I know that / is illegal in Linux, and the following are illegal in Windows (I think) * . " / \ [ ] : ; | ,

What else am I missing?

I need a comprehensive guide, however, and one that takes into account double-byte characters. Linking to outside resources is fine with me.

I need to first create a directory on the filesystem using a name that may contain forbidden characters, so I plan to replace those characters with underscores. I then need to write this directory and its contents to a zip file (using Java), so any additional advice concerning the names of zip directories would be appreciated.

Solution 1

  1. The forbidden printable ASCII characters are:

    • Linux/Unix:

        / (forward slash)
    • Windows:

        < (less than)
        > (greater than)
        : (colon - sometimes works, but is actually NTFS Alternate Data Streams)
        " (double quote)
        / (forward slash)
        \ (backslash)
        | (vertical bar or pipe)
        ? (question mark)
        * (asterisk)
  2. Non-printable characters

    If your data comes from a source that would permit non-printable characters then there is more to check for.

    • Linux/Unix:

        0 (NULL byte)
    • Windows:

        0-31 (ASCII control characters)

    Note: While it is legal under Linux/Unix file systems to create files with control characters in the filename, it might be a nightmare for the users to deal with such files.

  3. Reserved file names

    The following filenames are reserved:

    • Windows:

        CON, PRN, AUX, NUL 
        COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9
        LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, LPT9

      (both on their own and with arbitrary file extensions, e.g. LPT1.txt).

  4. Other rules

    • Windows:

      Filenames cannot end in a space or dot.

    • macOS:

      You didn't ask for it, but just in case: Colon : and forward slash / depending on context are not permitted (e.g. Finder supports slashes, terminal supports colons). (More details)

Solution 2

A comprehensive guide of forbidden filename characters is not going to work on Windows because it reserves filenames as well as characters. Yes, characters like * " ? and others are forbidden, but there are a infinite number of names composed only of valid characters that are forbidden. For example, spaces and dots are valid filename characters, but names composed only of those characters are forbidden.

Windows does not distinguish between upper-case and lower-case characters, so you cannot create a folder named A if one named a already exists. Worse, seemingly-allowed names like PRN and CON, and many others, are reserved and not allowed. Windows also has several length restrictions; a filename valid in one folder may become invalid if moved to another folder. The rules for naming files and folders are on the Microsoft docs.

You cannot, in general, use user-generated text to create Windows directory names. If you want to allow users to name anything they want, you have to create safe names like A, AB, A2 et al., store user-generated names and their path equivalents in an application data file, and perform path mapping in your application.

If you absolutely must allow user-generated folder names, the only way to tell if they are invalid is to catch exceptions and assume the name is invalid. Even that is fraught with peril, as the exceptions thrown for denied access, offline drives, and out of drive space overlap with those that can be thrown for invalid names. You are opening up one huge can of hurt.

Solution 3

Under Linux and other Unix-related systems, there are only two characters that cannot appear in the name of a file or directory, and those are NUL '\0' and slash '/'. The slash, of course, can appear in a pathname, separating directory components.

Rumour1 has it that Steven Bourne (of 'shell' fame) had a directory containing 254 files, one for every single letter (character code) that can appear in a file name (excluding /, '\0'; the name . was the current directory, of course). It was used to test the Bourne shell and routinely wrought havoc on unwary programs such as backup programs.

Other people have covered the rules for Windows filenames, with links to Microsoft and Wikipedia on the topic.

Note that MacOS X has a case-insensitive file system. Current versions of it appear to allow colon : in file names, though historically that was not always the case:

$ echo a:b > a:b
$ ls -l a:b
-rw-r--r--  1 jonathanleffler  staff  4 Nov 12 07:38 a:b

POSIX defines a Portable Filename Character Set consisting of:

a b c d e f g h i j k l m n o p q r s t u v w x y z
0 1 2 3 4 5 6 7 8 9 . _ -

Sticking with names formed solely from those characters avoids most of the problems, though Windows still adds some complications.

