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3.2. Orientation in the file system
3.2.1. The path
When you want the system to execute a command, you almost never have to give the full path to that command. For example, we know that the ls command is in the /bin directory (check with which -a ls), yet we don't have to enter the command /bin/ls for the computer to list the content of the current directory.
The PATH environment variable takes care of this. This variable lists those directories in the system where executable files can be found, and thus saves the user a lot of typing and memorizing locations of commands. So the path naturally contains a lot of directories containing bin somewhere in their names, as the user below demonstrates. The echo command is used to display the content ("$") of the variable PATH:
rogier:> echo $PATH
/opt/local/bin:/usr/X11R6/bin:/usr/bin:/usr/sbin/:/bin
rogier:> echo $PATH
/opt/local/bin:/usr/X11R6/bin:/usr/bin:/usr/sbin/:/bin
In this example, the directories /opt/local/bin, /usr/X11R6/bin, /usr/bin, /usr/sbin and
/bin are subsequently searched for the required program. As soon as a match is found, the search is stopped, even if not every directory in the path has been searched. This can lead to strange situations. In the first example below, the user knows there is a program called sendsms to send an SMS message, and another user on the same system can use it, but she can't. The difference is in the configuration of the PATH variable:
[jenny@blob jenny]$ sendsms bash: sendsms: command not found [jenny@blob jenny]$ echo $PATH
/bin:/usr/bin:/usr/bin/X11:/usr/X11R6/bin:/home/jenny/bin [jenny@blob jenny]$ su - tony
Password: tony:~>which sendsms
sendsms is /usr/local/bin/sendsms
tony:~>echo $PATH
/home/tony/bin.Linux:/home/tony/bin:/usr/local/bin:/usr/local/sbin:\
/usr/X11R6/bin:/usr/bin:/usr/sbin:/bin:/sbin
[jenny@blob jenny]$ sendsms bash: sendsms: command not found [jenny@blob jenny]$ echo $PATH
/bin:/usr/bin:/usr/bin/X11:/usr/X11R6/bin:/home/jenny/bin [jenny@blob jenny]$ su - tony
Password: tony:~>which sendsms
sendsms is /usr/local/bin/sendsms
tony:~>echo $PATH
/home/tony/bin.Linux:/home/tony/bin:/usr/local/bin:/usr/local/sbin:\
/usr/X11R6/bin:/usr/bin:/usr/sbin:/bin:/sbin
Note the use of the su (switch user) facility, which allows you to run a shell in the environment of another user, on the condition that you know the user's password.
A backslash indicates the continuation of a line on the next, without an Enter separating one line from the other.
In the next example, a user wants to call on the wc (word count) command to check the number of lines in a file, but nothing happens and he has to break off his action using the Ctrl+C combination:
jumper:~> wc -l test
(Ctrl-C)
jumper:~> which wc
wc is hashed (/home/jumper/bin/wc)
jumper:~> echo $PATH
/home/jumper/bin:/usr/local/bin:/usr/local/sbin:/usr/X11R6/bin:\
/usr/bin:/usr/sbin:/bin:/sbin
jumper:~> wc -l test
(Ctrl-C)
jumper:~> which wc
wc is hashed (/home/jumper/bin/wc)
jumper:~> echo $PATH
/home/jumper/bin:/usr/local/bin:/usr/local/sbin:/usr/X11R6/bin:\
/usr/bin:/usr/sbin:/bin:/sbin
The use of the which command shows us that this user has a bin-directory in his home directory, containing a program that is also called wc. Since the program in his home directory is found first when searching the
paths upon a call for wc, this "home-made" program is executed, with input it probably doesn't understand, so we have to stop it. To resolve this problem there are several ways (there are always several ways to solve a problem in UNIX/Linux): one answer could be to rename the user's wc program, or the user can give the full path to the exact command he wants, which can be found by using the -a option to the which command.
If the user uses programs in the other directories more frequently, he can change his path to look in his own directories last:
jumper:~> export PATH=/usr/local/bin:/usr/local/sbin:/usr/X11R6/bin:\
/usr/bin:/usr/sbin:/bin:/sbin:/home/jumper/bin
jumper:~> export PATH=/usr/local/bin:/usr/local/sbin:/usr/X11R6/bin:\
/usr/bin:/usr/sbin:/bin:/sbin:/home/jumper/bin
