Bug #21391: Inconsistent trailing slash behavior of File.join and Pathname#join with empty strings

• Status: Open
• Assignee: -
• Target version: -
• ruby -v: ruby 3.4.4 (2025-05-14 revision a38531fd3f) +PRISM [x86_64-darwin23]
• Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN

[lovro-bikic (Lovro Bikić)]

File.join('/usr', '')
# => "/usr/"

Pathname.new('/usr').join('').to_s
# => "/usr" # no trailing slash

File.join('/usr', ' ')
# => "/usr/ "

Pathname.new('/usr').join(' ').to_s
# => "/usr/ "

File.join with an empty string adds a trailing slash, Pathname#join doesn't.
When Pathname#join argument is a string with empty whitespace, a trailing slash is added (plus whitespace).

I think it's a common use-case to append a trailing slash to Pathname, and currently you have to resort to other methods such as string interpolation (e.g. in Rails, "#{Rails.root}/") or File.join (e.g. File.join(Rails.root, '')).

In other popular languages, both approaches have been taken:

• os.path.join in Python adds a trailing slash:

import os
os.path.join('/usr', '')
# '/usr/'

• Path.join in Rust adds a trailing slash:

use std::path::{Path};

fn main() {
    println!("{}", Path::new("/usr").join("").display());
    // prints "/usr/"
}

• path.join in Node doesn't add a trailing slash:

const path = require('path');

path.join('/usr', '');
// '/usr'

• filepath.Join in Go doesn't add a trailing slash:

package main

import ("fmt"; "path/filepath")

func main() {
  fmt.Println(filepath.Join("/usr", ""))
  // prints "/usr"
}

[lovro-bikic (Lovro Bikić)]

• Subject changed from Inconsistent trailing slash behavior of File#join and Pathname#join with empty strings to Inconsistent trailing slash behavior of File.join and Pathname#join with empty strings

[Dan0042 (Daniel DeLorme)]

It's not the only inconsistent behavior:

File.join("/usr","/var")               #=> "/usr/var"
Pathname.new("/usr").join("/var").to_s #=> "/var"

File.join("/usr","../var")               #=> "/usr/../var"
Pathname.new("/usr").join("../var").to_s #=> "/var"

File.join("/usr","/../var")               #=> "/usr/../var"
Pathname.new("/usr").join("/../var").to_s #=> "/../var"

File.join simply joins two strings together with a separator, whereas Pathname#join is a logical operation on two paths. It's normal for there to be differences.

That being said, I feel that Pathname.new('/usr').join('') is a nonsensical operation. It seems to result in a no-op, but it might be better to warn or raise an error.

[lovro-bikic (Lovro Bikić)]

Dan0042 (Daniel DeLorme) wrote in #note-2:

It's not the only inconsistent behavior: (absolute and relative paths example)

To clarify, I don't expect the result of the two to be equivalent in all cases, it's clearly documented what each method does.

What I am reporting is that there's undocumented and possibly inconsistent behavior when it comes to empty strings. Furthermore, the example with a whitespace string shows that Pathname#join is capable of adding trailing slashes under certain conditions.

File.join behavior has been tested for empty strings, but Pathname#join hasn't, so it's unclear if this is a bug or an expected difference in behavior.

Dan0042 (Daniel DeLorme) wrote in #note-2:

That being said, I feel that Pathname.new('/usr').join('') is a nonsensical operation. It seems to result in a no-op, but it might be better to warn or raise an error.

Perhaps, but it's already a common pattern with File.join. Whether it's a nonsensical operation is up for debate, but I think there should be a clear way for Pathname#join to allow appending trailing slashes to pathnames.

[Dan0042 (Daniel DeLorme)]

Dan0042 (Daniel DeLorme) wrote in #note-2:

That being said, I feel that Pathname.new('/usr').join('') is a nonsensical operation. It seems to result in a no-op, but it might be better to warn or raise an error.

Ah, looks like I might have to retract that statement. In bash, cd "" is a no-op

cd /usr
cd ""
pwd  #=> /usr

So Pathname#join, which is equivalent to cd, has the same behavior. Not sure it makes sense, but at least it's consistent.
Although I should note that Dir.chdir("") raises an error.

lovro-bikic (Lovro Bikić) wrote in #note-3:

Furthermore, the example with a whitespace string shows that Pathname#join is capable of adding trailing slashes under certain conditions.

That's not a trailing slash, that the file/directory " " inside "usr". But it's true that if you do .join("a/") the resulting Pathname has a trailing slash, so indeed Pathname#join is capable of adding trailing slashes under certain conditions.

I think there should be a clear way for Pathname#join to allow appending trailing slashes to pathnames.

Perhaps, but personally I don't think that joining with an empty string should be it. Maybe more like .join("./") ?

[akr (Akira Tanaka)]

I don't recommend trailing slash on a pathname because it is not portable between operating systems.

The behavior of Pathname (it doesn't add a trailing slash) reflects this my opinion.

https://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xbd_chap04.html

| Two types of implementation have been prevalent; those that ignored trailing characters on all pathnames regardless, and those that permitted them only on existing directories.

It seems the standard tries to fix this situation, though.