./

[235u at comcast.net (eric)]

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hi.  both answers i got were great, thanks.  i learned about '.' and
cleared up some confusion i had about PATH.  thanks.

Chris Devers wrote:
| On Mon, 10 Nov 2003, eric wrote:
|
|
|>hey. it's not easy teaching yourself linux.  here's one for somebody
|>who's bored tonight...  what does ./ mean?
|>if i cd to /usr/sbin and then type tripwire, nothing happens, but if i
|>type ./tripwire off we go.  why?
|
|
| It's a path hint. In most/all Unix filesystems -- as well as DOS/FAT --
| '.' is shorthand for the current directory, and '..' is shorthand for the
| directory one level up from the current directory. As a parallel, '~' is
| shorthand for your home dir.
|
| So, when you type "./foo", the shell expands that "." to whatever the
| current working directory is. If you're in /usr/local/bin, then that is
| what '.' would expand to.
|
| The reason you have to type this sometimes has to do with your shell's
| $PATH environment variable. For any command you type that is not specified
| with a full path -- such as /usr/local/bin/foo -- the shell searches for
| that command among the set of available programs in the directories that
| are mentioned in your $PATH variable.
|
| So for example, one one account my $PATH is as follows:
|
|    $ echo $PATH
|    /Users/cdevers/bin:/sw/bin:/usr/local/bin::/usr/bin:/usr/X11R6/bin:/bin
|
| To see how this works in practice, assume I type 'zgrep foo /tmp/bar.gz'.
| The first place my shell would look for the command is in my ~/bin
| directory.  If there isn't one there, it'll look in the next directory
| mentioned -- Fink's bin directory at /sw/bin. I've got the GNU version of
| gzip installed through Fink, so I have a /sw/bin/zgrep command -- that's
| what would be used. If I didn't have that package/file, thenthe shell
| would keep working down the list until it got to /usr/bin, where Apple's
| copy of /usr/bin/zgrep would be found and used.
|
| Note though that "." doesn't show up anywhere in my $PATH. This is a
| security consideration. It used to be common to prefix $PATH with '.', so
| that `echo $PATH` would begin ".:/usr/local/bin:..." etc. But keeping the
| current directory out of your path protects you from situations where an
| untrusted user puts a copy of a program with behavior defined by them into
| a directory where you might be working. So for example if someone breaks
| into your web server somehow, and uploads a malicious file named 'vi' to
| your /usr/local/apache/htdocs, then the next time you try to do this:
|
|     $ cd /usr/local/apache/htdocs
|     $ vi index.html
|
| You would end up running that person's program over your index file,
| rather than the editor you were probably expecting. That program can do
| anything -- attempt to do a `rm -rf /`, change your password, create a
| backdoor for them to get in, etc. If sneaky, it might then do what it
| seemed like it should have done -- open up vi in this case -- so that you
| might not even realize the damage until it was too late.
|
|
| So. If you use commands in /usr/sbin a lot, you may want to consider
| adding that directory to your $PATH, by editing the relevant login scripts
| (e.g. ~/.profile, ~/.bashrc, or ~/.tcshrc). Somewhere in there should be a
| line declaring your $PATH. You can either edited that line, or do
| something like this:
|
|     PATH=$PATH:/usr/sbin
|
| (with slightly different syntax for tcsh). On the other hand, sbin
| directories are generally reserved for commands that should only be run by
| a priviliged user, so leaving them out of your path might be safe anyway.
|
|
| Make sense?
|
|
|

- --
loki_the_doppelganger
http://home.comcast.net/~235u/
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