This seems rather simple, and in a way it is, but a lot goes on
behind the scenes that you never see. What’s important to understand
about bash is that its basic operation is to load
and execute programs. All the rest is just window dressing to get ready
to run programs. Sure there are shell variables and control statements
for looping and if/then/else
branching, and there are ways to control input and output, but they
are all icing on the cake of program execution.
So where does it get the program to run?
bash will use a shell variable called
$PATH to locate your executable. The $PATH variable is a list of directories. The
directories are separated by colons (:). bash will search in each
of those directories for a file with the name that you specified. The
order of the directories is important—bash looks at
the order in which the directories are listed in the variable, and takes
the first executable found.
$ echo $PATH /bin:/usr/bin:/usr/local/bin:. $
In the $PATH variable
shown above, four directories are included. The last
directory in that list is just a single dot (called the dot directory, or
just dot), which represents the current directory.
The dot is the name of the directory found within every directory on a
Linux or Unix file system—wherever you are, that’s the directory to
which dot refers. For example, when you copy a file from someplace to
dot (i.e., cp /other/place/file.),
you are copying the file into the current directory. By having the dot
directory listed in our path, bash will look for
commands not just in those other directories, but also in the current
directory (.).
Many people feel that putting dot on your $PATH is too great a security risk—some-one
could trick you and get you to run their own (malicious) version of a
command in place of one that you were expecting. Now if dot were listed
first, then someone else’s version of ls would supersede the
normal ls command and you might unwittingly run
that command. Don’t believe us? Try this:
$ bash $ cd $ touch ls $ chmod 755 ls $ PATH=".:$PATH" $ ls $
Suddenly, the ls appears not to work in your
home directory. You get no output. When you cd to
some other location (e.g., cd /tmp),
then ls will work, but not in your home directory.
Why? Because in that directory there is an empty file called
ls that is run (and does nothing—it’s empty)
instead of the normal ls command located at
/bin/ls. Since we started this example by running a
new copy of bash , you can exit from this mess by
exiting this subshell; but you might want to remove the bogus
ls command first:
$ cd $ rm ls $ exit $
Can you see the mischief potential of wandering into a strange directory with your path set to search the dot directory before anywhere else?
If you put dot as the last directory in your $PATH variable, at least you won’t be tricked
that easily. Of course, if you leave it off altogether it is arguably
even safer and you can still run commands in your local directory by
typing a leading dot and slash character, as in:
$ ./myscript
The choice is yours.
Warning
Never allow a dot or writable directories in
root’s $PATH.
For more, see Finding World-Writable Directories in Your $PATH and Adding the Current Directory to the $PATH.
Don’t forget to set the file’s permissions to execute permission before you invoke your script:
$ chmod a+x ./myscript $ ./myscript
You only need to set the permissions once. Thereafter you can invoke the script as a command.
A common practice among some bash experts is
to create a personal bin directory, analogous to
the system directories /bin and
/usr/bin where executables are kept. In your
personal bin you can put copies of your favorite
shell scripts and other customized or private commands. Then add your
home directory to your $PATH, even to
the front (PATH=~/bin:$PATH). That
way, you can still have your own customized favorites without the
security risk of running commands from strangers.
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