Adds an option to encode/decode url-safe base64

base64

###
# Shell script which adds the ability to encode/decode web-safe URLs in base64.  Do this by specifying
#
#!/bin/bash
ARGS=()
BAD_CHARS="+/="
SAFE_CHARS="-_ "
URL="n"; CONVERT="n"; DECODE="n"; IN="-"; OUT="-"; HELP="n"
while (($#)); do
    case "${1}" in
        -h | --help)
            HELP="y"
            ARGS+=("${1}")
            ;;
        -D | -d | --decode)
            DECODE="y"
            ARGS+=("${1}")
            ;;
        -i | --input=*)
            if [ "${1}" == "-i" ]; then shift; fi
            IN="${1#--input=}"
            ;;
        -o | --output=*)
            if [ "${1}" == "-o" ]; then shift; fi
            OUT="${1#--output=}"
            ;;
        -u | --url)
            URL="y"
            ;;
        -c | --convert)
            CONVERT="y"
            ;;
        *)
            ARGS+=("${1}")
            ;;
    esac
    shift
done

# Converts the stream (of unsafe characters) to safe characters
function to_safe {
    tr -- "${BAD_CHARS}" "${SAFE_CHARS}" | sed -e 's/ *$//g'
}
# Converts the stream (of safe characters) to unsafe characters
function from_safe {
    awk '{ L=length($1)/4; L=int((L==int(L))?L:int(L)+1)*4; printf "%-*s\n", L, $1; }' \
    | tr -- "${SAFE_CHARS}" "${BAD_CHARS}"
}

# Handle convert-only
if [ "${CONVERT}" == "y" ]; then
    if [ "${OUT}" == "-" ]; then OUT="/dev/stdout"; fi
    if [ "${DECODE}" == "y" ]; then
        cat "${IN}" | from_safe > "${OUT}"
    else
        cat "${IN}" | to_safe > "${OUT}"
    fi
    exit $?
fi

# Handle if we specified url-safe
if [ "${URL}" == "y" ]; then
    if [ "${DECODE}" == "y" ]; then
        cat "${IN}" | from_safe | /usr/bin/base64 "${ARGS[@]}" -o "${OUT}"
    else
        if [ "${OUT}" == "-" ]; then OUT="/dev/stdout"; fi
        /usr/bin/base64 "${ARGS[@]}" -i "${IN}" | to_safe > "${OUT}"
    fi
    exit $?
fi

# Just pass it through - but add our help
/usr/bin/base64 "${ARGS[@]}" -i "${IN}" -o "${OUT}"
RET=$?
if [ "${HELP}" == "y" ]; then 
    echo "  -u, --url      encode/decode url-safe strings" >&2
    echo "  -c, --convert  without -D, converts to url-safe strings" >&2
    echo "                 with -D, converts from url-safe strings" >&2
fi
exit ${RET}

Isn't using tr on the base64 encoded output breaking the encoding? Acording to the RFC, https://tools.ietf.org/html/rfc3548.html#page-3, the alphabet needs to be chosen in the tool.

The tr is used for doing "url-safe" base64 output instead of standard output...it is the way to actually choose the alphabet that is used. See https://datatracker.ietf.org/doc/html/rfc4648#section-5. That is all this script does - it adds the -u option to select "url-safe" operation, and then translates the output from the system-provided base64 command. All other command line options are passed through to the underlying base64 command.