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.
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)
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.
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).
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)
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.
Under Linux and other Unix-related systems, there were traditionally only two characters that could not 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 necessarily 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
$
However, at least with macOS Big Sur 11.7, the file system does not allow file names that are not valid UTF-8 strings. That means the file name cannot consist of the bytes that are always invalid in UTF-8 (0xC0, 0xC1, 0xF5-0xFF), and you can't use the continuation bytes 0x80..0xBF as the only byte in a file name. The error given is 92 Illegal byte sequence.
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
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.
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.
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
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.
Discussing different possible approaches
Difficulties with defining, what's legal and not were already adressed and whitelists were suggested. But not only Windows, but also many unixoid OSes support more-than-8-bit characters such as Unicode. You could here also talk about encodings such as UTF-8. You can consider Jonathan Leffler's comment, where he gives info about modern Linux and describes details for MacOS. 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 and it's only limited by the used font, which is very broad, 3903 characters for Windows default. 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.
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. You can read 4. for more info about the combining characters.
Instead of . (U+2E . full stop), one of these could be a good option, for example ⋅ U+22C5 dot operator.
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 ¹³.
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 slash¹³. 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.
Instead of \ (U+5C Reverse solidus), you can use ⧵ U+29F5 Reverse solidus operator (more) or u+20E5 ⃥ combining reverse solidus overlay¹³.
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).
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 wordspace¹³.
Instead of ; (u+3b ; semicolon), you can use U+037E ; GREEK QUESTION MARK (see here).
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 ⵏ Tifinagh Letter Yan. Also the box drawing characters contain various other options.
Instead of , (, U+002C COMMA), you can use for example ‚ U+201A SINGLE LOW-9 QUOTATION MARK (see here).
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 ‽ interrobang¹³.
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 ⵦ (u+2D66)¹³ 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 ⵏ (Tifinagh Letter Yan). To get all of its information, you can always search for this character (ⵏ) on a suited platform such as this Unicode Lookup (please add 0x when you search for hex) or that 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 ⵏ (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 (remember to set the code point base below to decimal or hexadecimal respectively)
a one-liner makro in Autohotkey: :?*:altpipe::{U+2D4F} to type ⵏ 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 answer¹³ 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 there¹³.
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
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
For anyone looking for a regex:
const BLACKLIST = /[<>:"\/\\|?*]/g;
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:
\ / : * ? " < > |
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)
{
builder.Append(match.Value);
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 '*':
builder.Append('_');
break;
case '\\':
builder.Append(containsFolder ? c : '_');
break;
default:
builder.Append(c);
break;
}
}
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(group.Value);
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)
{
builder.Append(built.Substring(index));
}
return builder.ToString();
}
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 $PATH to its variable value.
The .NET Framework System.IO provides the following functions for invalid file system characters:
Path.GetInvalidFileNameChars
Path.GetInvalidPathChars
Those functions should return appropriate results depending on the platform the .NET runtime is running in. That said, the Remarks in the documentation pages for those functions say:
The array returned from this method is not guaranteed to contain the
complete set of characters that are invalid in file and directory
names. The full set of invalid characters can vary by file system.
I always assumed that banned characters in Windows filenames meant that all exotic characters would also be outlawed. The inability to use ?, / and : in particular irked me. One day I discovered that it was virtually only those chars which were banned. Other Unicode characters may be used. So the nearest Unicode characters to the banned ones I could find were identified and MS Word macros were made for them as Alt+?, Alt+: etc. Now I form the filename in Word, using the substitute chars, and copy it to the Windows filename. So far I have had no problems.
Here are the substitute chars (Alt + the decimal Unicode) :
⃰ ⇔ Alt8432
⁄ ⇔ Alt8260
⃥ ⇔ Alt8421
∣ ⇔ Alt8739
ⵦ ⇔ Alt11622
⮚ ⇔ Alt11162
‽ ⇔ Alt8253
፡ ⇔ Alt4961
‶ ⇔ Alt8246
″ ⇔ Alt8243
As a test I formed a filename using all of those chars and Windows accepted it.
This is good enough for me in Python:
def fix_filename(name, max_length=255):
"""
Replace invalid characters on Linux/Windows/MacOS with underscores.
List from https://stackoverflow.com/a/31976060/819417
Trailing spaces & periods are ignored on Windows.
