There does not exist any possibility to edit parts of a file without first reading it completely, editing it and saving it again
You already received my answer here 1) Read .csv file from phone, 2) Seach for a keyword 'Apple4321' 3) If keyword found, then look for the nearest word if it is 'send' 4) If the word 'send' is found do some action like 'play music' - #9 by Taifun
It is a bad design choice to use a file as data storage if you have to edit data frequently, because it is cumbersome to work with files
You have to read it completely, edit what you want to edit and save it again...
Remember, you have to use SAF to read/write non media files your app does not own, see also Some basics on Android storage system
Also you might want to think about how you plan to solve the problem of 2 processes (Tasker and your app) writing at the same time
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See again my suggestion without Tasker and without using csv files 1) Read .csv file from phone, 2) Seach for a keyword 'Apple4321' 3) If keyword found, then look for the nearest word if it is 'send' 4) If the word 'send' is found do some action like 'play music' - #28 by Taifun
Taifun