Diff — LFS Temporary System

{{infobox

| title = LFS: The Temporary System & the Chroot

| chapter = Linux Handbook, chapter 4 — temp tools cross-compiled, then the walls go up

| stage | LFS book chapters 6–7 — building inside the not-yet-system

| milestone = chroot: the moment $LFS becomes /

| hardest_idea = The chroot pivot — the host vanishes, the target becomes root

| needs = Chapter 3's cross-toolchain complete and sanity-checked

| next = LFS III — the real system packages

}}

**Chapter 4 builds the temporary system** — the last packages compiled with the cross-toolchain, but now running *for* the future system and installed into it — and then performs the maneuver the whole setup was scaffolding: **chroot into `$LFS`**, where the partition becomes the root filesystem and the host ceases to exist for the build. After this chapter you're inside the system you're building — there's no longer any host to contaminate or be contaminated by.

## Why a temporary system at all

The cross-toolchain (chapter 3) *produces* code for LFS, but it *runs on the host* — linked against the host's glibc, invoking the host's coreutils, the host's bash. To build the real system packages we need build tools that run *inside* the future system — compiled for `x86_64-lfs-linux-gnu`, linked against the fresh `$LFS` glibc. That's the temporary system: ~15 packages (M4, Ncurses, Bash, Coreutils, Diffutils, File, Findutils, Gawk, Grep, Gzip, Make, Patch, Sed, Tar, Xz, Binutils pass 2, GCC pass 2) cross-compiled and installed into `$LFS` — the minimal toolkit needed to build everything else once we're inside.

## Pass 2 — binutils and gcc again, now for real

Binutils and GCC are built a **second time** — same tarballs, re-extracted fresh, but now `--host=$LFS_TGT` and linked against the new glibc. Pass 2's products land in `/tools`, and the build uses *them* — the pass-1 cross-toolchain's job is done.

```sh

cd binutils-*/build   # fresh tree, fresh build dir

../configure --prefix=/usr        \

             --build=$(../config.guess) \

             --host=$LFS_TGT      \

             --disable-nls        \

             --enable-shared      \

             --enable-gprofng=no  \

             --disable-werror     \

             --enable-64-bit-bfd  \

             --enable-new-dtags   \

             --enable-default-hash-style=gnu

make && make DESTDIR=$LFS install

# clean libtool archives that pin the cross path

rm -v $LFS/usr/lib/lib{bfd,ctf,ctf-nobfd,opcodes,sframe}.{a,la}

```

GCC pass 2 is the same shape — full languages, threads enabled, libstdc++ built properly — and it ends with the critical `cc` symlink:

```sh

ln -svf gcc $LFS/usr/bin/cc   # packages call 'cc' by convention

```

## The temp tools — the same ritual, ~15 times

Each temp package is the same pattern — cross-configure (`--host=$LFS_TGT`), `make`, `make DESTDIR=$LFS install`. A representative entry:

```sh

cd coreutils-*/

./configure --prefix=/usr     \

            --host=$LFS_TGT   \

            --build=$(build-aux/config.guess) \

            --enable-install-program=hostname \

            --enable-no-install-program=kill,uptime

make

make DESTDIR=$LFS install

# FHS placement fix-ups the book carries for several packages

mv -v $LFS/usr/bin/chroot $LFS/usr/sbin

mkdir -pv $LFS/usr/share/man/man8

mv -v $LFS/usr/share/man/man1/chroot.1 $LFS/usr/share/man/man8/chroot.8

sed -i 's/"usr\/bin"/"usr\/sbin"/' $LFS/usr/bin/chroot

```

The pattern repeats for M4, Ncurses (with its terminfo database and the `libncursesw` wide-char variants), Bash, Diffutils, File, Findutils, Gawk, Grep, Gzip, Make, Patch, Sed, Tar, Xz — each with its own handful of configure flags and FHS fix-ups. The book's chapter 6 is a checklist; the *skill* being taught is the ritual itself — by package ten you can nearly write the configure line yourself.

## The chroot — the pivot

With the temporary system installed, `$LFS` now contains a self-sufficient Linux userspace: its own bash, its own coreutils, its own make, its own gcc. Enter it:

```sh

# as ROOT — back to root for the pivot

chroot "$LFS" /usr/bin/env -i    \

    HOME=/root TERM="$TERM"      \

    PS1='(lfs chroot) \u:\w\$ '  \

    PATH=/usr/bin:/usr/sbin      \

    MAKEFLAGS="-j$(nproc)"       \

    TESTSUITEFLAGS="-j$(nproc)"  \

    /usr/bin/bash --login

```

Everything that follows runs inside. The prompt reads `(lfs chroot)` — the filesystem root *is* the partition. `$LFS` no longer exists as a path; it's now `/`. The `/tools` symlink trick pays off here: binaries compiled to look in `/tools/...` find exactly the directory they were built for.

