Unix shell

Influenced by:
Influenced:
Paradigm:
Typing discipline:
File extensions:
.sh
Versions and implementations (Collapse all | Expand all):
Programming language

Unix shell (usually just “shell” or “sh”) is a command language and scripting programming language for the Unix-like operating systems.

Unix shell language is a Turing-complete programming language and has instructions for loops, conditions, functions definitions, etc…

Distinctive feature of Unix shell language — many operations that are builtin in traditional programming languages use external programs in shell.

Elements of syntax:

Inline comments #
Case-sensitivity yes
Variable identifier regexp \$[A-Za-z0-9]+
Function identifier regexp [A-Za-z][A-Za-z0-9_]*
Variable assignment variable=value
Function definition functioname(){ ~ }
Function call `functioname a b c`
Function call with no parameters functionname
If - then if ~ then ~ fi
If - then - else if ~ then ~ else ~ fi
Loop forever while true; do ~ done
While condition do while condition; do ~ done

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