TOUCH
Section: POSIX Programmer's Manual (1P)
Updated: 2017
Page Index
PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
NAME
touch
--- change file access and modification times
SYNOPSIS
touch [-acm] [-r ref_file|-t time|-d date_time] file...
DESCRIPTION
The
touch
utility shall change the last data modification timestamps, the
last data access timestamps, or both.
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
-d
date_time
option-argument, as specified in the following sections. If none of
these are specified,
touch
shall use the current time.
For each
file
operand,
touch
shall perform actions equivalent to the following functions defined in
the System Interfaces volume of POSIX.1-2017:
- 1.
-
If
file
does not exist:
-
- a.
-
The
creat()
function is called with the following arguments:
-
- --
-
The
file
operand is used as the
path
argument.
- --
-
The value of the bitwise-inclusive OR of S_IRUSR, S_IWUSR,
S_IRGRP, S_IWGRP, S_IROTH, and S_IWOTH is used as the
mode
argument.
- b.
-
The
futimens()
function is called with the following arguments:
-
- --
-
The file descriptor opened in step 1a.
- --
-
The access time and the modification time, set as described in the
OPTIONS section, are used as the first and second elements of the
times
array argument, respectively.
- 2.
-
If
file
exists, the
utimensat()
function is called with the following arguments:
-
- a.
-
The AT_FDCWD special value is used as the
fd
argument.
- b.
-
The
file
operand is used as the
path
argument.
- c.
-
The access time and the modification time, set as described in the
OPTIONS section, are used as the first and second elements of the
times
array argument, respectively.
- d.
-
The
flag
argument is set to zero.
OPTIONS
The
touch
utility shall conform to the Base Definitions volume of POSIX.1-2017,
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.
- -d date_time
-
Use the specified
date_time
instead of the current time. The option-argument shall be a string of
the form:
-
-
YYYY-MM-DDThh:mm:SS[.frac][tz]
or:
-
YYYY-MM-DDThh:mm:SS[,frac][tz]
where:
- *
-
YYYY
are at least four decimal digits giving the year.
- *
-
MM,
DD,
hh,
mm,
and
SS
are as with
-t
time.
- *
-
T is the time designator, and can be replaced by a single
<space>.
- *
-
[.frac] and [,frac] are either empty, or a
<period>
('.')
or a
<comma>
(',')
respectively, followed by one or more decimal digits, specifying
a fractional second.
- *
-
[tz] is either empty, signifying local time, or the letter
'Z',
signifying UTC. If [tz] is empty, the resulting time shall
be affected by the value of the
TZ
environment variable.
If the resulting time precedes the Epoch, the behavior is
implementation-defined. If the time cannot be represented as the file's
timestamp,
touch
shall exit immediately with an error status.
- -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 this standard 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,
the behavior is implementation-defined. If the time is out of range for
the file's timestamp,
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 POSIX.1-2017,
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 POSIX.1-2017,
Section 4.16,
Seconds Since the Epoch).
It should be noted that implementations conforming to the System Interfaces volume of POSIX.1-2017 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.
If the
T
time designator is replaced by a
<space>
for the
-d
date_time
option-argument, the
<space>
must be quoted to prevent the shell from splitting the argument.
EXAMPLES
Create or update a file called
dwc;
the resulting file has both the last data modification and last data
access timestamps set to November 12, 2007 at 10:15:30 local time:
-
touch -d 2007-11-12T10:15:30 dwc
Create or update a file called
nick;
the resulting file has both the last data modification and last data
access timestamps set to November 12, 2007 at 10:15:30 UTC:
-
touch -d 2007-11-12T10:15:30Z nick
Create or update a file called
gwc;
the resulting file has both the last data modification and last data
access timestamps set to November 12, 2007 at 10:15:30 local time with
a fractional second timestamp of .002 seconds:
-
touch -d 2007-11-12T10:15:30,002 gwc
Create or update a file called
ajosey;
the resulting file has both the last data modification and last data
access timestamps set to November 12, 2007 at 10:15:30 UTC with a
fractional second timestamp of .002 seconds:
-
touch -d "2007-11-12 10:15:30.002Z" ajosey
Create or update a file called
cathy;
the resulting file has both the last data modification and last data
access timestamps set to November 12, 2007 at 10:15:00 local time:
-
touch -t 200711121015 cathy
Create or update a file called
drepper;
the resulting file has both the last data modification and last data
access timestamps set to November 12, 2007 at 10:15:30 local time:
-
touch -t 200711121015.30 drepper
Create or update a file called
ebb9;
the resulting file has both the last data modification and last data
access timestamps set to November 12, 2007 at 10:15:30 local time:
-
touch -t 0711121015.30 ebb9
Create or update a file called
eggert;
the resulting file has the last data access timestamp set to the
corresponding time of the file named
mark
instead of the current time. If the file exists, the last data
modification time is not changed:
-
touch -a -r mark eggert
RATIONALE
The functionality of
touch
is described almost entirely through references to functions in
the System Interfaces volume of POSIX.1-2017. 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 POSIX.1-2017 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 POSIX.1-2017 requires exit status
zero if no errors occur.
In previous version of the standard, if at least two operands are
specified, and the first operand is an eight or ten-digit decimal
integer, the first operand was assumed to be a
date_time
operand. This usage was removed in this version of the standard since
it had been marked obsolescent previously.
The
-d
date_time
format is an ISO 8601:2004 standard complete representation of date and time extended
format with an optional decimal point or
<comma>
followed by a string of digits following the seconds portion to specify
fractions of a second. It is not necessary to recognize
"[+/-]hh:mm"
and
"[+/-]hh"
to specify timezones other than local time and UTC. The
T
time designator in the ISO 8601:2004 standard extended format may be replaced by
<space>.
FUTURE DIRECTIONS
None.
SEE ALSO
date
The Base Definitions volume of POSIX.1-2017,
Section 4.16, Seconds Since the Epoch,
Chapter 8, Environment Variables,
Section 12.2, Utility Syntax Guidelines,
<sys_stat.h>
The System Interfaces volume of POSIX.1-2017,
creat(),
futimens(),
time(),
utime()
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 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 .
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
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https://www.kernel.org/doc/man-pages/reporting_bugs.html .