## But first — the virtual filesystems

A chrooted userspace needs the kernel's virtual filesystems or nothing works — `/proc`, `/sys`, `/dev` are kernel interfaces, not real directories. Before chrooting (or the book's equivalent ordering), they must be **bind-mounted into** `$LFS` from the host:

```sh

# as root — populate the device tree

mkdir -pv $LFS/{dev,proc,sys,run}

mount -v --bind /dev $LFS/dev

mount -v --bind /dev/pts $LFS/dev/pts

mount -vt proc proc $LFS/proc

mount -vt sysfs sysfs $LFS/sys

mount -vt tmpfs tmpfs $LFS/run

# shm + pts for modern packages

if [ -h $LFS/dev/shm ]; then mkdir -pv $LFS/$(readlink $LFS/dev/shm);

else mount -t tmpfs -o nosuid,nodev tmpfs $LFS/dev/shm; fi

```

This is the step people skip and then wonder why `configure` can't detect the system. **Inside the chroot there is no `/dev/null` unless you bound it** — testsuites, shells, everything needs it. The mounts are *bind mounts* — the host's kernel interfaces showing through into the chroot — so the temp system can interrogate the real hardware it's building for.

## Ownership handover

Before chrooting the book changes ownership of the entire `$LFS` tree **from lfs to root** — inside the chroot, root is the builder:

```sh

chown --from lfs -R root:root $LFS/{usr,lib,var,etc,bin,sbin,tools}

case $(uname -m) in

  x86_64) chown --from lfs -R root:root $LFS/lib64 ;;

esac

```

The lfs user's job ends here; the rest builds as chroot-root. (And later, the finished system's files get normalized to their real owners.)

## The directory skeleton

Inside the chroot, the FHS skeleton is laid before packages:

```sh

mkdir -pv /{boot,home,mnt,opt,srv}

mkdir -pv /etc/{opt,sysconfig}

mkdir -pv /lib/firmware

mkdir -pv /media/{floppy,cdrom}

mkdir -pv /usr/{,local/}{include,src}

mkdir -pv /usr/lib/locale

mkdir -pv /usr/local/{bin,lib,sbin}

mkdir -pv /usr/{,local/}share/{color,dict,doc,info,locale,man}

mkdir -pv /usr/{,local/}share/{misc,terminfo,zoneinfo}

mkdir -pv /usr/{,local/}share/man/man{1..8}

mkdir -pv /var/{cache,local,log,mail,opt,spool}

mkdir -pv /var/lib/{color,misc,locate}

ln -sfv /run /var/run && ln -sfv /run/lock /var/lock

install -dv -m 0750 /root

install -dv -m 1777 /tmp /var/tmp

```

Then the essential first files — `/etc/hosts`, `/etc/passwd` and `/etc/group` (the temporary versions), plus the log files programs expect:

```sh

echo "127.0.0.1 localhost $(hostname)" > /etc/hosts

cat > /etc/passwd << "EOF"

root:x:0:0:root:/root:/bin/bash

bin:x:1:1:bin:/dev/null:/usr/bin/false

daemon:x:6:6:Daemon User:/dev/null:/usr/bin/false

messagebus:x:18:18:D-Bus Message Daemon User:/run/dbus:/usr/bin/false

uuidd:x:80:80:UUID Generation Daemon User:/dev/null:/usr/bin/false

nobody:x:65534:65534:Unprivileged User:/dev/null:/usr/bin/false

EOF

# /etc/group: root, bin, daemon, sys, kmem, tape, tty, daemon again...

touch /var/log/{btmp,lastlog,faillog,wtmp}

chgrp -v utmp /var/log/lastlog

chmod -v 664 /var/log/lastlog

chmod -v 600 /var/log/btmp

```

## Where this chapter goes wrong

| Symptom | Cause |

| --- | --- |

| Inside chroot: `no such file` for basic tools | Virtual filesystems not bind-mounted — /dev empty |

| `configure` can't detect the system | `/proc`, `/sys` not mounted inside |

| Testsuites all fail | `/dev/null`, `/dev/pts` missing |

| Files owned by nobody/weird UIDs | Ownership handover skipped |

| `cc: command not found` inside | The `cc`→gcc symlink skipped in gcc pass 2 |

| chroot complains "failed to run command" | `/usr/bin/env` path wrong in the chroot command, or bash not installed |

| Host binaries leaking in | PATH not reset in the `env -i` chroot line |

## Exercises

1. Explain why `/dev` must be a bind mount rather than `mkdir`.

2. From inside the chroot, prove you're isolated: compare `cat /etc/hostname`, `ls /home`, `df` against the host's.

3. Find three packages in the temp set with FHS fix-ups; explain one.

4. Exit the chroot, re-enter it — write the full mount+chroot script from memory.

5. Break `PATH` inside chroot and observe which "commands" still work (builtins vs binaries).

## Timeline

| Step | Result |

| --- | --- |

| binutils/gcc pass 2 | The real toolchain, LFS-linked |

| ~15 temp packages | Coreutils, bash, make… installed to $LFS |

| Virtual FS binds | /dev /proc /sys /run visible inside |

| Ownership handover | lfs → root |

| chroot | $LFS becomes / — you're inside |

| Skeleton | FHS dirs, passwd/group, log files |

| next | LFS III — the real system packages |

## References

- linuxfromscratch.org — chapters 6–7: cross-compiling temporary tools; entering chroot

- `chroot(2)`, `mount --bind` — the kernel mechanisms

- Filesystem Hierarchy Standard — the skeleton being laid

## See also

- [[LFS Preparation]] · [[LFS Cross-Toolchain]] · [[Linux]]

[[Category:Linux]]

[[Category:LFS]]

[[Category:Handbook]]