5.8 Invoking guix time-machine

The 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 (see Invoking 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 DeLorean¹²! 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!

Note: 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.

Note: 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).

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 pull (see Invoking guix pull):

--url=url

--commit=commit

--branch=branch

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=file

-C file

Read the list of channels from file. file must contain Scheme code that evaluates to a list of channel objects. See Channels for more information.

--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) …

Note that guix time-machine can trigger builds of channels and their dependencies, and these are controlled by the standard build options (see Common Build Options).

If guix time-machine is executed without any command, it prints the file name of the profile that would be used to execute the command. This is sometimes useful if you need to get store file name of the profile—e.g., when you want to guix copy it.