Is their any way that i can pass variable value of python to my unix command

Note : var is a python variable which stores filename with path

var='/d/demo/f/f.txt'

Want to call var to my sed command

os.system(bash -c 'sed -i "1i\NAME" var ')

This throws me error at "1i\" syntax invalid

Use subprocess.run, not os.system(), and pass an array of individual argument-vector entries.

var = '/d/demo/f/f.txt'
subprocess.run(['sed', '-i', r'1i\NAME', var])

Each argument needs to be escaped according to Python rules, not shell rules; hence using r'1i\NAME' for a string with a literal backslash in it. You could also achieve the same effect with '1i\\NAME'.