chase - chase symbolic links

chase [ --verbose | --from-file | -f | --null | -0 | --loop-warn-threshold COUNT | -w COUNT | --loop-fail-threshold COUNT | -l COUNT | --disable-loop-detection | -D ] [file...] chase -v | --version chase -h | --help

Chase is a small utility that tracks down the name of the actual file that is truly accessed, when one hits a symbolic link. It chases the link, so to speak.

In normal operation, you give chase a name of an existing file. The program will then show you (or, more accurately, print to the standard output stream) the name of the real file that the original file referred to. It does not matter, if you give the program the name of a regular file; chase will just give you the same name back. You can give the utility the option --verbose, in order to have it be more verbose in its operation. You can also give the program a list of files from which it will read the file names to be chased (see the option --from-file).

I'll give you a good example of a case where chase is useful. In a Debian GNU/Linux system, software installation is handled by the program dpkg(8). There is a way to query dpkg(8), which package a file belongs to. Unfortunately, this does not not grok all symlinks. This is a problem, since the Debian system uses symbolic links extensively for handling several flavours of a program (say, XEmacs and GNU Emacs and the versions 19 and 20 of both). Now with chase, one can determine the real file easily and then using this result as an argument to dpkg(8). Observe:

ajk@ugh:~$ dpkg -S `which xemacs20`
dpkg: /usr/bin/xemacs20 not found.
ajk@ugh:~$ dpkg -S `which xemacs20 | xargs chase`
xemacs20-nomule: /usr/bin/xemacs-20.4-nomule
ajk@ugh:~$ 

When given multiple file names as arguments, chase will chase every one of them in order.

The following options are recognized by chase:

--verbose

Chat about what is being done.

-f, --from-file

Treat the file names on the command line as sources for names to be chased. If this option is present, chase will read in each of the files given on the command line (both a sole dash "-" and the absence of file names will be taken as meaning the standard input stream). Each line in the files is taken as a verbatim file name that will be chased like it had been given on the command line of a run without this option. There is no means for quoting the newline character; therefore only file names that do not contain newlines can be specified via the files. This restriction is lifted by the -0 option, though. However, literal spaces are preserved and are not interpreted as special.

-0, --null

This option implies the option -f, which reads file names from a file. The -0 option modifies the behviour -f so that instead of treating lines in the file as file names, the file names are expected to be separated by null characters. This allows for specifying file names with newlines in them with -f.

-w WCOUNT, --loop-warn-threshold=WCOUNT

-l LCOUNT, --loop-fail-threshold=LCOUNT

Set the threshold for warning about a possible symlink loop (WCOUNT) and for failing because of one (LCOUNT). There are default values, which you can find out by invoking chase with the argument --help. Using zero as WCOUNT or LCOUNT disables the check.

-D, --disable-loop-detection

Normally chase keeps a record of all symlinks it has visited when chasing the current file. It uses this record to reliably detect symlink loops. However, checking the record slows the process a little (more for long chains of symlinks) and the record requires a small amount of memory (more for long chains of symlinks), so it may be desirable in some cases to inhibit this check. This option disables this loop detection and the associated recording of visited symlinks. When this option is in effect (assuming no bugs in the related code) chase runs in constant space and linear time relative to the length of the symlink chains encountered.

-h, --help

Output a usage summary and exit successfully.

-v, --version

Show version information and exit successfully.

The exit status is 0 if and only if all files given in the command lines exist and none of them are dangling symlinks.

The following messages are emitted by chase when it suspects or detects a symlink loop:

quite many symlink hops, hope we're not looping...

This means that the chain of symlinks is longer than a given threshold. This may mean that the chain is infinite (and thus contains a loop somewhere). The threshold can be specificed by using the --loop-warn-threshold command line option.

too many symlink hops, giving up...

This means that the chain of symlinks is longer than a given threshold. This usually means that the chain is infinite (and thus contains a loop somewhere). The threshold can be specificed by using the --loop-fail-threshold command line option. If you see this message, it means that chase has given up on that file.

symlink loop detected, giving up...

The symlink chain is looping and therefore there is no result for the chase. Chase has given up on that file.

Additionally, Chase emits several error messages under problemous conditions. They all come from the system libraries, so the program has no control over the actual wording of the messages. They all follow the format "program name: file name: error message", where file name is the name of the offending file or directory.

Chase may have bugs. If you find them, please report them to <rotty@debian.org> or preferably via the Debian bugtracking system.

Chase has been written by Antti-Juhani Kaijanaho <gaia@iki.fi> and is currently maintained by Andreas Rottmann <rotty@debian.org>.

find(1), ln(1), namei(1), readlink(1), readlink(2), symlinks(1)