5.8 Invocación de guix time-machine

La orden guix time-machine proporciona acceso a otras revisiones de Guix, por ejemplo para instalar versiones antiguas de un paquete, o para reproducir una computación en un entorno idéntico. La revisión de Guix que se usará se define por el identificador de una revisión o por un archivo de descripción de canales creado con guix describe (see Invocación de guix describe).

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 runs its guix environment command to spawn an environment in a container running guile (guix environment has since been subsumed by guix shell; see Invoking guix shell). It’s like driving a DeLorean¹³! The first guix time-machine 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.

As for 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-machine provides 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-machine opens the door to security vulnerabilities. See --allow-downgrades.

guix time-machine raises an error when attempting to travel to commits older than “v0.16.0” (commit ‘4a0b87f0’), dated Dec. 2018. This is one of the oldest commits supporting the channel mechanism that makes “time travel” possible.

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 OpenSSL whose tests would fail after a certain date. This particular problem can be worked around by running a virtual build machine with its clock set to the right time (see Virtual Build Machines).

La sintaxis general es:

guix time-machine opciones… -- orden param…

donde orden and param… se proporcionan sin modificar a la orden guix de la revisión especificada. Las opciones que definen esta revisión son las mismas que se usan con guix pull (see Invocación de guix pull):

--url=url

--commit=revisión

--branch=rama

Use the 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.

--channels=archivo

-C archivo

Lee la lista de canales de archivo. archivo debe contener código Scheme que evalúe a una lista de objetos “channel”. See Canales, para más información.

--no-channel-files

-q

Inhibit loading of the user and system channel files, ~/.config/guix/channels.scm and /etc/guix/channels.scm.

Thus, guix time-machine -q is equivalent to the following Bash command, using the “process substitution” syntax (see Process Substitution in The GNU Bash Reference Manual):

guix time-machine -C <(echo %default-channels) …

Tenga en cuenta que guix time-machine puede desencadenar construcciones de canales y sus dependencias, y que pueden controlarse mediante las opciones de construcción estándar (see Opciones comunes de construcción).