Shell下syntax error: operand expected (error token is “-”)

在这个监控实时网口速率的脚本中，第21,22行存在错误：

#!/bin/bash

#Modified by [e-mail]

echo ===DTmobile NetSpeedMonitor===

sleep 1

echo loading...

sleep 1

ethn=$1

while true

do

  RXpre=$(cat /proc/net/dev | grep $ethn | sed 's/:/ /g' | awk '{print $2}')

  TXpre=$(cat /proc/net/dev | grep $ethn | sed 's/:/ /g' | awk '{print $10}')

  sleep 1

  RXnext=$(cat /proc/net/dev | grep $ethn | sed 's/:/ /g' | awk '{print $2}')

  TXnext=$(cat /proc/net/dev | grep $ethn | sed 's/:/ /g' | awk '{print $10}')

  clear

  echo -e "\t\t\t  RX \t\t   TX  \t\t\t TIME"

  RX=$((${RXnext}-${RXpre}))

  TX=$((${TXnext}-${TXpre}))

  if [ $RX -lt 1024 ];then

    RX="${RX}B/s"

  elif [ $RX -gt 1048576 ];then

    RX=$(echo $RX | awk '{print $1/1048576 "MB/s"}')

  else

    RX=$(echo $RX | awk '{print $1/1024 "KB/s"}')

  fi

  if [ $TX -lt 1024 ];then

    TX="${TX}B/s"

  elif [[ $TX -gt 1048576 ]];then

    TX=$(echo $TX | awk '{print $1/1048576 "MB/s"}')

  else

    TX=$(echo $TX | awk '{print $1/1024 "KB/s"}')

  fi

  echo -e "$ethn \t $RX   $TX   \t\t\t `date +%k:%M:%S` "

done

　　

修改后的文件

#!/bin/bash

#Modified by [e-mail]

echo ===DTmobile NetSpeedMonitor===

sleep 1

echo loading...

sleep 1

ethn=$1

while true

do

  RXpre=$(cat /proc/net/dev | grep $ethn | sed 's/:/ /g' | awk '{print $2}')

  TXpre=$(cat /proc/net/dev | grep $ethn | sed 's/:/ /g' | awk '{print $10}')

  sleep 1

  RXnext=$(cat /proc/net/dev | grep $ethn | sed 's/:/ /g' | awk '{print $2}')

  TXnext=$(cat /proc/net/dev | grep $ethn | sed 's/:/ /g' | awk '{print $10}')

  clear

  echo -e "\t\t\t  RX \t\t   TX  \t\t\t TIME"

  RX=$((RXnext-RXpre))

  TX=$((TXnext-TXpre))

  if [ $RX -lt 1024 ];then

    RX="${RX}B/s"

  elif [ $RX -gt 1048576 ];then

    RX=$(echo $RX | awk '{print $1/1048576 "MB/s"}')

  else

    RX=$(echo $RX | awk '{print $1/1024 "KB/s"}')

  fi

  if [ $TX -lt 1024 ];then

    TX="${TX}B/s"

  elif [[ $TX -gt 1048576 ]];then

    TX=$(echo $TX | awk '{print $1/1048576 "MB/s"}')

  else

    TX=$(echo $TX | awk '{print $1/1024 "KB/s"}')

  fi

  echo -e "$ethn \t $RX   $TX   \t\t\t `date +%k:%M:%S` "

done

　　

原因为，在$取值的时候，括号里面只需要跟变量即可（变量可自行进行计算），不需要对括号内进行运算的变量在进行取值操作。

  RX=$((${RXnext}-${RXpre}))

  TX=$((${TXnext}-${TXpre}))

  修改后：

  RX=$((RXnext-RXpre))

  TX=$((TXnext-TXpre))

　　

然后就没有然后了~~~