====== Characters in file names ======

Note : this page often uses "Unix" for non-Windows systems, because Mac OS and Linux are both Unix-based systems. And the Web standard also originated on Unix systems, so for example, it's path separator is the same "/".

===== Path separators =====
* / in Unix and on the web. Windows also accpts it in some contexts, but sometimes not.
* \ in Windows. But it's an escape character in Unix.
* : was used in the old Mac OS up to version 9. See the [[#Mac : / confusion]] below.

===== Escape characters =====

  * \ Unix escape
  * ^ Windows cmd.exe escape
  * ` Windows PowerShell escape

(see also https://en.wikipedia.org/wiki/Escape_character)

===== Windows Powershell =====
The Windows Powershell (v. 2 in Win7) has a function `GetInvalidFileNameChars()` :

<code>
[System.IO.Path]::GetInvalidFileNameChars() | ForEach-Object {$i=[int]$_; '{0,3}  {1:X2}  {2}' -f $i,$i,$_ } | Sort-Object
  0  00
  1  01  ☺
  2  02  ☻
  3  03  ♥
  4  04  ♦
  5  05  ♣
  6  06  ♠
  7  07
  8  08
  9  09
 10  0A

 11  0B  ♂
 12  0C  ♀
 13  0D
 14  0E  ♫
 15  0F  ☼
 16  10  ►
 17  11  ◄
 18  12  ↕
 19  13  ‼
 20  14  ¶
 21  15  §
 22  16  ▬
 23  17  ↨
 24  18  ↑
 25  19  ↓
 26  1A  →
 27  1B  ←
 28  1C  ∟
 29  1D  ↔
 30  1E  ▲
 31  1F  ▼
 34  22  "
 42  2A  *
 47  2F  /
 58  3A  :
 60  3C  <
 62  3E  >
 63  3F  ?
 92  5C  \
124  7C  |
</code>

So it lists the obvious control characters 0 to 31 (x00-x1F), plus
  * "
  * *
  * /
  * :
  * <
  * >
  * ?
  * \
  * |

===== At start or end of file name =====
  * space at the end of a file or directory name is ignored/removed by Windows. It is OK in Unix, but obviously a problem because it's invisible.
  * . dot at the end of a file or directory name is ignored/removed by Windows.
  * . dot at the start of a file or directory name makes it "hidden" in Unix. It will not show in normal directory listings and file managers.

===== Mac : / confusion =====
Mac OS in the eighties and nineties (up to version 9) was not a Unix-based system, and used ":" as a path separator. "/" had no special meaning for Macs, so it was accepted in file names like "report 25/3/1992".

When Mac replaced it's OS with Unix, "/" became an invalid character in file names, but on the other hand, ":" was now a perfectly normal character. So it decided to silently replace all "/" with ":", and made it's graphical interface handle the translation invisibly. So you can still see this "report 25/3/1992" file in the Finder, and you can have the impression that you can still write a file named "report 25/12/2019". However, the real file name in the file system will be "report 25:3:1992", and some Mac programs will only see the real file name and not "translate" it on the fly.