2005/04/27 PATH, command
Your PATH (echo $PATH) is where your shell will search for
executables. If a command you want to run isn't included in $PATH,
you have to be specific: /somewhere/someprog. One confusion
for Unix/Linux newcomers is that if PATH doesn't include "." (current
directory), then trying to run a program you can see right in front
of your nose with "ls" is going to generate a "command not found" message from
your shell. Unlike MSDOS, Unix shells don't usually assume they should look
in the current directory without being told to. Remember, this
is a function of the shell you are using and NOT Unix/Linux itself.
Someone COULD write a shell that automatically looked in the current directory
for commands - but they aren't likely to.
Why not? Because it's a bad idea. It's especially a bad idea
for root because of the very real possibility of being tricked into
executing the wrong program with (potentially) disastrous consequences.
It's true that your shell searches through PATH in order, so appending
the "." to the end is less dangerous than sticking it elsewhere, but
simple fat-fingered typing can catch you even with that. Especially
for root, it's best not to do that and just learm to type "./this" when
you really do want to rn the "this" in the current directory.
It's definitely a bad idea for root, and probably not a real good
idea for anyone. But it does raise a question in my mind - why
would you want "." in your PATH anyway? Yes, good old Microsoft
stuff defaults to that behaviour, but that shouldn't justify
defaulting non-root users to include that.. and does any system
still do that? I'm not sure I've seen one recently.. oops, yes I
have: SCO systems still set this by default (at least as of OpenServer 5.0.7)
At least it's at the end..
but other than that, I don't think I've seen "." added by default
anywhere. So what does determine your PATH? A default PATH is
probably compiled into your shell, but it's usually reset or at least
added to by startup files. In the case of bash, that would be
/etc/profile, /etc/bashrc and your own "dot" files. PATH should be
set in .bashrc (but see ./bashrc doesn't run automatically, .bashrc broken and
Shell Bashing) if you are changing or
adding to it.
The first place PATH is set is by login. As I mentioned, your shell
probably has a default compiled in also; it looks like this on
#define DEFAULT_PATH_VALUE \
Also defined is a failsafe PATH which you can get with "command -p" :
* The value for PATH when invoking `command -p'. This is only used when
the Posix.2 confstr () function, or CS_PATH define are not present. */
#define STANDARD_UTILS_PATH \
To see how that works, create a little script that just has "echo $PATH" in it. I called mine "p":
$ cat p
$ bash ./p
$ command ./p
$ command -p ./p
Notice that isn't what the defaults say it would be, so something
set that elsewhere during the compile. "command" is useful even without -p
because it does ignore aliases and functions, giving more assurance that
you are running the command you think you are.
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© 2012-07-19 Tony Lawrence