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touch(P) 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 ref-
erenced by the -r ref_file option-argument, or the
date_time operand, as specified in the following sec-
tions. 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:
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.
The utime() function is called with the following argu-
ments: <ol type="a">
The file operand is used as the path argument.
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 con-
cerning this condition.
-m Change the modification time of file. Do not
change the access time unless -a is also speci-
fied.
-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 num-
ber 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 spec-
ified.
OPERANDS
The following operands shall be supported:
file A pathname of a file whose times shall be modi-
fied.
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 internationaliza-
tion 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 cate-
gories.)
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 mes-
sages 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 interpret-
ing 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 conform-
ing 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 speci-
fied, -r ref_file is not specified, and the first oper-
and 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:
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 con-
struct solves these problems for future conforming
applications (note that the -t option is not historical
practice).
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 incre-
mented 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 Operat-
ing System Interface (POSIX), The Open Group Base Speci-
fications Issue 6, Copyright (C) 2001-2003 by the Insti-
tute 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.open-
group.org/unix/online.html .
POSIX 2003 touch(P)
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