lfs-book/chapter04/settingenviron.xml

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<sect1 id="ch-tools-settingenviron">
<title>Setting Up the Environment</title>
<?dbhtml filename="settingenvironment.html"?>

<para>Set up a good working environment by creating two new startup
files for the <command>bash</command> shell. While logged in as user
<emphasis>lfs</emphasis>, issue the
following command to create a new <filename>.bash_profile</filename>:</para>

<screen><userinput>cat &gt; ~/.bash_profile &lt;&lt; "EOF"
<literal>exec env -i HOME=$HOME TERM=$TERM PS1='\u:\w\$ ' /bin/bash</literal>
EOF</userinput></screen>

<para>When logged on as user <emphasis>lfs</emphasis>, the
initial shell is usually a <emphasis>login</emphasis> shell which reads the
<filename>/etc/profile</filename> of the host (probably containing
some settings and environment variables) and then
<filename>.bash_profile</filename>. The <command>exec env
-i.../bin/bash</command> command in the
<filename>.bash_profile</filename> file replaces the running shell
with a new one with a completely empty environment, except for the
<envar>HOME</envar>, <envar>TERM</envar>, and
<envar>PS1</envar> variables. This ensures that no unwanted and
potentially hazardous environment variables from the host system leak
into the build environment. The technique used here achieves the goal
of ensuring a clean environment.</para>

<para>The new instance of the shell is a <emphasis>non-login</emphasis>
shell, which does not read the <filename>/etc/profile</filename> or
<filename>.bash_profile</filename> files, but rather reads the
<filename>.bashrc</filename> file instead. Create the
<filename>.bashrc</filename> file now:</para>

<screen><userinput>cat &gt; ~/.bashrc &lt;&lt; "EOF"
<literal>set +h
umask 022
LFS=/mnt/lfs
LC_ALL=POSIX
PATH=/tools/bin:/bin:/usr/bin
export LFS LC_ALL PATH</literal>
EOF</userinput></screen>

<para>The <command>set +h</command> command turns off
<command>bash</command>'s hash function. Hashing is ordinarily a useful 
feature&mdash;<command>bash</command> uses a hash table to remember the
full path of executable files to avoid searching the <envar>PATH</envar> time
and again to find the same executable. However, the new tools
should be used as soon as they are installed. By switching off the
hash function, the shell will always search the <envar>PATH</envar> when a program is
to be run. As such, the shell will find the newly compiled
tools in <filename class="directory">$LFS/tools</filename> as soon as
they are available without remembering a previous version of the same
program in a different location.</para>

<para>Setting the user file-creation mask (umask) to 022 ensures that newly
created files and directories are only writable by their owner, but
are readable and executable by anyone (assuming default modes are used
by the open(2) system call, new files will end up with permission mode
644 and directories with mode 755).</para>

<para>The <envar>LFS</envar> variable should be set to the
chosen mount point.</para>

<para>The <envar>LC_ALL</envar> variable controls the
localization of certain programs, making their messages follow the
conventions of a specified country.  If the host system uses a version
of Glibc older than 2.2.4, having <envar>LC_ALL</envar> set to something other than
<quote>POSIX</quote> or <quote>C</quote> (during this chapter) may
cause issues if you exit the chroot environment and wish to return
later. Setting <envar>LC_ALL</envar> to <quote>POSIX</quote>
or <quote>C</quote> (the two are equivalent) ensures that
everything will work as expected in the chroot environment.</para>

<para>By putting <filename class="directory">/tools/bin</filename> ahead of the
standard <envar>PATH</envar>, all the programs installed in <xref
linkend="chapter-temporary-tools"/> are picked up by the shell immediately after
their installation. This, combined with turning off hashing, limits the risk
that old programs are used from the host when the same programs are available in
the chapter 5 environment.</para>

<para>Finally, to have the environment fully prepared for building the
temporary tools, source the just-created user profile:</para>

<screen><userinput>source ~/.bash_profile</userinput></screen>

</sect1>