Alejandro, Branden and Hendrik: Thank you for the useful suggestions and revisions. Patch v2, below, assimilates them. --------------------------------------------------------------------------- CHANGES IN v2 --------------------------------------------------------------------------- Hendrik Boom, G. Branden Robinson and Alejandro Colomar (1): Write "uppercase" and "lowercase" rather than "capital" and "small" Alejandro Colomar and G. Branden Robinson (2): Use semantic newlines Avoid \f, but rather use separate lines Alejandro Colomar (11): Use subsections instead of sections Use subsubsections instead of subsections Remove unnecessary .P after .S[HS] Use .PP rather than .P Fix indentation of paragraph, which continues talking about \0 Mention FAT For consistency, list "-" with "-name" and ".name"; s/a pair of/some/ Delete the redundant mention of "." By s/all but/almost/, avoid double negation Reference filesystems(5) under SEE ALSO Under CONFORMING TO, write only, "POSIX.1‐2001 and later." G. Branden Robinson (3): Write "letter case" rather than "capitalization" Reference section-3 pages not under SEE ALSO but only in passing Avoid \c Thaddeus H. Black (2): Reword subsubsect "Special semantics" to support Alejandro's change no. 7 Avoid beginning any subsect or subsubsect with specially formatted text --------------------------------------------------------------------------- GROFF SOURCE v2 (IN PATCH FORMAT) --------------------------------------------------------------------------- --- /dev/null 2021-10-17 12:05:11.393541700 +0000 +++ b/man7/filename.7 2021-10-17 20:21:24.938542310 +0000 @@ -0,0 +1,654 @@ +.\" Copyright (C) 2021 Thaddeus H. Black <thb@xxxxxxxxxx> +.\" +.\" %%%LICENSE_START(VERBATIM) +.\" Permission is granted to make and distribute verbatim copies of this +.\" manual provided the copyright notice and this permission notice are +.\" preserved on all copies. +.\" +.\" Permission is granted to copy and distribute modified versions of this +.\" manual under the conditions for verbatim copying, provided that the +.\" entire resulting derived work is distributed under the terms of a +.\" permission notice identical to this one. +.\" +.\" Since the Linux kernel and libraries are constantly changing, this +.\" manual page may be incorrect or out-of-date. The author(s) assume no +.\" responsibility for errors or omissions, or for damages resulting from +.\" the use of the information contained herein. The author(s) may not +.\" have taken the same level of care in the production of this manual, +.\" which is licensed free of charge, as they might when working +.\" professionally. +.\" +.\" Formatted or processed versions of this manual, if unaccompanied by +.\" the source, must acknowledge the copyright and authors of this work. +.\" %%%LICENSE_END +.\" +.\" 2021-10-18, Thaddeus H. Black <thb@xxxxxxxxxx> +.\" Wrote the manual page's initial version. +.\" +.TH FILENAME 7 2021-10-18 "Linux" "Linux Programmer's Manual" +.SH NAME +filename \- requirements and conventions for the naming of files +.SH DESCRIPTION +This manual page sets forth requirements for +and delineates conventions regarding filenames +on a Linux system, +where a +.I filename +is either (as the word suggests) the name of a regular file +or the name of another object held by the system's filesystem +such as a directory, symbolic link, named pipe or device. +.SS Legal filenames +A filename on a Linux system can consist +of almost any sequence of UTF-8 characters +or, indeed, almost any sequence of bytes. +The exceptions are as follows. +.TP +.B Reserved characters +.RS +The following characters are reserved. +.TP +.B / +The solidus is reserved to separate pathname components +as for example in +.IR /usr/share/doc , +each component being itself a filename. +For this reason, no filename may include a solidus. +More precisely, +no filename may include the byte that, +in ASCII and UTF-8, +exclusively represents the solidus. +.TP +.B \e0 +The null character is reserved for the filesystem to append +to terminate a filename's representation in memory. +For this reason, no filename may include a null character. +More precisely, +no filename may include the byte that, +in ASCII and UTF-8, +exclusively represents the null character. +(When appended by the filesystem +to terminate a filename's representation in memory, +the byte in question is called the +.I terminating null +.IR