trying to adb backup but dialog doesn't show I have sony xperia P device and I want to flash its ROM to get Marshmallow(I currently have android 4.0.4).  

I want to do full backup of my phone before doing this, I knew about adb backup command, I installed android-tools from my Arch linux repos and issued :  

adb backup -all -apk -shared -f momsmobile.ab   


it shows : Now unlock your device and confirm the backup operation. but Nothing shows at all, I have USB debugging enabled and tried to put a password the Developer option "Desktop password for backup".  

I also tried following Arch wiki and added udev rules, this is the rule file :  

SUBSYSTEM=="usb", ATTR{idVendor}=="xxxx", MODE="0666", GROUP="adbusers"
SUBSYSTEM=="usb",ATTR{idVendor}=="xxxx",ATTR{idProduct}=="yyyy",SYMLINK+="android_adb"
SUBSYSTEM=="usb",ATTR{idVendor}=="xxxx",ATTR{idProduct}=="yyyy",SYMLINK+="android_fastboot"


I'm sure the vendor's and product's id are correct(the real ones aren't provided for privacy).  

the above didn't work too, I still get no confirm prompt, although when I issue adb devices I can see mine there.  

Basically I have the same issue as this question, only this question has different device model(Sumsung vs Sony) and it's not answered.  

Lastly I tried to disable all my antivirus protections, I'm using avast mobile security and disabled every thing but still didn't work.

So what's the problem ?

PS

I don't want to root the device(at least not on backup phase) and I don't want to use any apps.
Might be a little late, but here's what worked for me: put single quotes around the parameters.

adb backup '-all -apk -shared -f momsmobile.ab'