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| -rw-r--r-- | doc/coreutils.texi | 144 |
1 files changed, 75 insertions, 69 deletions
diff --git a/doc/coreutils.texi b/doc/coreutils.texi index 825e452b1..2da543cc2 100644 --- a/doc/coreutils.texi +++ b/doc/coreutils.texi @@ -209,6 +209,7 @@ Free Documentation License''. * Delaying:: sleep * Numeric operations:: factor numfmt seq * File permissions:: Access modes +* File timestamps:: File timestamp issues * Date input formats:: Specifying date strings * Opening the software toolbox:: The software tools philosophy * GNU Free Documentation License:: Copying and sharing this manual @@ -342,7 +343,6 @@ Changing file attributes * chgrp invocation:: Change group ownership * chmod invocation:: Change access permissions * touch invocation:: Change file timestamps -* File timestamps:: File timestamp issues Disk usage @@ -467,6 +467,11 @@ Numeric operations * numfmt invocation:: Reformat numbers * seq invocation:: Print numeric sequences + +File timestamps + +* File timestamps:: File timestamp issues + File permissions * Mode Structure:: Structure of file mode bits @@ -10432,7 +10437,6 @@ These commands change file attributes. * chgrp invocation:: Change file groups. * chmod invocation:: Change access permissions. * touch invocation:: Change file timestamps. -* File timestamps:: File timestamp issues. @end menu @@ -11051,73 +11055,6 @@ For example, use @samp{touch ./12312359 main.c} or @samp{touch -t @exitstatus -@node File timestamps -@section File timestamps - -@cindex atime -@cindex birthtime -@cindex ctime -@cindex mtime -Standard POSIX files have three timestamps: the access timestamp -(atime) of the last read, the modification timestamp (mtime) of the -last write, and the status change timestamp (ctime) of the last change -to the file's meta-information. Some file systems support a -fourth time: the birth timestamp (birthtime) of when the file was -created; by definition, birthtime never changes. - -One common example of a ctime change is when the permissions of a file -change. Changing the permissions doesn't access the file, so 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. - -Naively, a file's atime, mtime, and ctime are set to the current time -whenever you read, write, or change the attributes of the file -respectively, and searching a directory counts as reading it. A -file's atime and mtime can also be set directly, via the -@command{touch} command (@pxref{touch invocation}). In practice, -though, timestamps are not updated quite that way. - -For efficiency reasons, many systems are lazy about updating atimes: -when a program accesses a file, they may delay updating the file's -atime, or may not update the file's atime if the file has been -accessed recently, or may not update the atime at all. Similar -laziness, though typically not quite so extreme, applies to mtimes and -ctimes. - -Some systems emulate timestamps instead of supporting them directly, -and these emulations may disagree with the naive interpretation. For -example, a system may fake an atime or ctime by using the mtime. - -@cindex clock skew -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 -updates 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''. - -@cindex file timestamp resolution -When the system updates a file timestamp to a desired time @var{t} -(which is either the current time, or a time specified via the -@command{touch} command), there are several reasons the file's -timestamp may be set to a value that differs from @var{t}. First, -@var{t} 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 -timestamp and 100-nanosecond resolution for modification timestamp, and the -operating system might use nanosecond resolution for the current time -and microsecond resolution for the primitive that @command{touch} uses -to set a file's timestamp to an arbitrary value. - - @node Disk usage @chapter Disk usage @@ -17669,8 +17606,77 @@ outputs 1.0000000000000000007 twice and skips 1.0000000000000000008. @chapter File permissions @include perm.texi + +@node File timestamps +@chapter File timestamps + +@cindex atime +@cindex birthtime +@cindex ctime +@cindex mtime +Standard POSIX files have three timestamps: the access timestamp +(atime) of the last read, the modification timestamp (mtime) of the +last write, and the status change timestamp (ctime) of the last change +to the file's meta-information. Some file systems support a +fourth time: the birth timestamp (birthtime) of when the file was +created; by definition, birthtime never changes. + +One common example of a ctime change is when the permissions of a file +change. Changing the permissions doesn't access the file, so 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. + +Naively, a file's atime, mtime, and ctime are set to the current time +whenever you read, write, or change the attributes of the file +respectively, and searching a directory counts as reading it. A +file's atime and mtime can also be set directly, via the +@command{touch} command (@pxref{touch invocation}). In practice, +though, timestamps are not updated quite that way. + +For efficiency reasons, many systems are lazy about updating atimes: +when a program accesses a file, they may delay updating the file's +atime, or may not update the file's atime if the file has been +accessed recently, or may not update the atime at all. Similar +laziness, though typically not quite so extreme, applies to mtimes and +ctimes. + +Some systems emulate timestamps instead of supporting them directly, +and these emulations may disagree with the naive interpretation. For +example, a system may fake an atime or ctime by using the mtime. + +@cindex clock skew +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 +updates 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''. + +@cindex file timestamp resolution +When the system updates a file timestamp to a desired time @var{t} +(which is either the current time, or a time specified via the +@command{touch} command), there are several reasons the file's +timestamp may be set to a value that differs from @var{t}. First, +@var{t} 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 +timestamp and 100-nanosecond resolution for modification timestamp, and the +operating system might use nanosecond resolution for the current time +and microsecond resolution for the primitive that @command{touch} uses +to set a file's timestamp to an arbitrary value. + + @include parse-datetime.texi + @c What's GNU? @c Arnold Robbins @node Opening the software toolbox |