PATH

Revision as of 12:33, 22 February 2005

PATH is a system variable which functions in unix much the same as it does in DOS; it contains the directories that your shell/command line interface checks for commands in. This can be set to a default in your skeleton_file or it can be set in your shell .profile or you can set it from the command line itself. You'll need to run the command set PATH and export the PATH; doing it this way means that you will not save your new path -- it'll be lost once you log out or reboot the machine. You can see your current PATH by asking the shell to echo it:

samizdata# echo $PATH
/sbin:/bin:/usr/sbin:/usr/bin:/usr/games:/usr/local/sbin:/usr/local/bin:/usr/X11R6/bin:/root/bin
samizdata# logout
dave@samizdata:~% echo $PATH
/sbin:/bin:/usr/sbin:/usr/bin:/usr/games:/usr/local/sbin:/usr/local/bin:/usr/X11R6/bin:/home/dave/bin
dave@samizdata:~%

There are, however, a couple of significant differences between the way PATH is utilized under DOSlike and Unixlike systems. First, DOSlikes always silently include ./ (the current directory) as part of the PATH, whereas Unixlikes do not (and should not, for security reasons). This means that on a DOSlike system, if you are in the same directory as program.exe, you can run it simply by typing program.exe. On a unixlike system, you would need to instead type in ./program.exe to specify the current directory directly.

Another significant difference is that under a DOSlike system, the PATH is interpreted fresh every time it's called - meaning that if you drop a new program into your PATH somewhere, you can immediately call it from anywhere on the system without specifying a path to it. While this is convenient, it does mean that it takes longer to execute any pathless command the longer the PATH becomes, and also makes it somewhat easier to get an unsuspecting user to unwittingly run a trojan horse. So Unixlike systems do it a little differently - under a Unixlike system, the PATH is read, and all programs within it have their pathnames cached at the startup of a shell session. To manually re-interpret the PATH so that new programs can be found, you need to use the rehash command.