Filenames That Cross Platforms

The most conservative file naming conventions provide the most cross-platform compatibility.


  • To compatible with old system shall follow 8.3 rule
  • Otherwise, limit filenames to the characters A-Z, a-z, 0-9, underscore ( _ ), period ( . ), and hyphen ( - ).

Raise the questions

I developed a module to our software, in which it collects images from a file system and concatenates its caption to file name, then zips all files. Users can download this zip file and unzip it.

Simple as? Nah! I need to answer the following questions

  1. How long can a directory name be?
  2. How long can a file name be?
  3. Can I use special characters in the directory name or file name?

Research on MAC, LINUX and Windows


  • Linux has a maximum filename length of 255 characters for most filesystems
  • eCryptfs recommends filenames shall no longer than 143, otherwise, it requires more than 255 characters to encrypt

Windows 10

  • Filename length is limited to 255 characters
  • MAX_PATH is defined as 260 characters
  • Reserved Characters:
    • < (less than),
    • > (greater than),
    • : (colon),
    • " (double quote),
    • / (forward slash),
    • \ (backslash),
    • | (vertical bar or pipe),
    • ? (question mark),
    • * (asterisk)

Give it a go on MAC

I’m using MAC with MAC OS X, so I just gave it a go.


  • Filename allows everything, except: /
  • Need to escape some special characters, like \?, \*. Otherwise, it behaves weirdly
  • Mac max directory name length: 255
  • Mac max file name length: 255

Not Allowed: /

  moss$ mkdir "/"
  mkdir: /: Is a directory
  moss$ mkdir "//"
  mkdir: //: Is a directory
  moss$ mkdir "/4"
  mkdir: /4: Permission denied
  moss$ mkdir haha/haha
  mkdir: haha: No such file or directory

Weird parts

If a file named ? exists, you cannot do mkdir *, which will return mkdir: ?: File exists

  moss$ ls -a
  .	..
  moss$ mkdir ?
  moss$ ls -a
  .	..	?
  moss$ mkdir *
  mkdir: ?: File exists
  moss$ ls -a
  .	..	?
  moss$ rm -rf ?
  moss$ mkdir *
  moss$ ls -a
  *	.	..

Mac max file name length: 255

touch [256_charactors_here] //File name too long
touch [255_charactors_here_including_extension] //File created

Research result:

  1. Most restrict rule is known as 8.3, which shall always work on any platform.
  2. Avoid using “special” non-alphanumeric characters, they may be reserved for special purposes depending on the OS.
  3. Avoid using white space characters such as spaces, tabs, new lines and embedded returns.
  4. Filenames must not lead with .(dot), -
  5. Directory and file names should not be too long, 140 is recommand.
  6. Using filename extension, like .jpg and .doc
  7. Must not use periods in directory (folder) names

Java implementation

Replace replace everything but [a-zA-Z0-9.-]

myString = myString.replaceAll("[^a-zA-Z0-9\\.\\-]", "_");