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
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.
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.