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One way to extract specific lines from a .txt file based on predetermined variable values in R is as follows:

1. Read in the .txt file using the readLines() function, which reads the file line by line and stores it as a character vector.

text <- readLines("file.txt")

1. Create a vector of predetermined variable values.

values <- c("value1", "value2", "value3")

1. Use the grep() function to search for the lines in the text vector that contain any of the predetermined values. The returned output is the index of each matched line.

matches <- grep(paste(values, collapse = "|"), text)

1. Extract the specific lines from the text vector based on the matched indices using the square bracket notation.

output <- text[matches]

The output variable will contain all the lines from the .txt file that contain any of the predetermined values.