+\1f
+File: wget.info, Node: Directory-Based Limits, Next: FTP Links, Prev: Types of Files, Up: Following Links
+
+Directory-Based Limits
+======================
+
+ Regardless of other link-following facilities, it is often useful to
+place the restriction of what files to retrieve based on the directories
+those files are placed in. There can be many reasons for this--the
+home pages may be organized in a reasonable directory structure; or some
+directories may contain useless information, e.g. `/cgi-bin' or `/dev'
+directories.
+
+ Wget offers three different options to deal with this requirement.
+Each option description lists a short name, a long name, and the
+equivalent command in `.wgetrc'.
+
+`-I LIST'
+`--include LIST'
+`include_directories = LIST'
+ `-I' option accepts a comma-separated list of directories included
+ in the retrieval. Any other directories will simply be ignored.
+ The directories are absolute paths.
+
+ So, if you wish to download from `http://host/people/bozo/'
+ following only links to bozo's colleagues in the `/people'
+ directory and the bogus scripts in `/cgi-bin', you can specify:
+
+ wget -I /people,/cgi-bin http://host/people/bozo/
+
+`-X LIST'
+`--exclude LIST'
+`exclude_directories = LIST'
+ `-X' option is exactly the reverse of `-I'--this is a list of
+ directories *excluded* from the download. E.g. if you do not want
+ Wget to download things from `/cgi-bin' directory, specify `-X
+ /cgi-bin' on the command line.
+
+ The same as with `-A'/`-R', these two options can be combined to
+ get a better fine-tuning of downloading subdirectories. E.g. if
+ you want to load all the files from `/pub' hierarchy except for
+ `/pub/worthless', specify `-I/pub -X/pub/worthless'.
+
+`-np'
+`--no-parent'
+`no_parent = on'
+ The simplest, and often very useful way of limiting directories is
+ disallowing retrieval of the links that refer to the hierarchy
+ "above" than the beginning directory, i.e. disallowing ascent to
+ the parent directory/directories.
+
+ The `--no-parent' option (short `-np') is useful in this case.
+ Using it guarantees that you will never leave the existing
+ hierarchy. Supposing you issue Wget with:
+
+ wget -r --no-parent http://somehost/~luzer/my-archive/
+
+ You may rest assured that none of the references to
+ `/~his-girls-homepage/' or `/~luzer/all-my-mpegs/' will be
+ followed. Only the archive you are interested in will be
+ downloaded. Essentially, `--no-parent' is similar to
+ `-I/~luzer/my-archive', only it handles redirections in a more
+ intelligent fashion.
+
+\1f
+File: wget.info, Node: FTP Links, Prev: Directory-Based Limits, Up: Following Links
+
+Following FTP Links
+===================
+
+ The rules for FTP are somewhat specific, as it is necessary for them
+to be. FTP links in HTML documents are often included for purposes of
+reference, and it is often inconvenient to download them by default.
+
+ To have FTP links followed from HTML documents, you need to specify
+the `--follow-ftp' option. Having done that, FTP links will span hosts
+regardless of `-H' setting. This is logical, as FTP links rarely point
+to the same host where the HTTP server resides. For similar reasons,
+the `-L' options has no effect on such downloads. On the other hand,
+domain acceptance (`-D') and suffix rules (`-A' and `-R') apply
+normally.
+
+ Also note that followed links to FTP directories will not be
+retrieved recursively further.
+
+\1f
+File: wget.info, Node: Time-Stamping, Next: Startup File, Prev: Following Links, Up: Top
+
+Time-Stamping
+*************
+
+ One of the most important aspects of mirroring information from the
+Internet is updating your archives.
+
+ Downloading the whole archive again and again, just to replace a few
+changed files is expensive, both in terms of wasted bandwidth and money,
+and the time to do the update. This is why all the mirroring tools
+offer the option of incremental updating.
+
+ Such an updating mechanism means that the remote server is scanned in
+search of "new" files. Only those new files will be downloaded in the
+place of the old ones.