>>> fix_filename(" COM1 ")
'_ COM1 _'
>>> fix_filename("COM10")
'COM10'
>>> fix_filename("COM1,")
'COM1,'
>>> fix_filename("COM1.txt")
'_.txt'
>>> all('_' == fix_filename(chr(i)) for i in list(range(32)))
True
"""
return re.sub(r'[/\\:|<>"?*\0-\x1f]|^(AUX|COM[1-9]|CON|LPT[1-9]|NUL|PRN)(?![^.])|^\s|[\s.]$', "_", name[:max_length], flags=re.IGNORECASE)
See also this outdated list for additional legacy stuff like = in FAT32.
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).
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.
When creating internet shortcuts in Windows, to create the file name, it skips illegal characters, except for forward slash, which is converted to minus.
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" -> "#"
};
I'm trying to get a handle on documentation/evidence around when PowerShell does conversions, and when a user needs to be explicit about what they want. A somewhat related question, here, has a broken link that possibly explained scenarios that the type would be adjusted. There are plenty of instances of similar problems though (namely, comparing a string to an int) - for example when you get a number via Read-Host, you're actually getting a string.
Specifically though I've found that certain mechanisms seem to handle a string representation of the number fine - but I want to understand if that is truly because they're handling it as a number, or if the output is correct in appearance, but wrong under the hood. Here's a specific example though, so the question is about Measure-Object and how it handles calculating the Sum of a string property.
PS > $Example = Import-Csv .\Example.csv
PS > $Example
Procedure_Code : 123456789
CPT_Code : J3490
Modifier :
Service_Date : 02/01/2020
Revenue_Code : 259
Units : -100.00
Amount : 55.00
Procedure_Code : 123456789
CPT_Code : J3490
Modifier :
Service_Date : 02/02/2020
Revenue_Code : 259
Units : 100.00
Amount : 55.00
PS > [Math]::Sign($Example[0].Units)
-1
PS > $Example | Measure-Object Amount -Sum
Count : 2
Sum : 110
Property : Amount
PS > $Example | ForEach-Object { $Sum = $Sum + $_.Amount} ; Write-Host "$($Sum)"
55.0055.00 #Plus sign concatenates strings
PS > $Example | ForEach-Object { $Sum = $Sum + [int]$_.Amount} ; Write-Host "$($Sum)"
110 #Expected result if casting the string into an integer type before adding
So basically it seems like [Math]::Sign() works (even though it doesn't accept strings as far as I can tell), and Measure-Object is smarter about strings than a simple plus sign. But is Measure-Object casting my string into an [int]? If I use an example with decimals, the answer is more precise, so maybe it is a [single]?
What is Measure-Object doing to get the right answer, and when shouldn't I trust it?
Measure-Object is a special case, because it is that cmdlet's specific logic that performs automatic type conversion, as opposed to PowerShell's own, general automatic conversions that happen in the context of using operators such as +, passing arguments to commands, calling .NET methods such as [Math]::Sign(), and using values as conditionals.
As your own answer hints at, Measure-Object calls the same code that PowerShell's general automatic type conversions use; these are discussed below.
Automatic type conversions in PowerShell:
In general, PowerShell always attempts automatic type conversion - and a pitfall to those familiar with C# is that even an explicit cast (e.g. [double] '1.2') may not be honored if the target type is ultimately a different one.[1]
Supported conversions:
In addition to supporting .NET's type conversions, PowerShell implements a few built-in custom conversions and supports user-defined conversions by way of custom conversion classes and attribute classes; also, it implicitly tries to call single-parameter target-type constructors and - for string input - a target type's static ::Parse() method, if present; see this answer for details.
To-number conversion and culture-sensitivity when converting to and from strings:
Typically, PowerShell uses culture-invariant conversion, so that, for instance, only . is recognized as the decimal mark in "number strings" that represent floating-point numbers.
String-to-number conversion, including how specific numeric data types are chosen, is covered in this answer.
To-string conversion (including from numbers) is covered in this answer.
Automatic type conversions by context:
Operands (operator arguments):
Some operators, such as -replace and -match, operate on strings only, in which case the operand(s) are invariably converted to strings (which always succeeds):
42 -replace 2, '!' yields '4!': both 42 and 2 were implicitly converted to strings.
Others, such as - and \, operate on numbers only, in which case the operand(s) are invariably converted to numbers (which may fail):
'10' - '2' yields 8, an [int]
Yet others, such as + and *, can operate on either strings or numbers, and it is the type of the LHS that determines the operation type:
'10' + 2 yields '102', the concatenation of string '10' with the to-string conversion of number 2.
By contrast, 10 + '2' yields 12, the addition of the number 10 and the to-number conversion of string '2'.
See this answer for more information.
