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使用 bash 从脚本中消除外部命令以生成 NMEA 0183 异或校验和的方法

tag-icon • tag-icon bash shell-script checksum
使用 bash 从脚本中消除外部命令以生成 NMEA 0183 异或校验和的方法

我最近需要使用 bash 生成和验证 NMEA 0183 校验和，但我找不到太多可以完成我所需要的内容。

NMEA 0183 句子以 $ 开头，以 * 结尾，以及两个字符，这两个字符是 $ 和 * 之间所有字节的十六进制异或。例子：

$INGGA,230501.547,2118.97946,N,15752.60495,W,2,08,1.1,5.17,M,,,0,0001*02

该实用程序会将字符串转换为十六进制并对其进行异或。它可用于验证已经存在的校验和，或为您正在生成的 NMEA 句子末尾生成校验和（它会从您提供的字符串中删除 $ 和 *..）。

#!/bin/bash

# =========================================================
# Reads a NMEA 0183 sentence and calculates the proper
# XOR checksum for the end.

# Will accept a string with or without a checksum on
# the end or $ on the front and calculate what the checksum
# should be.

# Sentence can be read as an argument but must be single quoted
# or preceded by a \ or the shell will try to interpret the
# talker as a variable and the result will be incorrect.
# Examples:

#     xor '$INHDT,207.7,T*27'
#     xor \$INHDT,207.7,T*27
#     xor INHDT,207.7,T

# If run with no arguments, will prompt user for data.  No
# quotes or backslash is needed then.

# Depends: xxd sed

# ===T.Young 09/2016=======================================

set -o pipefail
set -o errexit
set -o nounset

# Functions
# =========

depcheck() { # Checks that necessary external commands are present
             # and executable
    local DEPENDS="sed xxd"
    for PROG in $DEPENDS; do
        [[ -x "$(command -v $PROG)" ]] || {
            echo "$PROG MISSING!  Exiting."
            exit 0
            }
    done
    }

x_or() { # Here is where the magic happens
    # The next two lines strip out $ characters, or an
    # * and anything after it (checksum)
    HEAD="${SENTENCE%\**}"
    TAIL="${HEAD//\$}"

    # Convert ASCII string into hex and read into an array.
    # Each element in the array gets preceded by "0x"
    HEXVAL="$(xxd -pu <<< ${TAIL})"
    HEXARRAY=($(printf '%s' "${HEXVAL%0a}" | sed -e 's/../0x& /g'))

    # Loop through the array and do the xor, initially start $XOR at 0
    for (( x=0; x<"${#HEXARRAY[@]}"; x++ )); do
        XOR=0x$(printf '%02x' "$(( ${XOR:-0} ^ ${HEXARRAY[$x]} ))")
    done

    # Strip off the 0x from the result
    CLEAN=${XOR#0x}
    printf '%s\n' "${CLEAN^^}"
    }

main() {
    case "${1:-}" in
        "")  # No input specified, read from stdin
            depcheck
            read -r SENTENCE
            x_or
            ;;

        *) # Input was provided, use that
            depcheck
            SENTENCE="$1"
            x_or
            ;;
    esac
}

# Main
# ====

main "$@"

当编写 shell 脚本时，我总是试图找到方法来消除外部程序的使用，甚至是像 sed 或 xxd 这样常见的程序。如果有人知道仅使用 shell 内置函数执行上述操作的方法，请插话。

更新：这是一个考虑佐藤方法的新函数。它允许完全消除外部程序调用以及上面关联的 depcheck 函数。

x_or() { # Create a hex XOR checksum of all the bytes
    # Clean the line of $ character and anything before it
    TAIL="${SENTENCE##*$}"
    HEAD=${TAIL%\**}
    LEN=${#HEAD}

    # Loop through the string and do the xor
    # initially start $XOR at 0
    XOR=0
    for (( x=0; x<$LEN; x++ )); do
        (( XOR^=$(printf '%d' "'${HEAD:$x:1}'") ))
    done

    printf '%02X\n' "${XOR}"
    }

使用“LC_CTYPE=C”调用该函数。这里可能还可以做更多的事情，但这相当简洁。

答案1

就我个人而言，我会这样做：

#! /usr/bin/env bash

log() {
    {
        printf '%s: ' "${0##*/}"
        printf "$@"
        printf '\n'
    } >&2
}


cksum() {
    tot=${#1}
    let len=tot-4

    let res=0
    while [ $len -gt 0 ]; do
        let res^=$( LC_CTYPE=C printf '%d' "'${1:$len:1}'" )
        let len--
    done

    let ptr=tot-2
    if [ x"$( printf '%s' "${1:$ptr}" | tr a-f A-F )" != x"$( printf '%02X' $res )" ]; then
        log '%s: invalid checksum (found %02X)' "$1" $res
    fi
}


check () {
    if expr "$2" : '\$.*\*[0-9a-fA-F][0-9a-fA-F]$' >/dev/null; then
        cksum "$2"
    else
        log 'invalid input on line %d: %s' "$1" "$2"
    fi
}


let cnt=0
if [ $# -ne 0 ]; then
    while [ $# -gt 0 ]; do
        let cnt++
        check $cnt "$1"
        shift
    done
else
    while read -r str; do
        let cnt++
        check $cnt "$str"
    done
fi

shebang 线声称bash，但它应该仍然适用于ksh93r和zsh。不依赖于xxd.也没有声称是可遵循的脚本风格的示例。 :)

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