Filenames That Cross Platforms
The most conservative file naming conventions provide the most cross-platform compatibility.
TL;DR.
- 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
- How long can a directory name be?
- How long can a file name be?
- Can I use special characters in the directory name or file name?
Research on MAC, LINUX and Windows
Linux
- 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.
Conclusions:
- 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:
- Most restrict rule is known as 8.3, which shall always work on any platform.
- Avoid using “special” non-alphanumeric characters, they may be reserved for special purposes depending on the OS.
- Avoid using white space characters such as spaces, tabs, new lines and embedded returns.
- Filenames must not lead with
.(dot),- - Directory and file names should not be too long, 140 is recommand.
- Using filename extension, like
.jpgand.doc - 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\\.\\-]", "_");