environment: Allow setting environment variables with -E.

* guix/scripts/environment.scm (show-environment-options-help) [-E]: Document
the change.
* guix/scripts/environment.scm (%options) [-E]: If arg contains =, split it
and store under 'extra-env.
* guix/scripts/environment.scm (launch-environment, launch-environment/fork)
(launch-environment/container): Take and use `environment-variables' keyword argument.
* guix/scripts/environment.scm (guix-environment*): Process 'environment-variables opts.
* guix/profiles.scm (load-profile): Add `environment-variables' keyword argument.
* doc/guix.texi (Invoking guix shell): Document the change.
* tests/guix-environment.sh (GUIX_TEST_ZZZ): Test it.

Change-Id: Ib1db19ef73dda8f561b05731079a96eda076e365
Signed-off-by: Maxim Cournoyer <maxim@guixotic.coop>
Modified-by: Maxim Cournoyer <maxim@guixotic.coop>

1 files changed, 12 insertions, 5 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index c624c497d2a..05eac9bda96 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -6803,20 +6803,26 @@ creating an environment in which search paths only contain package inputs.
 
 @item --preserve=@var{regexp}
 @itemx -E @var{regexp}
+@itemx -E @var{var}=@var{value}
 When used alongside @option{--pure}, preserve the environment variables
 matching @var{regexp}---in other words, put them on a ``white list'' of
-environment variables that must be preserved.  This option can be repeated
-several times.
+environment variables that must be preserved.  This option can be
+repeated several times.
+
+When @var{regexp} contains a variable assignment (with a @samp{=}
+character), the @var{var} environment variable is directly set to the
+@var{value} in the environment.  This is used in the example below to
+set the number of threads OpenMPI should use:
 
 @example
-guix shell --pure --preserve=^SLURM openmpi @dots{} \
+guix shell --pure --preserve=^SLURM -E OMP_NUM_THREADS=16 openmpi @dots{} \
   -- mpirun @dots{}
 @end example
 
 This example runs @command{mpirun} in a context where the only environment
 variables defined are @env{PATH}, environment variables whose name starts
-with @samp{SLURM}, as well as the usual ``precious'' variables (@env{HOME},
-@env{USER}, etc.).
+with @samp{SLURM}, variable @env{TERM} set to @samp{no=color}, as well as
+the usual ``precious'' variables (@env{HOME}, @env{USER}, etc.).
 
 @item --search-paths
 Display the environment variable definitions that make up the
@@ -7319,6 +7325,7 @@ creating an environment in which search paths only contain package inputs.
 
 @item --preserve=@var{regexp}
 @itemx -E @var{regexp}
+@itemx -E @var{var}=@var{value}
 When used alongside @option{--pure}, preserve the environment variables
 matching @var{regexp}---in other words, put them on a ``white list'' of
 environment variables that must be preserved.  This option can be repeated