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Linux 2.6 - man page for chmod (linux section 1posix)

CHMOD(P)			    POSIX Programmer's Manual				 CHMOD(P)

NAME
       chmod - change the file modes

SYNOPSIS
       chmod [-R] mode file ...

DESCRIPTION
       The  chmod utility shall change any or all of the file mode bits of the file named by each
       file operand in the way specified by the mode operand.

       It is implementation-defined whether and how the chmod utility affects  any  alternate  or
       additional   file   access   control   mechanism  (see  the  Base  Definitions  volume  of
       IEEE Std 1003.1-2001, Section 4.4, File Access Permissions) being used for  the	specified
       file.

       Only  a process whose effective user ID matches the user ID of the file, or a process with
       the appropriate privileges, shall be permitted to change the file mode bits of a file.

OPTIONS
       The chmod utility shall conform to the Base Definitions	volume	of  IEEE Std 1003.1-2001,
       Section 12.2, Utility Syntax Guidelines.

       The following option shall be supported:

       -R     Recursively  change  file  mode bits. For each file operand that names a directory,
	      chmod shall change the file mode bits of the directory and all files  in	the  file
	      hierarchy below it.

OPERANDS
       The following operands shall be supported:

       mode   Represents the change to be made to the file mode bits of each file named by one of
	      the file operands; see the EXTENDED DESCRIPTION section.

       file   A pathname of a file whose file mode bits shall be modified.

STDIN
       Not used.

INPUT FILES
       None.

ENVIRONMENT VARIABLES
       The following environment variables shall affect the execution of chmod:

       LANG   Provide a default value for the internationalization variables that  are	unset  or
	      null. (See the Base Definitions volume of IEEE Std 1003.1-2001, Section 8.2, Inter-
	      nationalization Variables for the precedence of internationalization variables used
	      to determine the values of locale categories.)

       LC_ALL If  set  to a non-empty string value, override the values of all the other interna-
	      tionalization variables.

       LC_CTYPE
	      Determine the locale for the interpretation of sequences of bytes of text  data  as
	      characters  (for	example, single-byte as opposed to multi-byte characters in argu-
	      ments).

       LC_MESSAGES
	      Determine the locale that should be used to affect the format and contents of diag-
	      nostic messages written to standard error.

       NLSPATH
	      Determine the location of message catalogs for the processing of LC_MESSAGES .

ASYNCHRONOUS EVENTS
       Default.

STDOUT
       Not used.

STDERR
       The standard error shall be used only for diagnostic messages.

OUTPUT FILES
       None.

EXTENDED DESCRIPTION
       The  mode operand shall be either a symbolic_mode expression or a non-negative octal inte-
       ger. The symbolic_mode form is described by the grammar later in this section.

       Each clause shall specify an operation to be performed on the current file  mode  bits  of
       each  file.  The  operations  shall  be	performed  on each file in the order in which the
       clauses are specified.

       The who symbols u, g, and o shall specify the user, group, and other  parts  of	the  file
       mode bits, respectively. A who consisting of the symbol a shall be equivalent to ugo.

       The  perm  symbols  r, w, and x represent the read, write, and execute/ search portions of
       file mode bits, respectively. The perm symbol s shall represent the  set-user-ID-on-execu-
       tion  (when who contains or implies u) and set-group-ID-on-execution (when who contains or
       implies g) bits.

       The perm symbol X shall represent the execute/search portion of the file mode bits if  the
       file is a directory or if the current (unmodified) file mode bits have at least one of the
       execute bits (S_IXUSR, S_IXGRP, or S_IXOTH) set. It shall be ignored if the file is not	a
       directory and none of the execute bits are set in the current file mode bits.

       The  permcopy  symbols u, g, and o shall represent the current permissions associated with
       the user, group, and other parts of the file mode bits, respectively. For the remainder of
       this section, perm refers to the non-terminals perm and permcopy in the grammar.

       If  multiple actionlists are grouped with a single wholist in the grammar, each actionlist
       shall be applied in the order specified with that wholist. The op symbols shall	represent
       the operation performed, as follows:

       +      If perm is not specified, the '+' operation shall not change the file mode bits.

       If  who is not specified, the file mode bits represented by perm for the owner, group, and
       other permissions, except for those with corresponding bits in the file mode creation mask
       of the invoking process, shall be set.

       Otherwise,  the	file  mode bits represented by the specified who and perm values shall be
       set.

       -      If perm is not specified, the '-' operation shall not change the file mode bits.

       If who is not specified, the file mode bits represented by perm for the owner, group,  and
       other permissions, except for those with corresponding bits in the file mode creation mask
       of the invoking process, shall be cleared.

       Otherwise, the file mode bits represented by the specified who and perm	values	shall  be
       cleared.

