92 lines
4.0 KiB
HTML
92 lines
4.0 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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<HTML>
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<HEAD>
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<META NAME="GENERATOR" CONTENT="SGML-Tools 1.0.9">
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<TITLE>PATH HOWTO: General</TITLE>
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<LINK HREF="Path-4.html" REL=next>
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<LINK HREF="Path-2.html" REL=previous>
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<LINK HREF="Path.html#toc3" REL=contents>
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</HEAD>
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<BODY>
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<A HREF="Path-4.html">Next</A>
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<A HREF="Path-2.html">Previous</A>
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<A HREF="Path.html#toc3">Contents</A>
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<HR>
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<H2><A NAME="s3">3. General</A></H2>
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<P>
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<P>All the Unix processes contain an "environment". This is a list of
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variables that contain name and value, both just strings that can
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contain most characters. All Unix processes have a parent process -
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the process that created this process as child. Child processes
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inherit environment from parent process. They can make some
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modifications to the environment before passing it in turn to their
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child processes.
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<P>One important environment variable is PATH, a list of directories
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separated by colons (':'). These directories are searched through to
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find commands. If you try to invoke command 'foo', all the
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directories in PATH (in that order) are searched for an executable
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file 'foo' (one with x-bit on). If a file is found, it is executed.
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<P>In this howto, I use term 'command' to refer executable program that
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is meant to be called with short names, using the path mechanism.
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<P>In Linux, even the low level operating system calls to start
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processes (the exec family of calls) searches through directories in
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the PATH variable: you can use the path mechanism anywhere where you
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try to execute a command. If exec operating system call gets a file
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name that does not contain '/', it evaluates the PATH environment
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variable. Even if there is no variable PATH in the environment, at
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least directories /bin and /usr/bin are looked for suitable commands.
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<P>In sh you use export command to set environment, in csh you use setenv
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command. For example:
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<P>sh:
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<BLOCKQUOTE><CODE>
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<PRE>
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PATH=/usr/local/bin:/usr/bin:/bin:/usr/bin/X11:/usr/games:.
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</PRE>
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</CODE></BLOCKQUOTE>
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csh:
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<BLOCKQUOTE><CODE>
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<PRE>
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setenv PATH /usr/local/bin:/usr/bin:/bin:/usr/bin/X11:/usr/games:.
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</PRE>
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</CODE></BLOCKQUOTE>
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<P>C-programs can use setenv() library call to change environment. Perl
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has environment in an associative array %ENV, you can set PATH as
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$ENV{PATH}="/bin".
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<P>env command is the basic way of asking the current environment
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variables. It can be used to modify it as well.
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<P>More information of the basic environment mechanism can be found from
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manual pages 'environ', 'execl', 'setenv', info file 'env' and
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documentation of shells.
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<P>When Linux boots up, the first normal process that starts is the init
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process. It is a special process because it does not have parent.
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However, it is the ancestor of all the other processes. Init
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environment will remain as environment of all the processes if they do
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not touch it explicitly. Most processes do touch.
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<P>Init starts a group of processes. File /etc/inittab tells what
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processes the system starts. These processes work in the environment
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that is directly inherited from init - typically they are processes
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like 'getty', the program that writes 'login:' to console. If you
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start PPP connections here, you must remember that you are working in
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the init environment. The system initialization is often a script
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that is started here. In Debian 1.3 initialization script
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/etc/init.d/rc and it calls other initialization scripts in turn.
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<P>The system contains many running servers (daemons) that may or may not
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use the default environment. Most servers are started from the
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initialization scripts and thus they have the init environment.
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<P>When user logs in to the system, the environment is affected by the
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settings that are compiled into the programs, system wide
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initialization scripts and user initialization scripts. This is
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pretty complicated and the current situation is not completely
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satisfactory. It is totally different if user logs in from text
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console, XDM or from network.
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<P>
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<P>
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<HR>
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<A HREF="Path-4.html">Next</A>
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<A HREF="Path-2.html">Previous</A>
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<A HREF="Path.html#toc3">Contents</A>
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</BODY>
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</HTML>
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