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13.4 touch: Change file timestamps

touch changes the access and/or modification times of the specified files. Synopsis:

touch [option]... file...

On older systems, touch supports an obsolete syntax, as follows. If the first file would be a valid argument to the `-t' option and no timestamp is given with any of the `-d', `-r', or `-t' options and the `--' argument is not given, that argument is interpreted as the time for the other files instead of as a file name. POSIX 1003.1-2001 (see section 2.5 Standards conformance) does not allow this; use `-t' instead.

Any file that does not exist is created empty.

If changing both the access and modification times to the current time, touch can change the timestamps for files that the user running it does not own but has write permission for. Otherwise, the user must own the files.

Although touch provides options for changing two of the times -- the times of last access and modification -- of a file, there is actually a third one as well: the inode change time. This is often referred to as a file's ctime. The inode change time represents the time when the file's meta-information last changed. One common example of this is when the permissions of a file change. Changing the permissions doesn't access the file, so the atime doesn't change, nor does it modify the file, so the mtime doesn't change. Yet, something about the file itself has changed, and this must be noted somewhere. This is the job of the ctime field. This is necessary, so that, for example, a backup program can make a fresh copy of the file, including the new permissions value. Another operation that modifies a file's ctime without affecting the others is renaming. In any case, it is not possible, in normal operations, for a user to change the ctime field to a user-specified value.

The program accepts the following options. Also see 2. Common options.

Change the access time only.

Do not create files that do not exist.

Use time instead of the current time. It can contain month names, time zones, `am' and `pm', etc. See section 27. Date input formats.

Ignored; for compatibility with BSD versions of touch.

Change the modification time only.

`-r file'
Use the times of the reference file instead of the current time.

`-t [[CC]YY]MMDDhhmm[.ss]'
Use the argument (optional four-digit or two-digit years, months, days, hours, minutes, optional seconds) instead of the current time. If the year is specified with only two digits, then CC is 20 for years in the range 0 ... 68, and 19 for years in 69 ... 99. If no digits of the year are specified, the argument is interpreted as a date in the current year.

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