Command arguments:
If the target parameter has a specific type (other than [object] or [psobject]), PowerShell will attempt conversion.
Otherwise, for literal arguments, the type is inferred from the literal representation; e.g. 42 is passed as an [int], and '42' as a [string].
Values serving as conditionals, such as in if statements:
A conditional must by definition be a Boolean (type [bool]), and PowerShell automatically converts a value of any type to [bool], using the rules described in the bottom section of this answer.
Method arguments:
Automatic type conversion for .NET-method arguments can be tricky, because multiple overloads may have to be considered, and - if the argument's aren't of the expected type - PowerShell has to find the best overload based on supported type conversions - and it may not be obvious which one is chosen.
Executing [Math]::Sign (calling this as-is, without ()) reveals 8(!) different overloads, for various numeric types.
More insidiously, the introduction of additional .NET-method overloads in future .NET versions can break existing PowerShell code, if a new overload then happens to be the best match in a given invocation.
A high-profile example is the [string] type's Split() method - see the bottom section of this answer.
Therefore, for long-term code stability:
Avoid .NET methods in favor of PowerShell-native commands, if possible.
Otherwise, if type conversion is necessary, use casts (e.g. [Math]::Sign([int] '-42')) to guide method-overload resolution to avoid ambiguity.
[1] E.g., the explicit [double] is quietly converted to an [int] in the following statement: & { param([int] $i) $i } ([double] '1.2'). Also, casts to .NET interfaces generally have no effect in PowerShell - except to guide overload resolution in .NET method calls.
I'm not sure this is the be-all and end-all answer, but it works for what I was curious about. For a lot of 'quick script' scenarios where you might use something like Measure-Object - you'll probably get the correct answers you're looking for, albeit maybe slower than other methods.
Measure-Object specifically seems to use [double] for Sum, Average, and StandardDeviation and will indeed throw an error if one of the string values from a CSV Object can't be converted.
I'm still a little surprised that [Math]::Sign() works at all with strings, but seemingly glad it does
if (_measureAverage || _measureSum || _measureStandardDeviation)
{
double numValue = 0.0;
if (!LanguagePrimitives.TryConvertTo(objValue, out numValue))
{
_nonNumericError = true;
ErrorRecord errorRecord = new(
PSTraceSource.NewInvalidOperationException(MeasureObjectStrings.NonNumericInputObject, objValue),
"NonNumericInputObject",
ErrorCategory.InvalidType,
objValue);
WriteError(errorRecord);
return;
}
AnalyzeNumber(numValue, stat);
}
I have a script where function parameters are expressed like this:
param(
${param1},
${param2},
${param3}
)
What does it mean? I have been unable to find documentation on this.
What's the point of writing parameters that way instead of the more usual
param(
$param1,
$param2,
$param3
)
?
#MikeZ's answer is quite correct in explaining the example in the question, but as far as addressing the question title, there is actually more to say! The ${} notation actually has two uses; the second one is a hidden gem of PowerShell:
That is, you can use this bracket notation to do file I/O operations if you provide a drive-qualified path, as defined in the MSDN page Provider Paths.
(The above image comes from the Complete Guide to PowerShell Punctuation, a one-page wallchart freely available for download, attached to my recent article at Simple-Talk.com.)
They are both just parameter declarations. The two snippets are equivalent. Either syntax can be used here, however the braced form allows characters that would not otherwise be legal in variable names. From the PowerShell 3.0 language specification:
There are two ways of writing a variable name: A braced variable name, which begins with $, followed by a curly bracket-delimited set of one or more almost-arbitrary characters; and an ordinary variable name, which also begins with $, followed by a set of one or more characters from a more restrictive set than a braced variable name allows. Every ordinary variable name can be expressed using a corresponding braced variable name.
From about_Variables
To create or display a variable name that includes spaces or special characters, enclose the variable name in braces. This directs Windows PowerShell to interpret the characters in the variable name literally.
For example, the following command creates and then displays a variable named "save-items".
C:\PS> ${save-items} = "a", "b", "c"
C:\PS> ${save-items}
a
b
c
They are equivalent. It's just an alternative way of declaring a variable.
If you have characters that are illegal in a normal variable, you'd use the braces (think of it as "escaping" the variablename).
There is one additional usage.
One may have variable names like var1, var2, var11, var12, var101, etc.
Regardless if this is desirable variable naming, it just may be.
Using brackets one can precisely determine what is to be used:
assignment of $var11 may be ambiguous, using ${var1}1 or ${var11} leaves no room for mistakes.