Extract file extension from file path

Question

How can I extract the extension of a file given a file path as a character? I know I can do this via regular expression regexpr("\\.([[:alnum:]]+)$", x), but wondering if there's a built-in function to deal with this?

Solution 1

This is the sort of thing that easily found with R basic tools. E.g.: ??path.

Anyway, load the tools package and read ?file_ext .

Solution 2

Let me extend a little bit great answer from https://stackoverflow.com/users/680068/zx8754

Here is the simple code snippet

  # 1. Load library 'tools'
  library("tools")

  # 2. Get extension for file 'test.txt'
  file_ext("test.txt")

The result should be 'txt'.

Solution 3

simple function with no package to load :

getExtension <- function(file){ 
    ex <- strsplit(basename(file), split="\\.")[[1]]
    return(ex[-1])
} 

Solution 4

The regexpr above fails if the extension contains non-alnum (see e.g. https://en.wikipedia.org/wiki/List_of_filename_extensions) As an altenative one may use the following function:

getFileNameExtension <- function (fn) {
# remove a path
splitted    <- strsplit(x=fn, split='/')[[1]]   
# or use .Platform$file.sep in stead of '/'
fn          <- splitted [length(splitted)]
ext         <- ''
splitted    <- strsplit(x=fn, split='\\.')[[1]]
l           <-length (splitted)
if (l > 1 && sum(splitted[1:(l-1)] != ''))  ext <-splitted [l] 
# the extention must be the suffix of a non-empty name    
ext

}

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Comments

[IRTFM]

It doesn't show up with ??"extensions" although one would have expected that it would.

[Carl Witthoft]

@DWin: "patience, grasshopper" :-). I would also recommend package:sos . It's very cool.

[IRTFM]

Witthof: Color me puzzled on two accounts; how does pkg:sos address that lack of appearance of tools::fiie_ext with ??() when a reasonable person would expect it to; and one would certainly need patience obtain value from a search strategy that delivers 20 pages with 400 hits?

[Ben Bolker]

sos does a full text search. ?? only searches metadata (title, keywords, etc.) Furthermore, it's not that hard to skim the results. (I tried findFn("{file extension}"), "extract {file extension}", and "{extract file extension}", the first was best.)

[user5359531]

This would be more useful with an actual code sample

[Carl Witthoft]

@user5359531 Did you read ?file-ext ? one-line code sample is kind of silly

[Rich Scriven]

Please scroll up and read the accepted answer to this question.

[Andrii]

Thank you, Rich! I read this comment and add this code just to show how it looks in the simple code snippet. Maybe it will be helpful for someone.

[MichaelChirico]

The functions basename and dirname obviate some of the work here

[Mikko]

@Pisca46: I would like to use a function like this in an R package. Did you write the function? If not, could you add a reference in your answer?

[Pisca46]

Yes, I wrote the function myself.

[Roman Luštrik]

Can you comment how this is an improvement over tools::file_ext?

[Enrique Pérez Herrero]

You'd better use tools function

[Zachary Ryan Smith]

Note well: tools::file_ext('foo.csv.gz') return 'gz', not 'csv.gz'

[Carl Witthoft]

@ZacharyRyanSmith Well, that's because the extension in your example is "gz" so the code is doing the correct thing. There's no such concept as "double extensions"

[Zachary Ryan Smith]

@CarlWitthoft, that's true. I was just warning people who confound path suffixes with a file's extension (people like me).

[Dannid]

The other comment may have been first and accepted, but it is nice to see the solution written out. The accepted answer just tells you where you find the answer. This one actually answers the question.

[Serhii]

The proposed function works incorrectly if the file contains dots in the filename. The function splits the filename and outputs the second element, while it should output the last one. For the following filename 'file.name.txt' the output is 'name', not 'txt'. tools::file_ext works fine.

[bers]

Don't use library(tools) when you can simply use tools::file_ext, such as in tools::file_ext("test.txt").