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Filenames

The rules for valid filenames vary with the type of filesystem, but generally, filenames may include:

  • letters (a-z and A-Z)
  • numbers (0-9)
  • dashes (-)
  • underscores (_)
  • periods (.)

Other punctuation marks may be acceptable in some filesystems but not in others, and are therefore best avoided, especially if files may be transferred between different types of filesystems or between computers.

Characters from other alphabets are increasingly supported on modern filesystems (and you can even use emoji in many filename!), but older or more basic filesystems may not support them.

Spaces may be included in filenames, but may require quoting if accessed from the command-line, so that the shell does not interpret the filename as two or more separate filenames. For example, the filename "red leaf" may be interpreted as two separate filenames if written without quoting:

ls red leaf

When quotes are added, the ambiguity is removed, and the shell will correctly interpret the filename as a single name:

ls "red leaf"

For this reason, it is good practice to avoid using spaces in filenames (underscores are a good alternative).

Extensions​

Many operating systems use an extension at the end of a filename to denote the type of data stored in the file. These extensions are delimited by a period followed by one or more characters. For example, in the filename:

ops102_project.pdf

The extension is "pdf", denoting a file in Portable Document Format.

It is unusual to use multiple extensions on a Windows system, but not uncommon on a Linux (or other Unix-like) system. For example, on a Linux system, the filename

backup.tar.gz

has two extensions, "tar" indicating a archive created with the tar command, and "gz" indiciating that the file was compressed with the gzip command.

Case Sensitivity​

Some filesystems are case-sensitive, and UPPER- and lower-case letters are considered to be different. For example, the filenames MILK.PDF, Milk.pdf, and milk.pdf refer to three different files. This is the case with most Linux (and Unix-like) filesystems.

Most Windows filesystems are not case-sensitive, so the filenames MILK.PDF, Milk.pdf, and milk.pdf would refer to the same file.