I created the following sed in order to add the python command – exit() before the line LIST_ALL_SELECT_TOOL_PACKAGES_CMD in the file – yumrpm.py

 sed -i  '0,/LIST_ALL_SELECT_TOOL_PACKAGES_CMD/s//exit()n&/' yumrpm.py

Example ( after we run the above sed syntax )

exit()
LIST_ALL_SELECT_TOOL_PACKAGES_CMD = "yum list all" 

The problem is when we run the sed again and in this case we get addition exit()

example:

exit()
exit()
LIST_ALL_SELECT_TOOL_PACKAGES_CMD = "yum list all" 

What need to add in sed syntax command , in order to avoid adding of exit() when exit() already exists before the line LIST_ALL_SELECT_TOOL_PACKAGES_CMD?