Working with Files and Directories in Clojure

This cookbook covers working with files and directories from Clojure, using functions in the clojure.java.io namespace as well as parts of the JDK via interoperability.

This work is licensed under a Creative Commons Attribution 3.0 Unported License (including images & stylesheets). The source is available on Github.

Preliminaries

Note that for the examples below, "io" is an alias for clojure.java.io. That is, it's assumed your ns macro contains:

(:require [clojure.java.io :as io])

or else in the repl you've loaded it:

(require '[clojure.java.io :as io])

Recipes

Read a file into one long string

(def a-long-string (slurp "foo.txt"))

Note, you can pass urls to slurp as well. See also slurp at Clojuredocs.

Read a file one line at a time

Suppose you'd like to call my-func on every line in a file, and return the resulting sequence:

(with-open [rdr (io/reader "foo.txt")]
  (mapv my-func (line-seq rdr)))

Note: mapv is eager and returns a vector. If you use map here, the reader will be closed before the whole sequence is realized so you would need to wrap the call to map in a call to doall anyway. The lines that line-seq gives you have no trailing newlines (and empty lines in the file will yield empty strings ("")).

Write a long string out to a new file

(spit "foo.txt"
      "A long
multi-line string.
Bye.")

Overwrites the file if it already exists. To append, use

(spit "foo.txt" "file content" :append true)

Write a file one line at a time

Although you could do this by calling spit with :append true for each line, that would be inefficient (opening the file for appending for each call). Instead, use with-open to keep a file open while writing each line to it:

(with-open [wrtr (io/writer "foo.txt")]
  (doseq [i my-vec]
    (.write wrtr (str i "\n"))))

Check if a file exists

(.exists (io/file "filename.txt"))

Is it a directory? :

(.isDirectory (io/file "path/to/something"))

An io/file is a java.io.File object (a file or a directory). You can call a number of functions on it, including:

exists        Does the file exist?
isDirectory   Is the File object a directory?
getName       The basename of the file.
getParent     The dirname of the file.
getPath       Filename with directory.
mkdir         Create this directory on disk.

To read about more available methods, see the java.io.File docs.

Get a list of the files and dirs in a given directory

As File objects:

(.listFiles (io/file "path/to/some-dir"))

Same, but just the names (strings), not File objects:

(.list (io/file "path/to/some-dir"))

The results of those calls are seqable.

See also