Fire a pinch in/out command to Android phone using adb

You can do it using adb getevent and sendevent.
Connect you device using adb. Follow the steps below.

1. Identify your Input device:
   Open any image on your device. To list input devices, run

   $ adb shell getevent   
   add device 1: /dev/input/event7   
   name:     "msm8226-tapan9302-snd-card Headset Jack"
   add device 2: /dev/input/event6
     name:     "msm8226-tapan9302-snd-card Button Jack"
   add device 3: /dev/input/event2
     name:     "synaptics_dsx_i2c"
   add device 4: /dev/input/event4
     name:     "qpnp_pon"
   

   Pinch in/out on the image, you should see some continuous logs like

   /dev/input/event2: 0003 0030 00000005
   /dev/input/event2: 0000 0000 00000000
   /dev/input/event2: 0003 002f 00000000
   /dev/input/event2: 0003 0036 00000144
   /dev/input/event2: 0003 003a 00000079
   /dev/input/event2: 0000 0000 00000000
   /dev/input/event2: 0003 0036 00000142
   

   Confirms /dev/input/event2 is the input device name for my target device.

2. Get exact getevent and convert getevent to sendevent:

   Make sure your screen switched on and is open with some image,
   Run the below command on the prompt.

   $ adb shell getevent | grep dev/input/event2 > getevent_input.txt
   

   While the above is running, Pinch in/out on the image on your phone.

   Once completed, Kill the above command Ctrl + C
   Open file getevent_input.txt and delete first line "add device X: /dev/input/eventX" from it.

   Since getevent returns values in decimal, and sendevent takes value in hexadecimal.
   We have to do the above conversion.
   This script hex_to_dec.py here does the Job. Thanks to this guy!

   $./hex_to_dec.py  getevent_input.txt 
   

   Generates a file getevent_input.scr, Now rename this file to .sh

   $ mv getevent_input.scr  sendevent_input.sh
   

   Open file sendevent_input.sh and delete second line "echoing – drawing function" and save it.

3. Run on device.
   Transfer sendevent_input.sh to device.

   $ adb push sendevent_input.sh /sdcard/
   1615 KB/s (64379 bytes in 0.038s)
   

   Make sure you device has an image open, and screen is not off.

   $ adb shell sh /sdcard/sendevent_input.sh
   

Worked perfectly fine for me, If this what you looking for.
For visible results, enable Show touches in Developer Options.

Environment: Motorola Moto G (Android 4.4.4) with ADB (v 1.0.31) on Ubuntu 12.04.