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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...

Any file argument that does not exist is created empty, unless option --no-create (-c) or --no-dereference (-h) was in effect.

A file argument string of ‘-’ is handled specially and causes touch to change the times of the file associated with standard output.

By default, touch sets file timestamps to the current time. Because touch acts on its operands left to right, the resulting timestamps of earlier and later operands may disagree. Also, the determination of what time is “current” depends on the platform. Platforms with network file systems often use different clocks for the operating system and for file systems; because touch typically uses file systems' clocks by default, clock skew can cause the resulting file timestamps to appear to be in a program's “future” or “past”.

The touch command sets the file's timestamp to the greatest representable value that is not greater than the requested time. This can differ from the requested time for several reasons. First, the requested time may have a higher resolution than supported. Second, a file system may use different resolutions for different types of times. Third, file timestamps may use a different resolution than operating system timestamps. Fourth, the operating system primitives used to update timestamps may employ yet a different resolution. For example, in theory a file system might use 10-microsecond resolution for access time and 100-nanosecond resolution for modification time, and the operating system might use nanosecond resolution for the current time and microsecond resolution for the primitive that touch uses to set a file's timestamp to an arbitrary value.

When setting file timestamps to the current time, touch can change the timestamps for files that the user does not own but has write permission for. Otherwise, the user must own the files. Some older systems have a further restriction: the user must own the files unless both the access and modification times are being set to the current time.

Although touch provides options for changing two of the times—the times of last access and modification—of a file, there is actually a standard 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. Some operating systems and file systems support a fourth time: the birth time, when the file was first created; by definition, this timestamp never changes.

Time stamps assume the time zone rules specified by the TZ environment variable, or by the system default rules if TZ is not set. See Specifying the Time Zone with TZ. You can avoid ambiguities during daylight saving transitions by using UTC time stamps.

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

-a
--time=atime
--time=access
--time=use
Change the access time only.
-c
--no-create
Do not warn about or create files that do not exist.
-d
--date=time
Use time instead of the current time. It can contain month names, time zones, ‘am’ and ‘pm’, ‘yesterday’, etc. For example, --date="2004-02-27 14:19:13.489392193 +0530" specifies the instant of time that is 489,392,193 nanoseconds after February 27, 2004 at 2:19:13 PM in a time zone that is 5 hours and 30 minutes east of UTC. See Date input formats. File systems that do not support high-resolution time stamps silently ignore any excess precision here.
-f
Ignored; for compatibility with BSD versions of touch.
-h
--no-dereference
Attempt to change the timestamps of a symbolic link, rather than what the link refers to. When using this option, empty files are not created, but option -c must also be used to avoid warning about files that do not exist. Not all systems support changing the timestamps of symlinks, since underlying system support for this action was not required until POSIX 2008. Also, on some systems, the mere act of examining a symbolic link changes the access time, such that only changes to the modification time will persist long enough to be observable. When coupled with option -r, a reference timestamp is taken from a symbolic link rather than the file it refers to.
-m
--time=mtime
--time=modify
Change the modification time only.
-r file
--reference=file
Use the times of the reference file instead of the current time. If this option is combined with the --date=time (-d time) option, the reference file's time is the origin for any relative times given, but is otherwise ignored. For example, ‘-r foo -d '-5 seconds'’ specifies a time stamp equal to five seconds before the corresponding time stamp for foo. If file is a symbolic link, the reference timestamp is taken from the target of the symlink, unless -h was also in effect.
-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. On the atypical systems that support leap seconds, ss may be ‘60’.

On older systems, touch supports an obsolete syntax, as follows. If no timestamp is given with any of the -d, -r, or -t options, and if there are two or more files and the first file is of the form ‘mmddhhmm[yy]’ and this would be a valid argument to the -t option (if the yy, if any, were moved to the front), and if the represented year is in the range 1969–1999, that argument is interpreted as the time for the other files instead of as a file name. This obsolete behavior can be enabled or disabled with the _POSIX2_VERSION environment variable (see Standards conformance), but portable scripts should avoid commands whose behavior depends on this variable. For example, use ‘touch ./12312359 main.c’ or ‘touch -t 12312359 main.c’ rather than the ambiguous ‘touch 12312359 main.c’.

An exit status of zero indicates success, and a nonzero value indicates failure.