1 It was Kernighan & Pike in ['The Practice of Programming'](http://www.cs.princeton.edu/~bwk/tpop.webpage/) who said as much in Chapter 6, Testing, §6.5 Stress Tests:

When Steve Bourne was writing his Unix shell (which came to be known as the Bourne shell), he made a directory of 254 files with one-character names, one for each byte value except '\0' and slash, the two characters that cannot appear in Unix file names. He used that directory for all manner of tests of pattern-matching and tokenization. (The test directory was of course created by a program.) For years afterwards, that directory was the bane of file-tree-walking programs; it tested them to destruction.

Note that the directory must have contained entries . and .., so it was arguably 253 files (and 2 directories), or 255 name entries, rather than 254 files. This doesn't affect the effectiveness of the anecdote, or the careful testing it describes.

TPOP was previously at http://plan9.bell-labs.com/cm/cs/tpop and http://cm.bell-labs.com/cm/cs/tpop but both are now (2021-11-12) broken. See also Wikipedia on TPOP.

Solution 4

Instead of creating a blacklist of characters, you could use a whitelist. All things considered, the range of characters that make sense in a file or directory name context is quite short, and unless you have some very specific naming requirements your users will not hold it against your application if they cannot use the whole ASCII table.

It does not solve the problem of reserved names in the target file system, but with a whitelist it is easier to mitigate the risks at the source.

In that spirit, this is a range of characters that can be considered safe:

  • Letters (a-z A-Z) - Unicode characters as well, if needed
  • Digits (0-9)
  • Underscore (_)
  • Hyphen (-)
  • Space
  • Dot (.)

And any additional safe characters you wish to allow. Beyond this, you just have to enforce some additional rules regarding spaces and dots. This is usually sufficient:

  • Name must contain at least one letter or number (to avoid only dots/spaces)
  • Name must start with a letter or number (to avoid leading dots/spaces)
  • Name may not end with a dot or space (simply trim those if present, like Explorer does)

This already allows quite complex and nonsensical names. For example, these names would be possible with these rules, and be valid file names in Windows/Linux:

  • A...........ext
  • B -.- .ext

In essence, even with so few whitelisted characters you should still decide what actually makes sense, and validate/adjust the name accordingly. In one of my applications, I used the same rules as above but stripped any duplicate dots and spaces.

Solution 5

The easy way to get Windows to tell you the answer is to attempt to rename a file via Explorer and type in a backslash, /, for the new name. Windows will popup a message box telling you the list of illegal characters.

A filename cannot contain any of the following characters:
    \ / : * ? " < > | 

Microsoft Docs - Naming Files, Paths, and Namespaces - Naming Conventions

Solution 6

Well, if only for research purposes, then your best bet is to look at this Wikipedia entry on Filenames.

If you want to write a portable function to validate user input and create filenames based on that, the short answer is don't. Take a look at a portable module like Perl's File::Spec to have a glimpse to all the hops needed to accomplish such a "simple" task.

Solution 7

Discussing different possible approaches

Difficulties with defining, what's legal and not were already adressed and whitelists were suggested. But Windows supports more-than-8-bit characters. Wikipedia states, that (for example) the

modifier letter colon [(See 7. below) is] sometimes used in Windows filenames as it is identical to the colon in the Segoe UI font used for filenames. The [inherited ASCII] colon itself is not permitted.

Therefore, I want to present a much more liberal approach using Unicode Homoglyph characters to replace the "illegal" ones. I found the result in my comparable use-case by far more readable. Plus you can even restore the original content from the replacements.

Possible choices and research notes

To keep things organized, I will always give the character, it's name and the hexadecimal number representation. The latter is is not case sensitive and leading zeroes can be added or ommitted freely, so for example U+002A and u+2a are equivalent. If available, I'll try to point to more info or alternatives - feel free to show me more or better ones.

  1. Instead of * (U+2A * ASTERISK), you can use one of the many listed, for example U+2217 (ASTERISK OPERATOR) or the Full Width Asterisk U+FF0A . u+20f0 combining asterisk above from combining diacritical marks for symbols might also be a valid choice.

  2. Instead of . (U+2E . full stop), one of these could be a good option, for example U+22C5 dot operator

  3. Instead of " (U+22 " quotation mark), you can use U+201C english leftdoublequotemark, more alternatives see here. I also included some of the good suggestions of Wally Brockway's answer, in this case u+2036 reversed double prime and u+2033 double prime - I will from now on denote ideas from that source by .