byte . +Though familiar to\~C programmers, +the terminating null byte is usually invisible to users.) +.IP +Note +.RB that\~ \e0 , +the null character (or null byte), differs from +.RB from\~ 0 , +the printable digit-zero character. +The null character (or null byte) +is unprintable and registers in ASCII and UTF-8 +as the eight-bit pattern\~0x00, +whereas the printable digit zero registers as\~0x30 +[see the \(lqHex\(rq column in +.BR ascii (7)'s +character table]. +Nothing prevents a filename from including a printable digit zero, +as for instance the filename +.I intel-m10-bmc.h +from the kernel's source does. +.RE +.TP +.B Reserved names +.RS +The following names are reserved. +.TP +.B . +The filename consisting of a single full stop +is reserved to represent the current directory. +.TP +.B .. +The filename consisting of two full stops +is reserved to represent the parent directory. +.TP +(empty) +The empty filename, +consisting of no bytes at all +(except a terminating null byte), +is not allowed. +.PP +The aforementioned current and parent directories are the current +.I working +directory and its parent except when +.RB the\~ . +.RB or\~ .. +occurs in the middle or at the end of a pathname, +in which case the current and parent directories +are taken relative to preceding pathname elements. +For example, if the current working directory were +.IR /home/jsmith , +then +.I ../rjones +would mean +.I /home/rjones +but +.I foo/bar/../baz +would mean +.IR /home/jsmith/foo/baz , +whereas +.I foo/bar/./baz +would mean +.IR /home/jsmith/foo/bar/baz . +.RE +.TP +.B Long names +.RS +No filename may exceed\~255 bytes in length, +or\~256 bytes after counting the terminating null byte. +.RB ( Reserved +.B characters +above explains the terminating null byte.) +.RE +.TP +.B Non-UTF-8 names +.RS +Filenames need not consist of valid UTF-8 characters +(although, except where a non-UTF-8 legacy encoding is in use, +most filenames do). +As long as the requirements +of the preceding subsubsections +are met, +any sequence of bytes can legally serve as a filename. +.RE +.SS Conventional filenames +Merely because a filename is legal +does not make its use advisable, though. +Some legal filenames cause practical troubles. +For example, the legal filenames +.IR m=3 , +.IR \(tijsmith , +.I \-v +and +.I My\~Document.txt +are susceptible to misinterpretation by a shell. +Workarounds typically exist, +chiefly via quotation, escape +and the explicit termination of options processing +[see +.BR sh (1)]; +but when reprocessing of shell-command text +requires requotation and re-escape, +the workarounds become an inconvenient, confusing, error-prone hassle. +.PP +The use of conventional filenames averts the hassle. +It also makes filenames more recognizable to experienced users. +.PP +This subsection introduces broadly observed conventions for filenames. +.TP +.B The POSIX Portable Filename Character Set +.RS +In general contexts, +especially for international applications, +conventional filenames +are composed using the\~65 ASCII characters +of the POSIX Portable Filename Character Set. +The POSIX Portable Filename Character Set consists of the following. +.TP +.BR A \- Z +The\~26 uppercase or capital ASCII letters. +.TP +.BR a \- z +The\~26 lowercase or small ASCII letters. +.TP +.BR 0 \- 9 +The ten ASCII digits. +.TP +.B . \_ \- +These three ASCII punctuators: full stop; low line; hyphen-minus. +.PP +Special contexts often employ additional characters but, +in general contexts for international applications, +conventional filenames exclude characters other than the listed\~65. +(For noninternational applications, see +.B Locales and Unicode +below.) +.PP +Observe that the +.RB space\~ \(aq\0\(aq \~( \eu0020 ) +is not listed despite being an ASCII character. +Filenames that include spaces +are often encountered for various reasons in certain contexts, +but such filenames are unconventional in general +and are inconvenient to use with tools. +Within filenames, the low +.RB line\~ \_ +or +.RB hyphen-minus\~ \- +is conventionally employed as necessary instead of the space. +(See +.B Unconventional filenames +and, under +.B Soft +.BR conventions , +also +.B Low line versus hyphen-minus +below.) +.PP +Incidentally, uppercase and lowercase letters +are normally distinct within filenames on a Linux system; +so, for example, +.I README +and +.I readme +name two different