How do I scan for Wireless Access Points?

sudo iwlist wlan0 scanning | egrep 'Cell |Encryption|Quality|Last beacon|ESSID' should help.

It's the combination of sudo (run as root, do privileged operations), iwlist wlan0 scanning (produce some output on STDOUT), the pipe symbol "|" (connecting STDOUT of the command(s) to the left to the STDIN of the process on the right), and an egrep command with a "single quoted" (to prevent the shell from interpreting the "|" characters) Regular Expression to filter STDIN. See man bash, man sudo, man iwlist, man egrep, and man re_format for details.

ALWAYS do man whatever (as above) BEFORE you execute a command string from someone else. Self-education is much safer than blind trust.


Using iw

I don't have nm-tool installed so I use iw.

This command sorts access points by signal strength, strongest first:

sudo iw dev wlan0 scan | egrep "signal:|SSID:" | sed -e "s/\tsignal: //" -e "s/\tSSID: //" | awk '{ORS = (NR % 2 == 0)? "\n" : " "; print}' | sort

Each command explained:

iw dev wlan0 scan: Scan for access points reachable via interface wlan0

egrep "signal:|SSID:": Get the lines with signal strength and the SSIDs from iw's output. The output looks like this now:

        signal: -77.00 dBm 
        SSID: nameOfAccessPoint1
        signal: -71.00 dBm
        SSID: nameOfAccessPoint2

sed -e "s/\tsignal: //" -e "s/\tSSID: //": Reduce egrep's output to this:

-77.00 dBm 
nameOfAccessPoint1
-71.00 dBm
nameOfAccessPoint2

awk '{ORS = (NR % 2 == 0)? "\n" : " "; print}': Bring the signal strength and the SSID on the same line. More specifically, when the line number (NR) is even, i.e., we are on a line showing an access point, the output record separator (ORS) should be a line break. Otherwise, we are on the line containing signal strength, so we join the line by making ORS a simple space.

If we sort this output, we end up with a list of signal strengths and access points, showing the access point with the strongest signal on top:

-71.00 dBm nameOfAccessPoint2
-77.00 dBm nameOfAccessPoint1

Beware: Some access points can have an extended capability: Extended capabilities: * SSID List

So, grepping "SSID:" instead of "SSID" helps avoiding this extra ouput which would make the command fail otherwise.


nm-tool | grep "Freq.*Strength" | sed -ne "s|\(.*Strength \([0-9]\+\).*\)|\2}\1|p" | sort -n -r
  1. Use output of nm-tool to get list of Wireless Access Points
  2. Filter to get access points only
  3. Use sed to append signal level in front of each line
  4. sort output as numbers in reverse order (largest first)

nm-tool is part of "network-manager" package that is obviously installed in a typical Ubuntu system.

Tags:

Wireless

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