remove character on the last line that specific word appears

we have the following file example

we want to remove the , character on the last line that topic word exists

more file

{"topic":"life_is_hard","partition":84,"replicas":[1006,1003]},
{"topic":"life_is_hard","partition":85,"replicas":[1001,1004]},
{"topic":"life_is_hard","partition":86,"replicas":[1002,1005]},
{"topic":"life_is_hard","partition":87,"replicas":[1003,1006]},
{"topic":"life_is_hard","partition":88,"replicas":[1004,1001]},
{"topic":"life_is_hard","partition":89,"replicas":[1005,1002]},
{"topic":"life_is_hard","partition":90,"replicas":[1006,1004]},
{"topic":"life_is_hard","partition":91,"replicas":[1001,1005]},
{"topic":"life_is_hard","partition":92,"replicas":[1002,1006]},
{"topic":"life_is_hard","partition":93,"replicas":[1003,1001]},
{"topic":"life_is_hard","partition":94,"replicas":[1004,1002]},
{"topic":"life_is_hard","partition":95,"replicas":[1005,1003]},
{"topic":"life_is_hard","partition":96,"replicas":[1006,1005]},
{"topic":"life_is_hard","partition":97,"replicas":[1001,1006]},
{"topic":"life_is_hard","partition":98,"replicas":[1002,1001]},
{"topic":"life_is_hard","partition":99,"replicas":[1003,1002]},

expected output

{"topic":"life_is_hard","partition":84,"replicas":[1006,1003]},
{"topic":"life_is_hard","partition":85,"replicas":[1001,1004]},
{"topic":"life_is_hard","partition":86,"replicas":[1002,1005]},
{"topic":"life_is_hard","partition":87,"replicas":[1003,1006]},
{"topic":"life_is_hard","partition":88,"replicas":[1004,1001]},
{"topic":"life_is_hard","partition":89,"replicas":[1005,1002]},
{"topic":"life_is_hard","partition":90,"replicas":[1006,1004]},
{"topic":"life_is_hard","partition":91,"replicas":[1001,1005]},
{"topic":"life_is_hard","partition":92,"replicas":[1002,1006]},
{"topic":"life_is_hard","partition":93,"replicas":[1003,1001]},
{"topic":"life_is_hard","partition":94,"replicas":[1004,1002]},
{"topic":"life_is_hard","partition":95,"replicas":[1005,1003]},
{"topic":"life_is_hard","partition":96,"replicas":[1006,1005]},
{"topic":"life_is_hard","partition":97,"replicas":[1001,1006]},
{"topic":"life_is_hard","partition":98,"replicas":[1002,1001]},
{"topic":"life_is_hard","partition":99,"replicas":[1003,1002]}

we try to removed the character , from the the last line that contain topic word as the following sed cli but this syntax not renewed the ,

sed -i '${s/,[[:blank:]]*$//}' file

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sed (GNU sed) 4.2.2

Answer

In case you have control M characters in your Input_file then remove them by doing:

tr -d 'r' < Input_file > temp && mv temp Input_file

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Could you please try following once. From your question what I understood is you want to remove comma from very last line which has string topic in it, if this is the case then I am coming up with tac + awk solution here.

tac Input_file | 
awk '/topic/ && ++count==1{sub(/,$/,"")} 1' | 
tac

Once you are happy with above results then append > temp && mv temp Input_file to above command too, to save output into Input_file itself.

Explanation:

Atac will read Input_file from bottom line to first line then passing it’s output to awk where I am checking if first occurrence of topic is coming remove comma from last and rest of lines simply print then passing this output to tac again to make Input_file in original form again.