       =      Clear  the file mode bits specified by the who value, or, if no who value is speci-
	      fied, all of the file mode bits specified in this volume of IEEE Std 1003.1-2001.

       If perm is not specified, the '=' operation shall make no  further  modifications  to  the
       file mode bits.

       If  who is not specified, the file mode bits represented by perm for the owner, group, and
       other permissions, except for those with corresponding bits in the file mode creation mask
       of the invoking process, shall be set.

       Otherwise,  the	file  mode bits represented by the specified who and perm values shall be
       set.

       When using the symbolic mode form on a regular file, it is implementation-defined  whether
       or not:

	* Requests  to set the set-user-ID-on-execution or set-group-ID-on-execution bit when all
	  execute bits are currently clear and none are being set are ignored.

	* Requests to clear all execute bits also clear  the  set-user-ID-on-execution	and  set-
	  group-ID-on-execution bits.

	* Requests  to	clear the set-user-ID-on-execution or set-group-ID-on-execution bits when
	  all execute bits are currently clear are ignored. However, if the command  ls  -l  file
	  writes  an s in the position indicating that the set-user-ID-on-execution or set-group-
	  ID-on-execution is set, the commands chmod u-s file or chmod	g-s  file,  respectively,
	  shall not be ignored.

       When  using  the  symbolic  mode  form  on  other file types, it is implementation-defined
       whether or not requests to set or clear the set-user-ID-on-execution  or  set-group-ID-on-
       execution bits are honored.

       If  the	who symbol o is used in conjunction with the perm symbol s with no other who sym-
       bols being specified,  the  set-user-ID-on-execution  and  set-group-ID-on-execution  bits
       shall not be modified. It shall not be an error to specify the who symbol o in conjunction
       with the perm symbol s.

       The perm symbol t shall specify the S_ISVTX bit. When used with a file of type  directory,
       it  can	be used with the who symbol a, or with no who symbol. It shall not be an error to
       specify a who symbol of u, g, or o in conjunction with the perm symbol t, but the  meaning
       of  these  combinations	is unspecified.  The effect when using the perm symbol t with any
       file type other than directory is unspecified.

       For an octal integer mode operand, the file mode bits shall be set absolutely.

       For each bit set in the octal number, the corresponding file permission bit shown  in  the
       following table shall be set; all other file permission bits shall be cleared. For regular
       files, for each bit set in the octal number corresponding to the  set-user-ID-on-execution
       or the set-group-ID-on-execution, bits shown in the following table shall be set; if these
       bits are not set in the octal number, they are cleared. For other file types, it is imple-
       mentation-defined  whether or not requests to set or clear the set-user-ID-on-execution or
       set-group-ID-on-execution bits are honored.

		      Octal Mode Bit Octal Mode Bit Octal Mode Bit Octal Mode Bit
		      4000  S_ISUID  0400  S_IRUSR  0040  S_IRGRP  0004  S_IROTH
		      2000  S_ISGID  0200  S_IWUSR  0020  S_IWGRP  0002  S_IWOTH
		      1000  S_ISVTX  0100  S_IXUSR  0010  S_IXGRP  0001  S_IXOTH

       When bits are set in the octal number other than those listed  in  the  table  above,  the
       behavior is unspecified.

   Grammar for chmod
       The  grammar  and  lexical  conventions	in  this section describe the syntax for the sym-
       bolic_mode operand. The general conventions for this style of  grammar  are  described  in
       Grammar	Conventions . A valid symbolic_mode can be represented as the non-terminal symbol
       symbolic_mode in the grammar. This formal syntax shall take precedence over the	preceding
       text syntax description.

       The  lexical  processing  is based entirely on single characters. Implementations need not
       allow <blank>s within the single argument being processed.

	      %start	symbolic_mode
	      %%

	      symbolic_mode    : clause
			       | symbolic_mode ',' clause
			       ;

	      clause	       : actionlist
			       | wholist actionlist
			       ;

	      wholist	       : who
			       | wholist who
			       ;

	      who	       : 'u' | 'g' | 'o' | 'a'
			       ;

	      actionlist       : action
			       | actionlist action
			       ;

	      action	       : op
			       | op permlist
			       | op permcopy
			       ;

	      permcopy	       : 'u' | 'g' | 'o'
			       ;

	      op	       : '+' | '-' | '='
			       ;

	      permlist	       : perm
			       | perm permlist
			       ;

	      perm	       : 'r' | 'w' | 'x' | 'X' | 's' | 't'
			       ;

EXIT STATUS
       The following exit values shall be returned:

	0     The utility executed successfully and all requested changes were made.

       >0     An error occurred.

CONSEQUENCES OF ERRORS
       Default.

       The following sections are informative.

