touch (1p) - Linux Man Pages
touch: change file access and modification times
PROLOGThis 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.
touch - change file access and modification times
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:
- 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 arguments:
- The file operand is used as the path argument.
- The utimbuf structure members actime and modtime are determined as described in the OPTIONS section.
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:
- Change the access time of file. Do not change the modification time unless -m is also specified.
- Do not create a specified file if it does not exist. Do not write any diagnostic messages concerning this condition.
- Change the modification time of file. Do not change the access time unless -a is also specified.
- Use the corresponding time of the file named by the pathname ref_file instead of the current time.
Use the specified time instead of the current time. The option-argument
shall be a decimal number of the form:
where each two digits represents the following:
- The month of the year [01,12].
- The day of the month [01,31].
- The hour of the day [00,23].
- The minute of the hour [00,59].
- The first two digits of the year (the century).
- The second two digits of the year.
- 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: |
- 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.
The following operands shall be supported:
A pathname of a file whose times shall be modified.
The following environment variables shall affect the execution of touch:
- 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.)
- If set to a non-empty string value, override the values of all the other internationalization variables.
- 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).
- Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard error.
- Determine the location of message catalogs for the processing of LC_MESSAGES .
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.
The following exit values shall be returned:
- The utility executed successfully and all requested changes were made.
- An error occurred.
CONSEQUENCES OF ERRORS
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
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 construct 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 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.
COPYRIGHTPortions 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 .