files. +(Exception: the FAT filesystem on Linux is case-insensitive, +so uppercase and lowercase letters +are indistinct where FAT is in use. +For further observations regarding letter case, see +.B Letter case +under +.B Soft conventions +below.) +.RE +.TP +.B Special semantics +.RS +Besides the last subsubsection's POSIX convention, +some conventions derived from core utilities +are almost always respected, as well. +.TP +.B \- +The one-character name +consisting of a lone hyphen-minus +is sometimes understood by a shell +to refer to the previous working directory +and sometimes understood by tools +to refer to standard input or standard output. +Therefore, no conventional filename +consists of a lone hyphen-minus. +.TP +.BR \- name +A name (other than the just-mentioned one-character name) +that begins with a hyphen-minus +is usually interpreted by tools as a +command-line option rather than as a filename. +Therefore, no conventional filename +begins with the hyphen-minus. +.TP +.BR . name +Conventional filenames may indeed begin with the full stop. +This is normal and does not generally cause trouble. +However, filenames that begin with the full stop +conventionally designate +.I hidden files +(or hidden directories, etc.), +a familiar example being the +.I .profile +typically found in a user's home directory. +Hidden files behave normally but, by default, are ignored by +.BR ls (1) +and certain other tools. +.RE +.TP +.B The full stop to introduce a format extension +.RS +Other than at a filename's beginning +(a case the last subsubsection has discussed), +the full stop is employed in filenames +for various further conventional purposes. +No single rule governs all conventional uses of the full stop. +.PP +However, except at a filename's beginning, +the most common conventional use of the full stop +is to append to a filename's stem +an extension to indicate the format of the file's contents. +An example is the filename +.IR UnicodeData.txt , +in which +.I UnicodeData +is the stem and +.RI the\~ .txt +indicates that the file contains plain text. +Multiple format extensions are even appended to some filename stems, +as in +.I my-archive.tar.xz +for instance, which is the name of a tape archive +.I my-archive.tar +that, after archival, has subsequently been compressed by +.BR xz (1). +.PP +The format-extension convention is almost universally recognized. +Even nontechnical users are typically familiar with it. +However, many users employ full stops also +for various purposes unrelated to format extensions, as well; +and they do so often enough +that such unrelated usage can hardly be called unconventional. +Except at a filename's beginning, +convention supports free use of the full stop. +.PP +.I You +may reserve the full stop solely +to append format extensions if you wish, +of course. +Many users do. +.PP +.\" The next sentence has been corrected +.\" according to Charles Plessy's helpful advice +.\" [https://lists.debian.org/debian-devel/2021/08/msg00557.html]. +(If your machine is configured as a desktop or laptop +rather than as a server, +then you can probably find a fairly comprehensive catalog +of conventional filename extensions, +identifying the format each extension implies, +on your machine in a file such as +.I /etc/mime.types +or +.IR /usr/share/mime/globs .) +.RE +.SS Soft conventions +Further filenaming conventions are softer. +Though often observed, +such softer conventions can be bent or broken +without rendering filenames unconventional. +.PP +This subsection introduces soft conventions for filenames. +.TP +.B Low line versus hyphen-minus +.RS +Whether to use the low +.RB line\~ \_ +or the +.RB hyphen-minus\~ \- +in filenames is a matter of preference. +Except as stated above, +convention does not strongly prefer the one over the other. +.PP +If you would like advice, anyway, however, +then the kernel's source sets an example. +Most filenames in the kernel's source prefer the hyphen-minus. +You can do the same if you wish. +.PP +Even if you prefer the hyphen-minus, though, +some exceptions arise, as follows. +.IP \(bu +The contents of a program's source files usually designate various +.I entities +such as variables, functions, types and so forth. +In\~C and similar programming languages, +the hyphen-minus is a minus sign, +so the designations of entities must use the low line, instead. +Where a file is named after an