Why does `dir *.*` give me all files and folders?

Question

When I run dir *.* it produces unexpected results. Even files and folders without any dot in their names are listed. For example

C:\>dir *.*
 Volume in drive C is System
 Volume Serial Number is AC0A-29DA

 Directory of C:\

14-03-2017  05:17 PM             8,192 ntuser.dat
03-01-2017  07:10 PM    <DIR>          Perl520
28-03-2017  10:13 AM    <DIR>          Program Files
28-03-2017  10:13 AM    <DIR>          Program Files (x86)
04-01-2017  03:25 PM    <JUNCTION>     Python27 [C:\Program Files\Anaconda]
06-02-2017  10:16 PM    <DIR>          SAP
28-03-2017  04:10 PM               152 useragent.log
03-01-2017  03:04 PM    <DIR>          Users
16-03-2017  04:24 PM    <DIR>          VM
22-03-2017  11:13 AM    <DIR>          Windows
               2 File(s)          8,344 bytes
               8 Dir(s)  270,172,966,912 bytes free

Why is that? Is there any way to list only files with a dot?

Answer 1

I am writing this answer because OP has emphasized that:

what I'm interested is why *.* matches all files, as stated in the question

The DIR command comes from a time when:

1. Period (.) was not allowed as a character in file or folder names
2. File and folder names were restricted to 8 characters for name and 3 characters for extensions

Therefore, by that standard, *.* meant whatever the name and whatever the extension. It did not mean a string containing a ".", which may or may not have characters before or after the ".".

Microsoft policy is preserving backward compatibility. Hence, that interpretation of *.* is retained.

But in Windows PowerShell, *.* means a string containing a ".", which may or may not have characters before or after the ".".

Answer 2

Why is that?

One could find an answer to "Why is that?" in Wildcards article:

The * wildcard will match any sequence of characters
               (0 or more, including NULL characters)

The ? wildcard will match a single character
               (or a NULL at the end of a filename)

…

Wildcard matching rules

• * Generally matches any 0 or more characters, with one exception (see next rule). The non-greedy wildcard is free to match as many or as few characters as are necessary for the remainder of the mask to match.

• *. At end of mask matches any 0 or more characters except for {dot}. In actuality, the rule applies with any number of {dot} and {space} characters between the * and terminal {dot}. The regular expression for this term is "[*][. ]*[.]$"

• ? Match 0 or one character, except for {dot}. The only time it matches 0 characters is when it matches the end of the name, or the position before a {dot}. The question mark can also be used more than once to match more than one character.

Implication. The last {dot} in a file/folder name separates base name and extension. So

• dir *. displays all items with no extension, and
• dir *.* displays all items with extension of zero or more characters.

Strictly speaking, dir *. displays all items with no period (.) in name. (BTW, Naming Files, Paths, and Namespaces MSDN article says explicitly that "it is acceptable to specify a period as the first character of a name".)

Is there any way to list only files with a dot?

I don't think so. However, there is a workaround with a fitting regular expression.

PowerShell (full scope solution if used in a Powershell console):

:: PowerShell - no extension, full syntax
PowerShell -c "Get-ChildItem | Where-Object {$_.Name -match '^.[^\.]*$'}"
:: PowerShell -    extension, alias syntax
PowerShell -c "dir | ? {$_.Name -match '^..*\...*$'}"

Cmd (an idea only, might require some elaboration):

:: CMD/batch - no extension
for /F "delims=" %%G in ('dir /OGN /B ^| findstr "^.[^\.]*$"') do @echo %%~tG %%~aG %%~zG %%~nxG
:: CMD/batch -    extension
for /F "delims=" %%G in ('dir /OGN /B ^| findstr "^..*\...*$"') do @echo %%~tG %%~aG %%~zG %%~nxG

Addendum: a bonus and explanation

An intuitive guess that Name is concatenated BaseName and Extension does not hold. The following script proves it using cmd and PowerShell core features, and the weird ^..*\...*$ regex is derived from it's results.

@ECHO OFF
SETLOCAL EnableExtensions DisableDelayedExpansion
set "_workingDirectory=%~1"
if "%_workingDirectory%"=="%tmp%\tests_SU_1193102" (
  >NUL 2>&1 (
      mkdir "%_workingDirectory%"
      pushd "%_workingDirectory%"
      rem make directories
      mkdir     .Fldr-Ext
      mkdir     aFldr-Ext
      mkdir     .Fldr.Ext
      mkdir     aFldr.Ext
      rem create files
      copy  NUL .File-Ext 
      copy  NUL aFile-Ext 
      copy  NUL .File.Ext 
      copy  NUL aFile.Ext
      popd
  )
) else if "%_workingDirectory%"=="" set "_workingDirectory=%CD%"
pushd "%_workingDirectory%"
set "_first=ItemName  Attributes    BaseName Extension"
echo ON
::                        dir /OGN | findstr "Ext$"
for /F "delims=" %%G in ('dir /OGN /B') do @((if defined _first (echo %_first%&echo(&set "_first="))&echo %%~nxG %%~aG  %%~nG   %%~xG)
::   Get-ChildItem | Select-Object -Property Mode, BaseName, Extension, Name
PowerShell -c "dir | select -pr Name, Mode, BaseName, Extension | sort -pr @{Expression='Mode';Descending=$true}, @{Expression='Name';Descending=$false}"