  4. Instead of / (U+2F / SOLIDUS), you can use DIVISION SLASH U+2215 (others here), ̸ U+0338 COMBINING LONG SOLIDUS OVERLAY, ̷ COMBINING SHORT SOLIDUS OVERLAY U+0337 or u+2044 fraction slash. Be aware about spacing for some characters, including the combining or overlay ones, as they have no width and can produce something like -> ̸th̷is which is ̸th̷is. With added spaces you get -> ̸ th ̷ is, which is ̸ th ̷ is. The second one (COMBINING SHORT SOLIDUS OVERLAY) looks bad in the stackoverflow-font.

  5. Instead of \ (U+5C Reverse solidus), you can use U+29F5 Reverse solidus operator (more) or u+20E5 combining reverse solidus overlay

  6. To replace [ (U+5B [ Left square bracket) and ] (U+005D ] Right square bracket), you can use for example U+FF3B FULLWIDTH LEFT SQUARE BRACKET and U+FF3D FULLWIDTH RIGHT SQUARE BRACKET (from here, more possibilities here)

  7. Instead of : (u+3a : colon), you can use U+2236 RATIO (for mathematical usage) or U+A789 MODIFIER LETTER COLON, (see colon (letter), sometimes used in Windows filenames as it is identical to the colon in the Segoe UI font used for filenames. The colon itself is not permitted ... source and more replacements see here). Another alternative is this one: u+1361 ethiopic wordspace

  8. Instead of ; (u+3b ; semicolon), you can use U+037E ; GREEK QUESTION MARK (see here)

  9. For | (u+7c | vertical line), there are some good substitutes such as: U+2223 DIVIDES, U+0964 DEVANAGARI DANDA, U+01C0 ǀ LATIN LETTER DENTAL CLICK (the last ones from Wikipedia) or U+2D4F Tifinagh Letter Yan. Also the box drawing characters contain various other options.

  10. Instead of , (, U+002C COMMA), you can use for example U+201A SINGLE LOW-9 QUOTATION MARK (see here)

  11. For ? (U+003F ? QUESTION MARK), these are good candidates: U+FF1F FULLWIDTH QUESTION MARK or U+FE56 SMALL QUESTION MARK (from here and here). There are also two more from the Dingbats Block (search for "question") and the u+203d interrobang

  12. While my machine seems to accept it unchanged, I still want to include > (u+3e greater-than sign) and < (u+3c less-than sign) for the sake of completeness. The best replacement here is probably also from the quotation block, such as u+203a single right-pointing angle quotation mark and u+2039 single left-pointing angle quotation mark respectively. The tifinagh block only contains (u+2D66) to replace <. The last notion is less-than with dot u+22D6 and greater-than with dot u+22D7.

For additional ideas, you can also look for example into this block. You still want more ideas? You can try to draw your desired character and look at the suggestions here.

How do you type these characters

Say you want to type (Tifinagh Letter Yan). To get its information, you can always search for this character on a suited platform such as the Unicode Lookup (add 0x when you search for hex) or this Unicode Table (that only allows to search for the name, in this case "Tifinagh Letter Yan"). You should obtain its Unicode number U+2D4F and the HTML-code &#11599; (note that 2D4F is hexadecimal for 11599). With this knowledge, you have several options to produce these special characters including the use of

  • code points to unicode converter or again the Unicode Lookup to reversely, convert the numerical representation into the unicode character - set the code point base below to decimal or hexadecimal respectively
  • a one-liner makro in Autohotkey: :?*:altpipe::{U+2D4F} to type instead of the string altpipe - this is the way I input those special characters, my Autohotkey script can be shared if there is common interest
  • Alt Characters or alt-codes by pressing and holding alt, followed by the decimal number for the desired character (more info for example here, look at a table here or there). For the example, that would be Alt+11599. Be aware, that many programs do not fully support this windows feature for all of unicode (as of time writing). Microsoft Office is an exception where it usually works, some other OSes provide similar functionality. Typing these chars with Alt-combinations into MS Word is also the way Wally Brockway suggests in his answer that was already mentionted - if you don't want to transfer all the hexadecimal values to the decimal asc, you can find some of them there.
  • in MS Office, you can also use ALT + X as described in this MS article to produce the chars
  • if you rarely need it, you can of course still just copy-paste the special character of your choice instead of typing it