entity the file introduces, +the filename should use low lines as the entity's designation does. +Examples include the file +.IR lock\_events.h , +which introduces the entity +.IR lock\_events , +in the kernel's source. +.IP \(bu +Where distinct separators with different semantics are required, +a filename can use the low line as an alternate separator. +Examples include the file +.IR coreutils\_8.30-3\_amd64.deb , +which provides revision\~3 of the Debian binary package +that installs version\~8.30 of the GNU core utilities +for the amd64/x86-64 architecture. +.IP \(bu +Occasionally, the name of a file +that provides +private, internal, ephemeral, uninterfaceable or undocumented aspects +of an implementation +will +.I begin +with a low line to hint that the file +.RS +.IP + +does not require the user's or programmer's attention or +.IP + +is unsuitable for external agents to access directly. +.RE +.IP +Examples include the file +.\" On the author's PC using Groff's default output device, +.\" Groff typesets the next line's italicized low line inconsistently +.\" compared to the manual page's other italicized low lines. +.\" Presumably, Groff does this +.\" because the low line in question begins its word +.\" (though why Groff thinks beginning the word significant +.\" is unclear), but the inconsistency is slightly distracting. +.I \_sd-common.h +in systemd's source. +.IP \(bu +Sometimes, the low line +stands for an unspecified letter of the alphabet. +.PP +Otherwise, +despite that the low line and the hyphen-minus +are both conventional, +if you want advice: +prefer the hyphen-minus. +.RE +.TP +.B Letter case +.RS +A loosely observed convention +favors lowercase letters in filenames +where no reason to use uppercase exists. +Many exceptions occur, though, as for example the oft-encountered +.I Makefile +that instructs +.BR make (1) +how to build an executable program or other autogeneratable file. +.PP +The reason convention favors lowercase +is that the general use of lowercase +leaves uppercase to be employed for emphasis. +Where the default\~C (or C.UTF-8) locale is in use, +the uppercase ASCII letters +are collated before all the lowercase ones, +whereby +.BR ls (1) +lists filenames like +.I Makefile +and +.I README +before filenames like +.I a.out +and +.IR foo.c . +[If your locale causes +.BR ls (1) +to collate differently +when you would have preferred the just-described default collation, +then try +.B LC\_ALL=C ls +or +.B LC\_ALL=C.UTF-8 ls +to suppress the locale. +See +.BR locale (7).] +.PP +Programming styles in languages like\~C++ and Python +occasionally uppercase some letters +in the names of types and of certain other entities. +Such casing can spill over to affect filenames, +so it is hard to state a general rule. +.RE +.SS Locales and Unicode +.\" If another subsubsection were added to the manual page, +.\" then this subsection might be demoted to a subsubsection and, +.\" if appropriate, grouped with the new subsubsection together +.\" under a new subsection entitled "Further considerations." +If your application is local rather than international, +then you can relax POSIX's aforementioned character-set convention +at your discretion +by including graphic Unicode characters; +specifically, by including non-ASCII Unicode characters for which +.BR iswgraph (3) +returns true +in your locale +or (if your system has it) in the C.UTF-8 locale. +[For the relationship between +.BR unicode (7), +.BR utf-8 (7) +and +.BR ascii (7), +see the respective manual pages. +Approximately, in brief, +Unicode is a character set, +UTF-8 is a byte-oriented scheme +by which Unicode characters can be encoded, +and ASCII is both a character set +and a byte-oriented scheme +that is a subset of both Unicode and UTF-8.] +.PP +To suggest an exact noninternational filenaming rule, other than the +.BR iswgraph (3) +rule, for every locale would exceed the scope of this manual page; +but approximately, +in a Japanese or French application for instance, +a filename might respectively +include kanji ideographs or accented Latin letters. +Filenames that include kanji ideographs or accented Latin letters +might be hard for international users to read or type, +but insofar as such filenames +exclude spaces, control characters, ASCII symbols +.RB (like\~ $ +.RB or\~ = ), +and ASCII punctuators +other than the three punctuators POSIX recommends, +such filenames