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 wlan0 scan | grep -Po '(signal|SSID):\K.*' | sed 's/ $/ [unknown SSID]/' | paste -d ' ' - - | cut -c2- | sort -gr

Each command explained:

iw wlan0 scan

Scan for access points reachable via interface wlan0.

grep -Po '(signal|SSID):\K.*'

Grep gets the text after "signal:" or "SSID:". See this answer to learn more about the used options and \K.

Reduces the output of iw wlan0 scan to something like this:

 -77.00 dBm
 nameOfAccessPoint1
 -69.00 dBm
 
 -71.00 dBm
 nameOfAccessPoint2

Note that the access point with signal strength -69 dBm doesn't broadcast an SSID, the output of grep is a single space on that line.

sed 's/ $/ [unknown SSID]/'

To make the fact that the SSID is unknown explicit, we replace the single space with [unknown SSID]. The output of sed is:

 -77.00 dBm
 nameOfAccessPoint1
 -69.00 dBm
 [unknown SSID]
 -71.00 dBm
 nameOfAccessPoint2

paste -d ' ' - -

With paste we join two consecutive lines with a space in between. This produces:

 -77.00 dBm nameOfAccessPoint1
 -69.00 dBm [unknown SSID]
 -71.00 dBm nameOfAccessPoint2

cut -c2- | sort -gr

With cut -c2- we remove the leading space and sort orders the lines by dBm, smallest numerical value first. Which means the access points are sorted by signal strength, strongest first.

-69.00 dBm [unknown SSID]
-71.00 dBm nameOfAccessPoint2
-77.00 dBm nameOfAccessPoint1

Alternatively

Here's another way to get access points sorted by signal strength. Mind that this command does not take care of access points with an empty SSID:

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

Each command explained:

iw 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: //"

We 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.

sort -gr

If we sort this output by numerical value (-g) and reverse it (-r), 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

This means grepping "SSID:" instead of "SSID" helps avoiding this extra output 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.

You can also use nmcli, I found it myself a few months back and it's the easiest one I've used so far.

nmcli device wifi

Example output:

IN-USE  BSSID              SSID                             MODE   CHAN  RATE        SIGNAL  BARS  SECURITY  
        18:7B:CB:42:EF:8E  FRITZ!Box 7530 MS                Infra  11    260 Mbit/s  100     ▂▄▆█  WPA2      
*       D1:12:C8:87:D7:F1  FRITZ!Box 7530 MS                Infra  6     130 Mbit/s  75      ▂▄▆_  WPA2      
        DD:72:2D:64:04:C9  example network                  Infra  11    130 Mbit/s  65      ▂▄▆_  WPA1 WPA2 
        2C:A1:F2:9E:97:2E  FRITZ!Box 7530 MS                Infra  11    130 Mbit/s  57      ▂▄▆_  WPA2      
        11:22:F5:89:71:9C  some_other_network               Infra  6     130 Mbit/s  55      ▂▄__  WPA2      

If you can install software on your machine, I would recommend wavemon.

To install it run:

sudo apt-get update 
sudo apt-get install wavemon

To run it

sudo wavemon 

and then press F3 for the SCAN screen. You should see something similar to this: