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Nahydrin
Asked: 2012-01-26 17:55:25 +0800 CST

Filter results into a usable variable

  • 772

I run the following command:

sudo iwlist eth0 scan

and get output that looks like this:

Cell 01 - Address: AB:CD:EF:12:34:56
          ESSID:"name"
          Protocol:IEEE 802.11g
          Mode:master
          Frequency:2.417 GHz (Channel 2)
          Encryption key:on

I won't bother filling it all out, as I'm trying to fix a laptop and I cannot simply copy the output.

How can I use built-in tools such as grep, awk, sed, etc. to fetch information given certain criteria? For example:

I want to grab the mac address when knowing the essid. I also don't want to rely on the Cell # or line positions. Knowing common information, like the word Address before the mac is fine.

Expected output:

AB:CD:EF:12:34:56

I want to use it in a variable, like so:

sudo iwconfig eth1 ap $(command)

Where command would result in the expected result. If there's another way of pushing the result as a variable using > or something, that works as well. (command > sudo iwconfig eth1 ap $1)

Thanks.

grep

3 Answers

  1. My personal preference for these things is pure bash. The script below should do what you are looking for.

    get_mac_of_essid() {
 target_essid="$1"
 [ -z "$target_essid" ] && return
 interface=eth0

 # Ask for password now instead of in the middle of pipe below
 sudo -v

 mac="" ; essid=""
 {
  while read line ; do
   set -- $line
   if [ "$1" = Cell ] ; then
    # New network, so reset variables
    mac="" ; essid=""
    shift 3
   fi
   case "$1" in
   Address:)
    mac="$2" # MAC is on this line, second field
    ;;
   ESSID:*)
    essid="${1#ESSID:}" # ESSID is on this line, first field after ':'
    essid="${essid#\"}" # Remove duoble quote at start
    essid="${essid%\"}" # Remove double quote at end
    ;;
   esac
   if [ "$essid" = "$target_essid" ] && [ ! -z "$mac" ] ; then
    echo "$mac" ; return
   fi
  done
 } < <(sudo iwlist $interface scan)
}

    After pasting the above into your terminal, you should be able to run get_mac_of_essid name to get the MAC address associated with network "name" as many times as needed.

    • 2

  2. I suggest learning the cut command. It makes this task easy. Forgive the echo, but I wanted to demonstrate on the original data. 

    $ echo 'Cell 01 - Address: AB:CD:EF:12:34:56  
          ESSID:"name"  
          Protocol:IEEE 802.11g  
          Mode:master  
          Frequency:2.417 GHz (Channel 2)  
          Encryption key:on' | egrep Address | cut '-d:' -f2-  
 AB:CD:EF:12:34:56  

$ echo 'Cell 01 - Address: AB:CD:EF:12:34:56  
          ESSID:"name"  
          Protocol:IEEE 802.11g  
          Mode:master  
          Frequency:2.417 GHz (Channel 2)  
          Encryption key:on' | egrep ESSID: | cut '-d"' -f2
name
    • 1
  3. Best Answer

    AWK is very powerful and perfect for this. Basically, you want to:

    1. Save the address in the address line
    2. If the ESSID matches your value, print it

    The shortest version I could think of:

    sudo iwlist wlan0 scan|awk '/Address:/{a=$5}/ESSID:"name"/{print a}'

    /pattern/ is a regular expression that is compared with input. If it matches, the part after the curly brackets ({...}) is executed.

    The awk command processes input line by line:

    1. If a line matches Address:, the fifth field is stored in a variable named a. Fields are whitepace-delimited lines.
    2. If a line matches ESSID:"name", variable a is printed (the last address that matched).

    Both rules are executed, but 1 comes before 2 so a is always set.

    Manual page for awk

    • 1