Bash Base64 encoding / decoding functions with rotation (rotXX / rot13)

base64_rotation.sh

#!/bin/bash

function rot_base64_e(){
    USAGE="\nrot_base64_e <input_file_or_string> <rotation> [<output> empty for print]\nUsage example: rot_base64_e file.txt 13 output.txt\n"

    if [ -f "$1" ]; then
        CONTENT=$(cat $1)
    elif [ -n "$1" ]; then
        CONTENT=$1
    else
        printf "$USAGE"
        echo "Invalid file or string input"
        return
    fi

    if [[ $2 =~ ^-?[0-9]+$ ]]; then
        ROT=$2
    else
        printf "$USAGE"
        echo "Invalid integer rotation"
        return
    fi
    
    for (( i = 0; i < $ROT; i++ )); do
        CONTENT=$(echo "$CONTENT" | base64 -w 0)
    done
    
    if [ -z "$3" ]; then
        echo $CONTENT
    else
        OUTPUT=$3
        echo $CONTENT > $OUTPUT
    fi
}

function rot_base64_d(){
    USAGE="\nrot_base64_d <input_file_or_string> <rotation> [<output> empty for print]\nUsage example: rot_base64_d file.txt 13 output.txt\n"

    if [ -f "$1" ]; then
        CONTENT=$(cat $1)
    elif [ -n "$1" ]; then
        CONTENT=$1
    else
        printf "$USAGE"
        echo "Invalid file or string input"
        return
    fi

    if [[ $2 =~ ^-?[0-9]+$ ]]; then
        ROT=$2
    else
        printf "$USAGE"
        echo "Invalid integer rotation"
        return
    fi

    for (( i = 0; i < $ROT; i++ )); do
        CONTENT=$(echo "$CONTENT" | base64 -w 0 -d)
    done
    
    if [ -z "$3" ]; then
        echo $CONTENT
    else
        OUTPUT=$3
        echo $CONTENT > $OUTPUT
    fi
}

example.sh

# Example
echo "This is a encoded text" > test.txt

# Encodes a text 4 times in base64
rot_base64_e test.txt 4 out.txt

# Shows Base64 encoding results
cat out.txt

# Decodes Base64 encoding with rotation of 4 (this example without output file)
rot_base64_d out.txt 4