Solution 8

For Windows you can check it using PowerShell

$PathInvalidChars = [System.IO.Path]::GetInvalidPathChars() #36 chars

To display UTF-8 codes you can convert

$enc = [system.Text.Encoding]::UTF8
$PathInvalidChars | foreach { $enc.GetBytes($_) }

$FileNameInvalidChars = [System.IO.Path]::GetInvalidFileNameChars() #41 chars

$FileOnlyInvalidChars = @(':', '*', '?', '\', '/') #5 chars - as a difference

Solution 9

For anyone looking for a regex:

const BLACKLIST = /[<>:"\/\\|?*]/g;

Solution 10

In Windows 10 (2019), the following characters are forbidden by an error when you try to type them:

A file name can't contain any of the following characters:

\ / : * ? " < > |

Solution 11

Here's a c# implementation for windows based on Christopher Oezbek's answer

It was made more complex by the containsFolder boolean, but hopefully covers everything

/// <summary>
/// This will replace invalid chars with underscores, there are also some reserved words that it adds underscore to
/// </summary>
/// <remarks>
/// https://stackoverflow.com/questions/1976007/what-characters-are-forbidden-in-windows-and-linux-directory-names
/// </remarks>
/// <param name="containsFolder">Pass in true if filename represents a folder\file (passing true will allow slash)</param>
public static string EscapeFilename_Windows(string filename, bool containsFolder = false)
    StringBuilder builder = new StringBuilder(filename.Length + 12);

    int index = 0;

    // Allow colon if it's part of the drive letter
    if (containsFolder)
        Match match = Regex.Match(filename, @"^\s*[A-Z]:\\", RegexOptions.IgnoreCase);
        if (match.Success)
            index = match.Length;

    // Character substitutions
    for (int cntr = index; cntr < filename.Length; cntr++)
        char c = filename[cntr];

        switch (c)
            case '\u0000':
            case '\u0001':
            case '\u0002':
            case '\u0003':
            case '\u0004':
            case '\u0005':
            case '\u0006':
            case '\u0007':
            case '\u0008':
            case '\u0009':
            case '\u000A':
            case '\u000B':
            case '\u000C':
            case '\u000D':
            case '\u000E':
            case '\u000F':
            case '\u0010':
            case '\u0011':
            case '\u0012':
            case '\u0013':
            case '\u0014':
            case '\u0015':
            case '\u0016':
            case '\u0017':
            case '\u0018':
            case '\u0019':
            case '\u001A':
            case '\u001B':
            case '\u001C':
            case '\u001D':
            case '\u001E':
            case '\u001F':

            case '<':
            case '>':
            case ':':
            case '"':
            case '/':
            case '|':
            case '?':
            case '*':

            case '\\':
                builder.Append(containsFolder ? c : '_');


    string built = builder.ToString();

    if (built == "")
        return "_";

    if (built.EndsWith(" ") || built.EndsWith("."))
        built = built.Substring(0, built.Length - 1) + "_";

    // These are reserved names, in either the folder or file name, but they are fine if following a dot
    // CON, PRN, AUX, NUL, COM0 .. COM9, LPT0 .. LPT9
    builder = new StringBuilder(built.Length + 12);
    index = 0;
    foreach (Match match in Regex.Matches(built, @"(^|\\)\s*(?<bad>CON|PRN|AUX|NUL|COM\d|LPT\d)\s*(\.|\\|$)", RegexOptions.IgnoreCase))
        Group group = match.Groups["bad"];
        if (group.Index > index)
            builder.Append(built.Substring(index, match.Index - index + 1));

        builder.Append("_");        // putting an underscore after this keyword is enough to make it acceptable

        index = group.Index + group.Length;

    if (index == 0)
        return built;

    if (index < built.Length - 1)

    return builder.ToString();

Solution 12

Though the only illegal Unix chars might be / and NULL, although some consideration for command line interpretation should be included.

For example, while it might be legal to name a file 1>&2 or 2>&1 in Unix, file names such as this might be misinterpreted when used on a command line.

Similarly it might be possible to name a file $PATH, but when trying to access it from the command line, the shell will translate $PATH to its variable value.

