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Read the contents of a file into a string

clojure
(slurp "/tmp/foobar")

Process a file one line at a time

Open the source file to your solution and print each line in the file, prefixed by the line number, like:
1> First line of file
2> Second line of file
3> Third line of file
clojure
(defn read-line-by-line [fn]
(reduce str (map (partial format "%d> %s\n")
(iterate inc 1)
(read-lines fn))))

Write a string to a file

clojure
(with-out-writer "output.txt" (println "Hello file!"))

Append to a file

clojure
(with-out-append-writer "output.txt" (println "This is appended to the file"))

Process each file in a directory

clojure
; (defn process-file [f] "process one file" body...)
(map process-file (.listFiles (File. ".")))

Process each file in a directory recursively

clojure
; (defn process-file [f] "process one file" body...)
(map process-file (file-seq (File. ".")))