guix time-machine command provides access to other revisions
of Guix, for example to install older versions of packages, or to reproduce
a computation in an identical environment. The revision of Guix to be used
is defined by a commit or by a channel description file created by
guix describe (veja Invocando
Let’s assume that you want to travel to those days of November 2020 when
version 1.2.0 of Guix was released and, once you’re there, run the
guile of that time:
guix time-machine --commit=v1.2.0 -- \ environment -C --ad-hoc guile -- guile
The command above fetches Guix 1.2.0 (and possibly other channels
specified by your channels.scm configuration files—see below) and
guix environment command to spawn an environment in a
guix environment has since been
guix shell; veja Invoking
guix shell). It’s like
driving a DeLorean12! The first
invocation can be expensive: it may have to download or even build a large
number of packages; the result is cached though and subsequent commands
targeting the same commit are almost instantaneous.
guix pull, in the absence of any options,
time-machine fetches the latest commits of the channels specified
in ~/.config/guix/channels.scm, /etc/guix/channels.scm, or the
default channels; the -q option lets you ignore these configuration
files. The command:
guix time-machine -q -- build hello
will thus build the package
hello as defined in the main branch of
Guix, without any additional channel, which is in general a newer revision
of Guix than you have installed. Time travel works in both directions!
Nota: The history of Guix is immutable and
guix time-machineprovides the exact same software as they are in a specific Guix revision. Naturally, no security fixes are provided for old versions of Guix or its channels. A careless use of
guix time-machineopens the door to security vulnerabilities. Veja --allow-downgrades.
guix time-machine relying on the “inferiors” mechanism
(veja Inferiores), the oldest commit it can travel to is commit
‘6298c3ff’ (“v1.0.0”), dated May 1st, 2019, which is the first
release that included the inferiors mechanism. An error is returned when
attempting to navigate to older commits.
Nota: Although it should technically be possible to travel to such an old commit, the ease to do so will largely depend on the availability of binary substitutes. When traveling to a distant past, some packages may not easily build from source anymore. One such example are old versions of Python 2 which had time bombs in its test suite, in the form of expiring SSL certificates. This particular problem can be worked around by setting the hardware clock to a value in the past before attempting the build.
The general syntax is:
guix time-machine options… -- command arg…
where command and arg… are passed unmodified to the
guix command of the specified revision. The options that
define this revision are the same as for
guix channel from the specified url, at the given
commit (a valid Git commit ID represented as a hexadecimal string or
the name of a tag), or branch.
Read the list of channels from file. file must contain Scheme code that evaluates to a list of channel objects. Veja Canais for more information.
Inhibit loading of the user and system channel files, ~/.config/guix/channels.scm and /etc/guix/channels.scm.
guix time-machine -q is equivalent to the following Bash
command, using the “process substitution” syntax (veja Process
Substitution em The GNU Bash Reference Manual):
guix time-machine -C <(echo %default-channels) …
guix time-machine can trigger builds of channels and
their dependencies, and these are controlled by the standard build options
(veja Opções de compilação comum).
If you don’t know what a DeLorean is, consider traveling back to the 1980’s.