=1; $n<=17; $((n++)) ));
do
  echo "$((n-1))! = $f"
  f=$((f*n))
done