what you say is for the GUI version brother. In ubuntu i don't have GUI version. Moreover, i use rpc. Hence i require the command line to be added on teh bitcoin config file. Please guide me to implement it on the ubuntu server.
For Bitcoin Core, you should really try the dynamic fee. It has a fairly decent estimate of the fees/byte to include based on the mempool info. To enable this, you can include txconfirmtarget=X into the Bitcoin.conf, where X is a positive integer and the lesser it is, the more fee it will pay and the faster it would confirm. Else, you can put -txconfirmtarget=x into the command line.
If you really want to set your own TX fee, put paytxfee=XX into the Bitcoin.conf, where XX is the fee/KB. Eg 0.001BTC/KB will be paid for each transaction. Similarly, you can set -paytxfee=XX using your command line.
Take note that paytxfee will always override txconfirmtarget. You shouldn't put paytxfee if you want to use dynamic fee(txconfirmtarget).