pack: Clarify the /bin/sh requirement for Singularity.

* guix/scripts/pack.scm (squashfs-image)[symlinks*]: New variable.
[build]: Use it instead of SYMLINKS.
(guix-pack): Emit a warning and a hint when "bash" and "bash-minimal"
are missing and PACK-FORMAT is 'squashfs.
* doc/guix.texi (Invoking guix pack): Document the /bin/sh requirement
for Singularity.

1 files changed, 16 insertions, 1 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 3f646cd2ca..25efba118f 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -5053,7 +5053,7 @@ Yet another option is to produce a SquashFS image with the following
 command:
 
 @example
-guix pack -f squashfs guile emacs geiser
+guix pack -f squashfs bash guile emacs geiser
 @end example
 
 @noindent
@@ -5088,6 +5088,21 @@ package names passed on the command line or in the manifest file.
 This produces a SquashFS image containing all the specified binaries and
 symlinks, as well as empty mount points for virtual file systems like
 procfs.
+
+@quotation Note
+Singularity @emph{requires} you to provide @file{/bin/sh} in the image.
+For that reason, @command{guix pack -f squashfs} always implies @code{-S
+/bin=bin}.  Thus, your @command{guix pack} invocation must always start
+with something like:
+
+@example
+guix pack -f squashfs bash @dots{}
+@end example
+
+If you forget the @code{bash} (or similar) package, @command{singularity
+run} and @command{singularity exec} will fail with an unhelpful ``no
+such file or directory'' message.
+@end quotation
 @end table
 
 @cindex relocatable binaries