Solution 13

The .NET Framework System.IO provides the following functions for invalid file system characters:

Those functions should return appropriate results depending on the platform the .NET runtime is running in. That said, the Remarks in the documentation pages for those functions say:

The array returned from this method is not guaranteed to contain the complete set of characters that are invalid in file and directory names. The full set of invalid characters can vary by file system.

Solution 14

I always assumed that banned characters in Windows filenames meant that all exotic characters would also be outlawed. The inability to use ?, / and : in particular irked me. One day I discovered that it was virtually only those chars which were banned. Other Unicode characters may be used. So the nearest Unicode characters to the banned ones I could find were identified and MS Word macros were made for them as Alt+?, Alt+: etc. Now I form the filename in Word, using the substitute chars, and copy it to the Windows filename. So far I have had no problems.

Here are the substitute chars (Alt + the decimal Unicode) :

  • Alt8432
  • Alt8260
  • Alt8421
  • Alt8739
  • Alt11622
  • Alt11162
  • Alt8253
  • Alt4961
  • Alt8246
  • Alt8243

As a test I formed a filename using all of those chars and Windows accepted it.

Solution 15

This is good enough for me in Python:

def fix_filename(name, max_length=255):
    Replace invalid characters on Linux/Windows/MacOS with underscores.
    List from https://stackoverflow.com/a/31976060/819417
    Trailing spaces & periods are ignored on Windows.
    >>> fix_filename("  COM1  ")
    '_ COM1 _'
    >>> fix_filename("COM10")
    >>> fix_filename("COM1,")
    >>> fix_filename("COM1.txt")
    >>> all('_' == fix_filename(chr(i)) for i in list(range(32)))
    return re.sub(r'[/\\:|<>"?*\0-\x1f]|^(AUX|COM[1-9]|CON|LPT[1-9]|NUL|PRN)(?![^.])|^\s|[\s.]$', "_", name[:max_length], flags=re.IGNORECASE)

See also this outdated list for additional legacy stuff like = in FAT32.

Solution 16

As of 18/04/2017, no simple black or white list of characters and filenames is evident among the answers to this topic - and there are many replies.

The best suggestion I could come up with was to let the user name the file however he likes. Using an error handler when the application tries to save the file, catch any exceptions, assume the filename is to blame (obviously after making sure the save path was ok as well), and prompt the user for a new file name. For best results, place this checking procedure within a loop that continues until either the user gets it right or gives up. Worked best for me (at least in VBA).

Solution 17

In Unix shells, you can quote almost every character in single quotes '. Except the single quote itself, and you can't express control characters, because \ is not expanded. Accessing the single quote itself from within a quoted string is possible, because you can concatenate strings with single and double quotes, like 'I'"'"'m' which can be used to access a file called "I'm" (double quote also possible here).

So you should avoid all control characters, because they are too difficult to enter in the shell. The rest still is funny, especially files starting with a dash, because most commands read those as options unless you have two dashes -- before, or you specify them with ./, which also hides the starting -.

If you want to be nice, don't use any of the characters the shell and typical commands use as syntactical elements, sometimes position dependent, so e.g. you can still use -, but not as first character; same with ., you can use it as first character only when you mean it ("hidden file"). When you are mean, your file names are VT100 escape sequences ;-), so that an ls garbles the output.

Solution 18

When creating internet shortcuts in Windows, to create the file name, it skips illegal characters, except for forward slash, which is converted to minus.

Solution 19

I had the same need and was looking for recommendation or standard references and came across this thread. My current blacklist of characters that should be avoided in file and directory names are:

$CharactersInvalidForFileName = {
    "pound" -> "#",
    "left angle bracket" -> "<",
    "dollar sign" -> "$",
    "plus sign" -> "+",
    "percent" -> "%",
    "right angle bracket" -> ">",
    "exclamation point" -> "!",
    "backtick" -> "`",
    "ampersand" -> "&",
    "asterisk" -> "*",
    "single quotes" -> "",
    "pipe" -> "|",
    "left bracket" -> "{",
    "question mark" -> "?",
    "double quotes" -> "",
    "equal sign" -> "=",
    "right bracket" -> "}",
    "forward slash" -> "/",
    "colon" -> ":",
    "back slash" -> "\\",
    "lank spaces" -> "b",
    "at sign" -> "@"