How to fetch address and modify the value in key/value format file?

Question

I'm currently writing a shell-script for my Raspberry Pi so it sets up an access-point with a unique SSID. To do this I want to use the mac-address of the WiFi dongle as my SSID. For doing that I want to use awk, with

awk '{ gsub(/:/, ""); print > "file" }' /sys/class/net/wlan0/address

I can write the properly formatted mac address to a file, but how can I insert it into a specific line/place at another file? I have in mind something like

awk '{ gsub(/:/, ""); "replace ssid= at /etc/hostapd/hostapd.conf"  }' /sys/class/net/wlan0/address

Any help plus explanation for a newbie like me is appreciated.

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My input file is like:

$ cat /sys/class/net/wlan0/address
01:23:45:67:89:ab

and I'm trying to fetch it and replace the value of ssid in the file which is like:

interface=wlan0 driver=nl80211 ssid=7cdd907f6b07 hw_mode=g channel=11 macaddr_acl=0 auth_algs=1 ignore_broadcast_ssid=0 wpa=1 wpa_passphrase=My_Passphrase wpa_key_mgmt=WPA-PSK wpa_pairwise=TKIP rsn_pairwise=CCMP

Answer 1

 awk 'NR==FNR{gsub(":","");macadd=$0;next}{sub(/^ssid=.*/,"ssid="macadd)}1' /sys/class/net/wlan0/address /etc/hostapd/hostapd.conf

Current record no. (NR) equals current file record no. just for the first file, then :'s substitution is done, subsequently the line is placed on the macadd variable, and next skips line output.

The other instructions are applied to the hostapd.conf file; final 1 stands for "true", thus lines are printed.

If everyting works fine, you may redirect command output to a new file, and replace hostapd.conf.

Notes:

• files order does matter, as you can tell
• gawk features in-place replacement starting from 4.1.0

Answer 2

sed -i.bak "s/^ssid=.*/ssid=$(tr -d ':'</sys/class/net/wlan0/address|tee file)/" /etc/hostapd/hostapd.conf

Once tested, you can remove the .bak part, which creates a backup of hostapd.conf. I think you are also going to delete |tee file.

The ^ sign in regexps stands for "beginning of line"; the s operator in sed substitutes a regexp with a given string. Given string here has a subshell called thru $(...), getting address content, purging it from :'s with tr, and returning the output after a copy is stored into the file file by the tee command.

Answer 3

Then load and save it in-place using ex editor, try:

ex +'let @m = substitute(system("cat /sys/class/net/wlan0/address"), "[^0-9a-z]", "", "g")' +'%s/ssid=\zs[a-f0-9]\{12}/\=@m/' -scwq /etc/hostapd/hostapd.conf

which:

• takes MAC address from the file (system("cat file") which is equivalent to readfile('file')
• substitute output by removing everything exempt [0-9a-z]
• then assign it into @m register

• then substitute on hostapd.conf by finding ssid and replacing \{12} characters of [a-f0-9] with value of m register (\=@m),

  in other words, it's like: %s/^key=\zs.\+/new_value/

• then save the file in-place (-cwq)

To debug it, you may:

• add extra +%p (to print it) and change -scwq into -scq! for dry-run (without changing the file)
• add -V1 for more verbose

---

To fetch just MAC address from ifconfig it's easy to do with grep, like:

ifconfig wlan0 | egrep -om1 "..(:..){5}"

or:

grep -o ..:..:..:..:..:.. <(ifconfig wlan0)