Check user input is correct [duplicate]

Answer 1

Use a while loop

number=""
while [[ ! $number =~ ^[0-9]+$ ]]; do
    echo Please enter your age
    read number
done
echo You are $number years old

Answer 2

Allows space before and after the number, _33, or 33___ , but not 3__3. and no letters. 0 or

-- Get input from user until Correct

    # -- get input until Correct
    unset get_num
    while [[ ! ${get_num} =~ ^[0-9]+$ ]]; do
        echo "Please enter in a number:"
        read get_num
    done
    echo This is a number :  ${get_num}

-- Get input until Correct (within range)

    # -- get input until Correct (within range)
    unset get_num
    while [[ ! ${get_num} =~ ^[0-9]+$ ]]; do
        echo "Please enter in a number within range of (1-30):"
        read get_num
        ! [[ ${get_num} -ge 1 && ${get_num} -le 30  ]] && unset get_num
    done
    echo This is a number withn a range :  ${get_num}

-- get input until Correct (within range) (full regex way)

"Since regular expressions deal with text rather than with numbers, matching a number in a given range takes a little extra care. You can't just write [0-255] to match a number between 0 and 255. Though a valid regex, it matches something entirely different. [0-255] is a character class with three elements: the character range 0-2, the character 5 and the character 5 (again). This character class matches a single digit 0, 1, 2 or 5, just like [0125]." ~ http://www.regular-expressions.info/numericranges.html

    # -- get input until Correct (within range) (another way)
    unset get_num
    while [[ ! ${get_num} =~ ^([1-9]|1[0-9]|2[0-9]|30)$ ]]; do
        echo "Please enter in a number within range of (1-30):"
        read get_num
    done
    echo This is a number withn a range :  ${get_num}

-- Get input, and check it only (no while loop)

    # -- get input, and check it only (no while loop)
    unset get_num
    echo "Please enter in a number:"
    read get_num
    if [[ ! ${get_num} =~ ^[0-9]+$ ]] ;then 
        echo "${get_num} isn't a number" 
    else
        echo "${get_num} is a number"
    fi

Answer 3

And to avoid the heavy regex engine, use a simple glob:

if [[ ! $input || $input = *[^0-9]* ]]; then
    echo "Error: '$input' is not a number." >&2
fi