In this script for monitoring the real-time network port rate, there is an error on lines 21 and 22:

#!/bin/bash
#Modified by [email protected]
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

Revised document

#!/bin/bash
#Modified by [email protected]
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

The reason is that when the value of $is taken, only variables need to be followed in the brackets (variables can be calculated by themselves), and it is not necessary to take the value of variables in the brackets

RX=$((${RXnext}-${RXpre}))
  TX=$((${TXnext}-${TXpre}))
  
change to:
  
  RX=$((RXnext-RXpre))
  TX=$((TXnext-TXpre))

And then there’s no error~~~