Understanding the `-t` Option in `rsync`: Negating the `--compare-dest` Option
`rsync` is a powerful command-line tool used for copying and synchronizing files and directories remotely as well as locally in Linux/Unix systems. It is widely used for backups and mirroring tasks due to its efficient handling of large files and recursive directories.
The `--compare-dest` Option
The `--compare-dest` option in `rsync` is used to compare the source directory with a previously existing destination directory, instead of the current destination directory, to determine what files need to be transferred. This is particularly useful when you want to update a backup or mirror of a directory, but you don't want to transfer files that haven't changed since the last backup.
The `-t` Option and Its Negation
The `-t` option in `rsync` is used to preserve the timestamps of the files and directories being transferred. However, when used with the `--compare-dest` option, the `-t` option can have a different meaning.
When the `--compare-dest` option is used, the `-t` option negates the effect of `--compare-dest`. This means that `rsync` will not compare the timestamps of the files in the source and destination directories, but will instead transfer all files, regardless of whether they have changed or not. This behavior can be useful in certain situations, such as when you want to force an update of all files in the destination directory, even if they haven't changed since the last backup.
Example
Consider the following example:
rsync -av --compare-dest=/path/to/old_backup /path/to/source /path/to/destinationIn this example, `rsync` will compare the files in the `/path/to/source` directory with the files in the `/path/to/old_backup` directory, and only transfer the files that have changed since the last backup. However, if we add the `-t` option, like this:
rsync -avt --compare-dest=/path/to/old_backup /path/to/source /path/to/destination`rsync` will transfer all files, regardless of whether they have changed or not, effectively negating the effect of the `--compare-dest` option.
The `-t` option in `rsync` can be used to negate the effect of the `--compare-dest` option, causing `rsync` to transfer all files, regardless of whether they have changed or not. This behavior can be useful in certain situations, but it's important to understand the implications of using the `-t` option with `--compare-dest` to avoid unexpected results.
References
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