will not normally cause trouble for tools +and, thus, may be regarded as conventional within the local context. +.PP +The use of nonbreaking spaces +.RB like\~ \eu00A0 , +.BR \eu2007 , +.B \eu202F +.RB or\~ \euFEFF +in filenames is probably inadvisable for most locales, despite that +.BR iswgraph (3) +returns true. +[The use of ordinary, breaking spaces +.RB like\~ \eu0020 +(the familiar ASCII space), +.BR \eu1680 , +.B \eu2000 +.RB through\~ \eu2006 , +.BR \eu2008 , +.BR \eu2009 , +.BR \eu200A , +.B \eu205F +.RB and\~ \eu3000 +is probably also inadvisable, but +.BR iswgraph (3) +returns false for those, anyway.] +.PP +If calling +.BR iswgraph (3) +from a program, incidentally, see also +.BR mbrtowc (3) +and +.BR wcrtomb (3). +.SS Unconventional filenames +More than a few files on a typical Linux system, +occasionally even including standard files +employed by and/or automatically installed +by an operating-system distribution, +have unconventional filenames. +For example, on a Debian GNU/Linux system, +some names of files that supply software packages use the +.RB characters\~ + +.RB and\~ \(ti +which, though unconventional in general, +are normal and expected within that context. +For another example, +in the kernel's source, certain filenames use the +.RB character\~ , +to separate a device's designator +from the name of the device's manufacturer. +You may have noticed the unconventionally-named +.I lost+found +directory lurking at a filesystem's root on your computer; +and there are further examples, as well. +.PP +There are many reasons to use unconventional filenames. +.PP +It is hard to give a general rule, +with respect to a particular context, +as to which unconventional filenames +are likely to cause practical troubles +and which are not. +If unsure, you can avoid troubles by adhering to convention; +but if you wish or need to depart from convention, +then the only suggestions this manual page would make are +.IP \(bu +that unconventional filenames not be used without context; +.IP \(bu +that unconventional filenames not be used without reason; +.IP \(bu +that, even where filenames are unconventional, the recommendations of +.B Special semantics +above still be followed if practicable; +.IP \(bu +that, where several unconventionally named files are collected, +the use of unconventional characters be systematic +(for example, +.IR 16:30.log , +.IR 16:45.log , +.I 17:00.log +and so on); +.IP \(bu +that, even if unconventional symbols or punctuators +are employed within filenames, +one think twice before +.I beginning +a filename with an unconventional symbol or punctuator; +specifically, before beginning a filename +with a nonalphanumeric ASCII character +other than the full stop or low line +(consider for example a filename that began with +.RB the\~ \(ti +.RB or\~ $ +symbol, which a shell might misinterpret as it were a reference +to a home directory or shell parameter); +.IP \(bu +that, even if non-POSIX characters are used, +non-ASCII characters be avoided +to the extent to which the application is international; +.IP \(bu +that the shell's four standard globbing +.RB characters\~ *?[] +be avoided in most instances; and +.IP \(bu +even if none of the other suggestions is followed, +that control characters be avoided in any event, +.I control characters +being characters, including the +.RB tab\~ \et +and +.RB line-feed\~ \en +characters, for which +.BR iswcntrl (3) +returns true. +(Note that, +although the use of the space in filenames +contravenes POSIX and anyway annoys many Linux users, +the space is the sole nongraphic ASCII character +that, by definition, is not a control character. +Spaces in filenames are unconventional and perhaps inadvisable, +but they are hardly unusual; +whereas tabs and line feeds are, +for good reason, +practically never seen.) +.SH CONFORMING TO +POSIX.1-2001 and later. +.SH SEE ALSO +.BR ls (1), +.BR sh (1), +.BR ext4 (5), +.BR filesystems (5), +.BR ascii (7), +.BR locale (7), +.BR unicode (7), +.BR utf-8 (7) +.PP +info +.B coreutils +.\" The author, Thaddeus H. Black, thanks +.\" his wife Kristie, daughter Naomi and son George +.\" for their review and proofreading +.\" of various parts of this manual page; and also thanks +.\" Alejandro Colomar, G. Branden Robinson and Hendrik Boom +.\" for their subsequent inspection, discussion and revision.
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