mjhjmtu

My file data Pattern is below and i need output as 6 or 3 or 8 or 4 based on the variable value which is eth0 and eth1

eth0RX:6:eth0TX:3|eth1RX:8:eth1TX:4|

Below code works well

sed 's/.*eth0RX:([0-9]+).*/1/g' $EMSTATE/packetdrop.txt

But fails when I use dynamic variable like below

rxfile=sed 's/.*$iRX:([0-9]+).*/1/g' $EMSTATE/packetdrop.txt

1. Inside single quotes, everything is what it is. 
   $i is a dollar sign followed by an i. 
   If you want to use a variable,
   you have to put the string into double quotes, somewhat like this:
   
   

   "s/.*$iRX:([0-9]+).*/1/g"

   
   


2. But that won’t work, because it will look for a variable called iRX. 
   The simplest way to fix that is to put the variable name in braces, like this:
   
   

   "s/.*$iRX:([0-9]+).*/1/g"

   
   


3. You should put $EMSTATE into double quotes, too. 
   You can use "$EMSTATE"/packetdrop.txt or "$EMSTATE/packetdrop.txt",
   whichever you prefer.

Variable expansion doesn't happen in single quoting. Use double quote instead. And if you want to set result to a variable, you need to run sed within command substitution syntax $(...) and better to quote this as well "$(...)". Plus as $iRX can be a valid variable name better do $i.

rxfile="$(sed "s/.*$iRX:([0-9]+).*/1/g" "$EMSTATE/packetdrop.txt" )"