使用R，如何在不打开文件的情况下找到文件中特定短语的行号?

I am new to R. I have a large (3.2 Gb) txt file containing two columns. First column has the human genome sequence position and the Second column has a value corresponding to each position. I want to find the line numbers for specific positions from the first column and then read those lines into a table in R. I cannot import the file because of memory issue. here is an example of the R code I tried to get the line number of one specific position from the first column of my data file. the data file is called my.data.I am new to R. I have a large (3.2 Gb) txt file