Bash

example of condition:

if [[ -z $1 ]]
then
    echo "First argument is empty"
elif [[ -n $1 ]]
then
    echo "Second argument is not empty"
else
    echo "unreachable statement"
fi

File related conditions:

• -e <path> - file exists
• -f <path> - is file
• -d <path> - is directory
• -s <path> - filesize greater than 0
• -x <path> - file is executable

Example of case construction:

case $1 in
    0) echo "No students";;
    1) echo "1 student";;
    [2-4]) echo "$1 students";;
    *) echo "A lot of students";;
esac

Example of while loop:

#!/usr/bin/bash

while true
do
    echo "enter your name:"
    read name

    if [[ -z $name ]] 
    then
        echo "bye"
        break
    fi

    echo "enter your age:"
    read age

    if [[ $age -eq 0 ]] 
    then
        echo "bye"
        break
    fi

    group="adult"
    if [[ $age -le 16 ]] 
    then
        group="child"
    elif [[ $age -le 25 ]] 
        then
        group="youth"
    fi

    echo "$name, your group is $group"

done

Example of for loop:

#!/usr/bin/bash

for i in 1 2 3 4
do
    echo $i
done

Another example with case and arithmetic operations:

#!/usr/bin/bash

while [ true ]
do
    read left operator right
    if [ $left == "exit" ]
    then
        echo "bye"
        exit 0
    fi

    if [ -z $right ]
    then
        echo "error"
        exit 0
    fi

    case "$operator" in
        "+"|"-"|"*"|"**"|"/"|"%") let a="$left$operator$right"; echo $a;;
        *) echo "error1"; exit 0;;
    esac

done

Example of function:

#!/usr/bin/bash

while [ true ]
do
    read n1 n2
    if [ -z $n1 ]
    then
        echo "bye"
        break
    fi
    gcd() {
        remainder=1
        if [ $n2 -eq 0 ]
        then
            echo "bye"
            break
        fi

        while [ $remainder -ne 0 ]
        do
            remainder=$((n1%n2))
            n1=$n2
            n2=$remainder
        done
    }
    gcd $1 $2
    echo "GCD is $n1"
done