Output:

==> D:\bat\BaseName_vs_Extension.bat "%tmp%\tests_SU_1193102"

==> for /F "delims=" %G in ('dir /OGN /B') do @((if defined _first (echo ItemName  Attributes   BaseName Extension  & echo(  & set "_first=" ) )  & echo %~nxG %~aG     %~nG    %~xG )
ItemName  Attributes    BaseName Extension

.Fldr.Ext d----------   .Fldr   .Ext
.Fldr-Ext d----------           .Fldr-Ext
aFldr.Ext d----------   aFldr   .Ext
aFldr-Ext d----------   aFldr-Ext
.File.Ext --a--------   .File   .Ext
.File-Ext --a--------           .File-Ext
aFile.Ext --a--------   aFile   .Ext
aFile-Ext --a--------   aFile-Ext

==> PowerShell -c "dir | select -pr Name, Mode, BaseName, Extension | sort -pr @{Expression='Mode';Descending=$true}, @{Expression='Name';Descending=$false}"

Name      Mode   BaseName  Extension
----      ----   --------  ---------
.Fldr.Ext d----- .Fldr.Ext .Ext
.Fldr-Ext d----- .Fldr-Ext .Fldr-Ext
aFldr.Ext d----- aFldr.Ext .Ext
aFldr-Ext d----- aFldr-Ext
.File.Ext -a---- .File     .Ext
.File-Ext -a----           .File-Ext
aFile.Ext -a---- aFile     .Ext
aFile-Ext -a---- aFile-Ext

Compare definition of BaseName property, different for files and folders:

PS D:\PShell> Get-ChildItem | Get-Member -Name BaseName | Format-List -property TypeName, Definition


TypeName   : System.IO.DirectoryInfo
Definition : System.Object BaseName {get=$this.Name;}

TypeName   : System.IO.FileInfo
Definition : System.Object BaseName {get=if ($this.Extension.Length -gt 
             0){$this.Name.Remove($this.Name.Length - 
             $this.Extension.Length)}else{$this.Name};}

---

My original answer was based on unforgivable misunderstanding:

Read dir /?, use dir /A:-D:

/A          Displays files with specified attributes.
attributes   D  Directories                R  Read-only files
             H  Hidden files               A  Files ready for archiving
             S  System files               I  Not content indexed files
             L  Reparse Points             -  Prefix meaning not

Another approach: apply findstr regex as dir *.* | findstr /V "<.*>"

Answer 3

Is there any way to list only files with a dot?

You can match filenames with a dot by using the undocumented wildcard <, which matches either a sequence of 0 or more characters in the basename or a sequence of 0 or more characters in the extension:

dir "<.<"

Remember that < is also a metacharacter, so you need to quote or escape it whenever using it in a pattern from the command shell.

Answer 4

The Command Line Interpreter will see "." as "All base filenames" with "all extensions". A blank extension is still an extension, so will be matched and returned with the list. As others have explained, this is a hangover from earlier versions of Windows and MS-DOS.

If you're happy to look using PowerShell, finding these files is a lot easier. Note that in PowerShell, "dir" is an alias for the "Get-ChildItem" cmdlet.

To find all files (not directories/folders) that have an extension (e.g. "myfile.txt", but not "FileWithNoExt"):

dir -af | Where {$_.Name -match "\."}

the "-af" switch is shorthand for "-File" which restricts the return objects to files only. "-ad" would only get directories. The "\." means "a literal dot character". Without the backslash, the dot would match any character, as per standard regex.

To get all files where there was a dot in the name without the extension, as well as an extension (e.g. "myDocument_v1.1.doc" but not "myfile.txt"):

dir -af | Where {$_.BaseName -match "\."}

Note however, that this will exclude a file with the name such as ".archive" with a preceding dot. This is because it's treated as having no basename and an extension of "archive". If you want to include these:

dir -af | Where {$_.BaseName -match "\.|^$"}

Here, the match pattern says "A literal dot, OR(|) BeginningOfString(^) followed by EndOfString($) (i.e. an empty string)". Since a file can't have both an empty basename and extension, it will find the files starting with a dot.

Answer 5

If you type above command, then here: * (this is a wild card which means any sequence of characters) followed by a dot(.) followed by * this could be interpreted as show all files which have name like (any sequence of characters).(any sequence of characters) which further means that show all files with any extension.