What characters should be restricted from a Unix file name?

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+2 Q:

What characters should be restricted from a Unix file name?

Consider a Save As dialog with a free text entry where the user enters a file name as free text, then clicks a Save button. The software then validates the file name, and saves the file if the name is valid.

On a Unix file system, what rules should be applied in the validation such that:

The name will not be difficult to manipulate later in terms of escaping special characters, etc.
The rules are not so restrictive that saving a file becomes non-user-friendly.

So basically, what is the minimum set of characters that should be restricted from a Unix file name?

Gavin Miller 2009-01-19 15:36:35

Then what characters should be whitelisted?

Pim Jager 2009-01-19 15:37:18

The characters given so far sound good. A hyphen and a period would be good additions as well.

workmad3 2009-01-19 15:42:42

@workmad3 - good suggestions; I made the answer wiki, feel free to add.

Gavin Miller 2009-01-19 15:46:29

Newlines are a nuisance. Commas are pretty harmless. Colon would do no damage in Unix, but are problematic if the name is copied to Windows - or if the 'file' is a directory that might need to be added to PATH.

Jonathan Leffler 2009-01-19 15:53:39

There is some room to argue that any characters classified as 'isalpha()' in the current locale are OK - that allows people to use accented characters in the names. It complicates the story, though.

Jonathan Leffler 2009-01-19 15:57:34

i for one will regard anything that probits accented characters as user-unfriendly

hop 2009-01-19 16:45:37

+4 A:

The minimum are slash ('/') and NULL ('\0')

mouviciel 2009-01-19 15:37:25

The minimum is /, ; and | to avoid the user running arbitrary commands (assuming it's not escaped :))

workmad3 2009-01-19 15:43:28

This. No characters besides '/' should be disallowed.

Andrew Medico 2009-01-19 15:47:05

And ASCII NUL '\0' since that marks the end of the file name :D

Jonathan Leffler 2009-01-19 15:47:46

This is the rigourous answer. The application should be coded to assume that the user was this unconstrained (so when opening files, it should accept any name). It isn't such a good answer for saving (new) files; it is reasonable to put some limits on the file names.

Jonathan Leffler 2009-01-19 15:56:05

+2 A:

Let the user enter whatever name he wants. Artificially restricting the range of characters will only annoy the users and serve no real purpose.

Bombe 2009-01-19 15:38:57

sounds good... I'll enter a file called 'blah;rm -rf /' ;)

workmad3 2009-01-19 15:41:13

+1 for the comment!

Gavin Miller 2009-01-19 15:42:27

Or, better: '$(rm -fr $HOME)' (minus the single quotes) as the file name? That will wreak havoc sooner rather than later. Backticks and $(...) are particularly pernicious as they 'work' when the file name is quoted, unlike most of the other special characters. Embedded quotes are tricky, too.

Jonathan Leffler 2009-01-19 15:51:46

Those are all non-issues when saving the filename. fopen() doesn’t care about your filenames. When using a graphical shell (e.g. konqueror) it doesn’t care about your filenames. When you use auto-completion in the shell it doesn’t care about your filenames. So what are your points? :)

Bombe 2009-01-19 15:53:29

@Bombe, what one user might want in many cases will alienate other users, regardless of the havoc it plays with your UI development process. Bad idea.

le dorfier 2009-01-19 16:52:00

That’s my point: choosing strange names will not wreak havoc with anything—unless your “anything” is badly written. None of the standard tools of UNIX is badly written. Again: what’s your point?

Bombe 2009-01-19 17:13:13

+2 A:

Do not forget the dot (.) so that you can hide files and folders... Otherwise, I'd follow a UN*X name convention (from wikipedia):

Most UNIX file systems

Case handling: case-sensitive case-preservation
Allowed character set: any
Reserved characters: / null
Max length: 255
Notes: A leading . indicates that ls and file managers will not by default show the file

Link to wikipedia article about file names

Tobias Wärre 2009-01-19 15:49:02

Often forgotten: the colon (:) is not a good idea, since it's commonly used in stuff like $PATH, i.e. the list of directories where executables are found "automatically". This can cause confusion with DOS/Windows directory names, where of course the colon is used in drive names.

unwind 2009-01-19 16:49:39

Please don't use spaces! They work, but can be a big pain to work with on the command line.

Judge Maygarden 2009-01-19 16:52:09

ansaurus

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What characters should be restricted from a Unix file name?

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