APPLICATION USAGE
       Some implementations of the chmod utility change the mode of a directory before the  files
       in the directory when performing a recursive ( -R option) change; others change the direc-
       tory mode after the files in the directory. If an application  tries  to  remove  read  or
       search  permission  for	a  file  hierarchy, the removal attempt fails if the directory is
       changed first; on the other hand, trying to re-enable permissions to a restricted  hierar-
       chy  fails if directories are changed last. Users should not try to make a hierarchy inac-
       cessible to themselves.

       Some implementations of chmod never used the process' umask when changing  modes;  systems
       conformant  with this volume of IEEE Std 1003.1-2001 do so when who is not specified. Note
       the difference between:

	      chmod a-w file

       which removes all write permissions, and:

	      chmod -- -w file

       which removes write permissions that would be allowed if file was created  with	the  same
       umask.

       Conforming  applications  should  never assume that they know how the set-user-ID and set-
       group-ID bits on directories are interpreted.

EXAMPLES
			    Mode    Results
			    a+=     Equivalent to a+, a=; clears all file
				    mode bits.
			    go+-w   Equivalent to go+, go- w; clears group
				    and other write bits.
			    g=o-w   Equivalent to g= o, g- w; sets group bit
				    to match other bits and then clears
				    group write bit.
			    g-r+w   Equivalent to g- r, g+ w; clears group
				    read bit and sets group write bit.
			    uo=g    Sets owner bits to match group bits and
				    sets other bits to match group bits.

RATIONALE
       The functionality of chmod is  described  substantially	through  references  to  concepts
       defined	in  the  System  Interfaces volume of IEEE Std 1003.1-2001. In this way, there is
       less duplication of effort required for describing the interactions of  permissions.  How-
       ever,  the behavior of this utility is not described in terms of the chmod() function from
       the System Interfaces volume of IEEE Std 1003.1-2001 because that  specification  requires
       certain	side  effects  upon  alternate	file  access control mechanisms that might not be
       appropriate, depending on the implementation.

       Implementations that support mandatory file and record locking as specified  by	the  1984
       /usr/group  standard  historically  used the combination of set-group-ID bit set and group
       execute bit clear to indicate mandatory locking. This condition is usually set or  cleared
       with  the  symbolic  mode  perm	symbol	l instead of the perm symbols s and x so that the
       mandatory locking mode is not changed without explicit indication that that was	what  the
       user  intended.	Therefore,  the details on how the implementation treats these conditions
       must be defined in the documentation. This volume of IEEE Std 1003.1-2001 does not require
       mandatory  locking  (nor  does  the System Interfaces volume of IEEE Std 1003.1-2001), but
       does allow it as an extension. However, this volume of IEEE Std 1003.1-2001  does  require
       that  the  ls  and chmod utilities work consistently in this area. If ls -l file indicates
       that the set-group-ID bit is set, chmod g-s file must clear it (assuming appropriate priv-
       ileges exist to change modes).

       The  System  V and BSD versions use different exit status codes. Some implementations used
       the exit status as a count of the number of errors that occurred; this practice is unwork-
       able  since it can overflow the range of valid exit status values. This problem is avoided
       here by specifying only 0 and >0 as exit values.

       The System Interfaces volume of IEEE Std 1003.1-2001 indicates that implementation-defined
       restrictions  may  cause  the  S_ISUID  and  S_ISGID  bits  to  be ignored. This volume of
       IEEE Std 1003.1-2001 allows the chmod utility to choose to modify these bits before  call-
       ing  chmod()  (or  some function providing equivalent capabilities) for non-regular files.
       Among other things, this allows implementations that use the set-user-ID and  set-group-ID
       bits  on directories to enable extended features to handle these extensions in an intelli-
       gent manner.

       The X perm symbol was adopted from BSD-based systems because it provides commonly  desired
       functionality  when  doing  recursive ( -R option) modifications. Similar functionality is
       not provided by the find utility. Historical BSD versions of  chmod,  however,  only  sup-
       ported  X with op+; it has been extended in this volume of IEEE Std 1003.1-2001 because it
       is also useful with op=. (It has also been added for op- even though it duplicates  x,  in
       this case, because it is intuitive and easier to explain.)

       The grammar was extended with the permcopy non-terminal to allow historical-practice forms
       of symbolic modes like o= u -g (that is, set the "other" permissions to the permissions of
       "owner" minus the permissions of "group").

FUTURE DIRECTIONS
       None.

SEE ALSO
       ls , umask , the System Interfaces volume of IEEE Std 1003.1-2001, chmod()

COPYRIGHT
       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2003 Edition, Standard for Information Technology  --  Portable	Operating  System
       Interface  (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
       the Institute of Electrical and Electronics Engineers, Inc and  The  Open  Group.  In  the
       event  of  any  discrepancy  between this version and the original IEEE and The Open Group
       Standard, the original IEEE and The Open Group Standard is the referee document. The orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003					 CHMOD(P)


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