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

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

NAME
       touch - change file access and modification times

SYNOPSIS
       touch [-acm][ -r ref_file| -t time] file...

DESCRIPTION
       The touch utility shall change the modification times, access times, or both of files. The
       modification time shall be equivalent to the value of the  st_mtime  member  of	the  stat
       structure   for	 a   file,   as   described   in   the	 System   Interfaces   volume  of
       IEEE Std 1003.1-2001; the access time shall be equivalent to the value of st_atime.

       The time used can be specified by the -t  time  option-argument,  the  corresponding  time
       fields  of  the file referenced by the -r ref_file option-argument, or the date_time oper-
       and, as specified in the following sections. If none of these are specified,  touch  shall
       use  the current time (the value returned by the equivalent of the time() function defined
       in the System Interfaces volume of IEEE Std 1003.1-2001).

       For each file operand, touch shall perform actions equivalent to the  following	functions
       defined in the System Interfaces volume of IEEE Std 1003.1-2001:

	1. If  file does not exist, a creat() function call is made with the file operand used as
	   the path argument and the value of  the  bitwise-inclusive  OR  of  S_IRUSR,  S_IWUSR,
	   S_IRGRP, S_IWGRP, S_IROTH, and S_IWOTH used as the mode argument.

	2. The utime() function is called with the following arguments:

	    a. The file operand is used as the path argument.

	    b. The  utimbuf  structure	members actime and modtime are determined as described in
	       the OPTIONS section.

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

       The following options shall be supported:

       -a     Change  the  access  time of file. Do not change the modification time unless -m is
	      also specified.

       -c     Do not create a specified file if it does not exist. Do not  write  any  diagnostic
	      messages concerning this condition.

       -m     Change  the  modification  time of file. Do not change the access time unless -a is
	      also specified.

       -r  ref_file
	      Use the corresponding time of the file named by the pathname  ref_file  instead  of
	      the current time.

       -t  time
	      Use  the specified time instead of the current time. The option-argument shall be a
	      decimal number of the form:

	      [[CC]YY]MMDDhhmm[.SS]

       where each two digits represents the following:

       MM
	      The month of the year [01,12].

       DD
	      The day of the month [01,31].

       hh
	      The hour of the day [00,23].

       mm
	      The minute of the hour [00,59].

       CC
	      The first two digits of the year (the century).

       YY
	      The second two digits of the year.

       SS
	      The second of the minute [00,60].

       Both CC and YY shall be optional. If neither is given, the current year shall be  assumed.
       If YY is specified, but CC is not, CC shall be derived as follows:

					If YY is:   CC becomes:
					[69,99]     19
					[00,68]     20

       Note:
	      It is expected that in a future version of IEEE Std 1003.1-2001 the default century
	      inferred from a 2-digit year will change. (This would apply to all commands accept-
	      ing a 2-digit year as input.)

       The  resulting  time shall be affected by the value of the TZ environment variable. If the
       resulting time value precedes the Epoch, touch shall exit immediately with an  error  sta-
       tus.   The  range  of  valid  times past the Epoch is implementation-defined, but it shall
       extend to at least the time 0 hours, 0 minutes, 0 seconds, January  1,  2038,  Coordinated
       Universal Time. Some implementations may not be able to represent dates beyond January 18,
       2038, because they use signed int as a time holder.

       The range for SS is [00,60] rather than [00,59] because of leap seconds. If SS is 60,  and
       the  resulting  time, as affected by the TZ environment variable, does not refer to a leap
       second, the resulting time shall be one second after a time where SS is 59. If SS  is  not
       given a value, it is assumed to be zero.

       If  neither the -a nor -m options were specified, touch shall behave as if both the -a and
       -m options were specified.

OPERANDS
       The following operands shall be supported:

       file   A pathname of a file whose times shall be modified.

STDIN
       Not used.

INPUT FILES
       None.

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

       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 .

       TZ     Determine  the timezone to be used for interpreting the time option-argument. If TZ
	      is unset or null, an unspecified default timezone shall be used.

ASYNCHRONOUS EVENTS
       Default.

STDOUT
       Not used.

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

OUTPUT FILES
       None.

EXTENDED DESCRIPTION
       None.

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
       The interpretation of time is taken to be seconds since the Epoch (see  the  Base  Defini-
       tions volume of IEEE Std 1003.1-2001, Section 4.14, Seconds Since the Epoch). It should be
       noted   that   implementations	conforming   to   the	System	 Interfaces   volume   of
       IEEE Std 1003.1-2001  do  not  take leap seconds into account when computing seconds since
       the Epoch. When SS=60 is used, the resulting time always refers to 1  plus  seconds  since
       the Epoch for a time when SS=59.

       Although the -t time option-argument specifies values in 1969, the access time and modifi-
       cation time fields are defined in terms of seconds since the Epoch (00:00:00 on 1  January
       1970  UTC). Therefore, depending on the value of TZ when touch is run, there is never more
       than a few valid hours in 1969 and there need not be any valid times in 1969.

       One ambiguous situation occurs if -t time is not specified, -r ref_file is not  specified,
       and the first operand is an eight or ten-digit decimal number. A portable script can avoid
       this problem by using:

	      touch -- file

       or:

	      touch ./file

       in this case.

EXAMPLES
       None.

RATIONALE
       The functionality of touch is described almost entirely through references to functions in
       the System Interfaces volume of IEEE Std 1003.1-2001. In this way, there is no duplication
       of effort required for describing such side effects as the relationship of user IDs to the
       user database, permissions, and so on.

       There  are  some  significant  differences  between  the  touch	utility in this volume of
       IEEE Std 1003.1-2001 and those in System V and BSD systems.  They  are  upwards-compatible
       for historical applications from both implementations:

	1. In  System  V,  an ambiguity exists when a pathname that is a decimal number leads the
	   operands; it is treated as a time value. In BSD, no time value is allowed;  files  may
	   only  be  touched to the current time. The -t time construct solves these problems for
	   future conforming applications (note that the -t option is not historical practice).

	2. The inclusion of the century digits, CC, is also new. Note that a ten-digit time value
	   is  treated	as if YY, and not CC, were specified. The caveat about the range of dates
	   following the Epoch was included as recognition that some implementations are not able
	   to  represent  dates  beyond  18  January  2038  because they use signed int as a time
	   holder.

       The -r option was added because several comments requested this	capability.  This  option
       was  named  -f  in an early proposal, but was changed because the -f option is used in the
       BSD version of touch with a different meaning.

       At least one historical implementation of touch incremented the exit code if -c was speci-
       fied  and the file did not exist. This volume of IEEE Std 1003.1-2001 requires exit status
       zero if no errors occur.

FUTURE DIRECTIONS
       Applications should use the -r or -t options.

SEE ALSO
       date , the System Interfaces volume of IEEE Std 1003.1-2001, creat(), time(), utime(), the
       Base Definitions volume of IEEE Std 1003.1-2001, <sys/stat.h>

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					 TOUCH(P)


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