+
+ A file is considered new if one of these two conditions are met:
+
+ 1. A file of that name does not already exist locally.
+
+ 2. A file of that name does exist, but the remote file was modified
+ more recently than the local file.
+
+ To implement this, the program needs to be aware of the time of last
+modification of both remote and local files. Such information are
+called the "time-stamps".
+
+ The time-stamping in GNU Wget is turned on using `--timestamping'
+(`-N') option, or through `timestamping = on' directive in `.wgetrc'.
+With this option, for each file it intends to download, Wget will check
+whether a local file of the same name exists. If it does, and the
+remote file is older, Wget will not download it.
+
+ If the local file does not exist, or the sizes of the files do not
+match, Wget will download the remote file no matter what the time-stamps
+say.
+
+* Menu:
+
+* Time-Stamping Usage::
+* HTTP Time-Stamping Internals::
+* FTP Time-Stamping Internals::
+
+\1f
+File: wget.info, Node: Time-Stamping Usage, Next: HTTP Time-Stamping Internals, Prev: Time-Stamping, Up: Time-Stamping
+
+Time-Stamping Usage
+===================
+
+ The usage of time-stamping is simple. Say you would like to
+download a file so that it keeps its date of modification.
+
+ wget -S http://www.gnu.ai.mit.edu/
+
+ A simple `ls -l' shows that the time stamp on the local file equals
+the state of the `Last-Modified' header, as returned by the server. As
+you can see, the time-stamping info is preserved locally, even without
+`-N'.
+
+ Several days later, you would like Wget to check if the remote file
+has changed, and download it if it has.
+
+ wget -N http://www.gnu.ai.mit.edu/
+
+ Wget will ask the server for the last-modified date. If the local
+file is newer, the remote file will not be re-fetched. However, if the
+remote file is more recent, Wget will proceed fetching it normally.
+
+ The same goes for FTP. For example:
+
+ wget ftp://ftp.ifi.uio.no/pub/emacs/gnus/*
+
+ `ls' will show that the timestamps are set according to the state on
+the remote server. Reissuing the command with `-N' will make Wget
+re-fetch *only* the files that have been modified.
+
+ In both HTTP and FTP retrieval Wget will time-stamp the local file
+correctly (with or without `-N') if it gets the stamps, i.e. gets the
+directory listing for FTP or the `Last-Modified' header for HTTP.
+
+ If you wished to mirror the GNU archive every week, you would use the
+following command every week:
+
+ wget --timestamping -r ftp://prep.ai.mit.edu/pub/gnu/
+
+\1f
+File: wget.info, Node: HTTP Time-Stamping Internals, Next: FTP Time-Stamping Internals, Prev: Time-Stamping Usage, Up: Time-Stamping
+
+HTTP Time-Stamping Internals
+============================
+
+ Time-stamping in HTTP is implemented by checking of the
+`Last-Modified' header. If you wish to retrieve the file `foo.html'
+through HTTP, Wget will check whether `foo.html' exists locally. If it
+doesn't, `foo.html' will be retrieved unconditionally.
+
+ If the file does exist locally, Wget will first check its local
+time-stamp (similar to the way `ls -l' checks it), and then send a
+`HEAD' request to the remote server, demanding the information on the
+remote file.
+
+ The `Last-Modified' header is examined to find which file was
+modified more recently (which makes it "newer"). If the remote file is
+newer, it will be downloaded; if it is older, Wget will give up.(1)
+
+ When `--backup-converted' (`-K') is specified in conjunction with
+`-N', server file `X' is compared to local file `X.orig', if extant,
+rather than being compared to local file `X', which will always differ
+if it's been converted by `--convert-links' (`-k').
+
+ Arguably, HTTP time-stamping should be implemented using the
+`If-Modified-Since' request.
+
+ ---------- Footnotes ----------
+
+ (1) As an additional check, Wget will look at the `Content-Length'
+header, and compare the sizes; if they are not the same, the remote
+file will be downloaded no matter what the time-stamp says.
+
projects / wget / blobdiff