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