Linux From Scratch - Version 11.0-systemd

Chapter 4. Final Preparations

4.4. Setting Up the Environment

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

cat > ~/.bash_profile << "EOF"
exec env -i HOME=$HOME TERM=$TERM PS1='\u:\w\$ ' /bin/bash
EOF

When logged on as user lfs, the initial shell is usually a login shell which reads the /etc/profile of the host (probably containing some settings and environment variables) and then .bash_profile. The exec env -i.../bin/bash command in the .bash_profile file replaces the running shell with a new one with a completely empty environment, except for the HOME, TERM, and PS1 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.

The new instance of the shell is a non-login shell, which does not read, and execute, the contents of /etc/profile or .bash_profile files, but rather reads, and executes, the .bashrc file instead. Create the .bashrc file now: 

cat > ~/.bashrc << "EOF"
set +h
umask 022
LFS=/mnt/lfs
LC_ALL=POSIX
LFS_TGT=$(uname -m)-lfs-linux-gnu
PATH=/usr/bin
if [ ! -L /bin ]; then PATH=/bin:$PATH; fi
PATH=$LFS/tools/bin:$PATH
CONFIG_SITE=$LFS/usr/share/config.site
export LFS LC_ALL LFS_TGT PATH CONFIG_SITE
EOF

The meaning of the settings in .bashrc

set +h

The set +h command turns off bash's hash function. Hashing is ordinarily a useful feature—bash uses a hash table to remember the full path of executable files to avoid searching the PATH 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 PATH when a program is to be run. As such, the shell will find the newly compiled tools in $LFS/tools as soon as they are available without remembering a previous version of the same program in a different location.

umask 022

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).

LFS=/mnt/lfs

The LFS variable should be set to the chosen mount point.

LC_ALL=POSIX

The LC_ALL variable controls the localization of certain programs, making their messages follow the conventions of a specified country. Setting LC_ALL to “POSIX” or “C” (the two are equivalent) ensures that everything will work as expected in the chroot environment.

LFS_TGT=(uname -m)-lfs-linux-gnu

The LFS_TGT variable sets a non-default, but compatible machine description for use when building our cross compiler and linker and when cross compiling our temporary toolchain. More information is contained in Toolchain Technical Notes.

PATH=/usr/bin

Many modern linux distributions have merged /bin and /usr/bin. When this is the case, the standard PATH variable needs just to be set to /usr/bin/ for the Chapter 6 environment. When this is not the case, the following line adds /bin to the path.

if [ ! -L /bin ]; then PATH=/bin:$PATH; fi

If /bin is not a symbolic link, then it has to be added to the PATH variable.

PATH=$LFS/tools/bin:$PATH

By putting $LFS/tools/bin ahead of the standard PATH, the cross-compiler installed at the beginning of Chapter 5 is picked up by the shell immediately after its installation. This, combined with turning off hashing, limits the risk that the compiler from the host be used instead of the cross-compiler.

CONFIG_SITE=$LFS/usr/share/config.site

In Chapter 5 and Chapter 6, if this variable is not set, configure scripts may attempt to load configuration items specific to some distributions from /usr/share/config.site on the host system. Override it to prevent potential contamination from the host.

export ...

While the above commands have set some variables, in order to make them visible within any sub-shells, we export them.

[Important]

Important

Several commercial distributions add a non-documented instantiation of /etc/bash.bashrc to the initialization of bash. This file has the potential to modify the lfs user's environment in ways that can affect the building of critical LFS packages. To make sure the lfs user's environment is clean, check for the presence of /etc/bash.bashrc and, if present, move it out of the way. As the root user, run: 

[ ! -e /etc/bash.bashrc ] || mv -v /etc/bash.bashrc /etc/bash.bashrc.NOUSE

After use of the lfs user is finished at the beginning of Chapter 7, you can restore /etc/bash.bashrc (if desired).

Note that the LFS Bash package we will build in Section 8.34, “Bash-5.1.8” is not configured to load or execute /etc/bash.bashrc, so this file is useless on a completed LFS system.

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

source ~/.bash_profile