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
touch to change the times of the file associated with
touch sets file timestamps to the current time.
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”.
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,
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
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
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
environment variable, or by the system default rules if
not set. See Specifying the Time Zone with
TZ in The GNU C Library Reference Manual.
You can avoid ambiguities during
daylight saving transitions by using UTC time stamps.
The program accepts the following options. Also see Common options.
Change the access time only.
Do not warn about or create files that do not exist.
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.
Ignored; for compatibility with BSD versions of
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.
Change the modification time only.
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.
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.