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 $PATH
/opt/local/bin:/usr/X11R6/bin:/usr/bin:/usr/sbin/:/bin
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In this example, the directories /opt/local/bin, /usr/X11R6/bin, /usr/bin,
/usr/sbin and /bin are subsequently searched for the required
program. As soon as a match is found, the search is stopped, even
if not every directory in the path has been searched. This can lead
to strange situations. In the first example below, the user knows
there is a program called sendsms to send an
SMS message, and another user on the same system can use it, but
she can't. The difference is in the configuration of the PATH variable:
[jenny@blob jenny]$ 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
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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
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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
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Changes are not
permanent! |
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Note that when using the export command
in a shell, the changes are temporary and only valid for this
session (until you log out). Opening new sessions, even while the
current one is still running, will not result in a new path in the
new session. We will see in
Section 7.2 how we can make these kinds of changes to the environment
permanent, adding these lines to the shell configuration files.
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