Ubuntu Manpage: touch - change file access and modification times

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 operand, 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 accepting 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 status. 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, Internationalization  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  internationalization
              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 arguments).

       LC_MESSAGES
              Determine the locale that should be used to affect the format and contents of diagnostic  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  Definitions  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  modification  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 specified 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.

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
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group                                   2003                                              TOUCH(P)