home: services: environment-variables: Add support for literal strings.

* gnu/home/services.scm (<literal-string>): New record type.
(environment-variable-shell-definitions): Split 'shell-quote' into
'quote-string' and 'shell-double-quote'.  Add 'shell-single-quote'.
Add clause for 'literal-string' records.
* tests/guix-home.sh: Test it.
* doc/guix.texi (Essential Home Services): Document it.

doc/guix.texi

@@ -41007,13 +41007,15 @@ The easiest way to extend a service type, without defining a new service
 type is to use the @code{simple-service} helper from @code{(gnu
 services)}.
 
+@findex literal-string
 @lisp
 (simple-service 'some-useful-env-vars-service
 		home-environment-variables-service-type
 		`(("LESSHISTFILE" . "$XDG_CACHE_HOME/.lesshst")
                   ("SHELL" . ,(file-append zsh "/bin/zsh"))
                   ("USELESS_VAR" . #f)
-                  ("_JAVA_AWT_WM_NONREPARENTING" . #t)))
+                  ("_JAVA_AWT_WM_NONREPARENTING" . #t)
+                  ("LITERAL_VALUE" . ,(literal-string "$@{abc@}"))))
 @end lisp
 
 If you include such a service in you home environment definition, it
@@ -41021,11 +41023,17 @@ will add the following content to the @file{setup-environment} script
 (which is expected to be sourced by the login shell):
 
 @example
-export LESSHISTFILE=$XDG_CACHE_HOME/.lesshst
-export SHELL=/gnu/store/2hsg15n644f0glrcbkb1kqknmmqdar03-zsh-5.8/bin/zsh
+export LESSHISTFILE="$XDG_CACHE_HOME/.lesshst"
+export SHELL="/gnu/store/2hsg15n644f0glrcbkb1kqknmmqdar03-zsh-5.8/bin/zsh"
 export _JAVA_AWT_WM_NONREPARENTING
+export LITERAL_VALUE='$@{abc@}'
 @end example
 
+Notice that @code{literal-string} above lets us declare that a value is
+to be interpreted as a @dfn{literal string}, meaning that ``special
+characters'' such as the dollar sign will not be interpreted by the
+shell.
+
 @quotation Note
 Make sure that module @code{(gnu packages shells)} is imported with
 @code{use-modules} or any other way, this namespace contains the

gnu/home/services.scm

@@ -1,6 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2021 Andrew Tropin <andrew@trop.in>
 ;;; Copyright © 2021 Xinglu Chen <public@yoctocell.xyz>
+;;; Copyright © 2022 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -33,6 +34,7 @@
   #:use-module (guix i18n)
   #:use-module (guix modules)
   #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-9)
   #:use-module (ice-9 match)
   #:use-module (ice-9 vlist)
 
@@ -47,6 +49,10 @@
             home-run-on-change-service-type
             home-provenance-service-type
 
+            literal-string
+            literal-string?
+            literal-string-value
+
             environment-variable-shell-definitions
             home-files-directory
             xdg-configuration-files-directory
@@ -171,32 +177,50 @@ packages, configuration files, activation script, and so on.")))
 configuration files that the user has declared in their
 @code{home-environment} record.")))
 
+;; Representation of a literal string.
+(define-record-type <literal-string>
+  (literal-string str)
+  literal-string?
+  (str literal-string-value))
+
 (define (environment-variable-shell-definitions variables)
   "Return a gexp that evaluates to a list of POSIX shell statements defining
 VARIABLES, a list of environment variable name/value pairs.  The returned code
 ensures variable values are properly quoted."
-  #~(let ((shell-quote
-           (lambda (value)
-             ;; Double-quote VALUE, leaving dollar sign as is.
-             (let ((quoted (list->string
-                            (string-fold-right
+  #~(let* ((quote-string
+            (lambda (value quoted-chars)
+              (list->string (string-fold-right
                              (lambda (chr lst)
-                               (case chr
-                                 ((#\" #\\)
-                                  (append (list chr #\\) lst))
-                                 (else (cons chr lst))))
+                               (if (memq chr quoted-chars)
+                                   (append (list chr #\\) lst)
+                                   (cons chr lst)))
                              '()
                              value))))
-               (string-append "\"" quoted "\"")))))
+           (shell-double-quote
+            (lambda (value)
+              ;; Double-quote VALUE, leaving dollar sign as is.
+              (string-append "\"" (quote-string value '(#\" #\\))
+                             "\"")))
+           (shell-single-quote
+            (lambda (value)
+              ;; Single-quote VALUE to enter a literal string.
+              (string-append "'" (quote-string value '(#\' #\\))
+                             "'"))))
       (string-append
        #$@(map (match-lambda
                  ((key . #f)
                   "")
                  ((key . #t)
                   #~(string-append "export " #$key "\n"))
-                 ((key . value)
+                 ((key . (? string? value))
                   #~(string-append "export " #$key "="
-                                   (shell-quote #$value) "\n")))
+                                   (shell-double-quote #$value)
+                                   "\n"))
+                 ((key . (? literal-string? value))
+                  #~(string-append "export " #$key "="
+                                   (shell-single-quote
+                                    #$(literal-string-value value))
+                                   "\n")))
                variables))))
 
 (define (environment-variables->setup-environment-script vars)

tests/guix-home.sh

@@ -81,7 +81,8 @@ trap 'chmod -Rf +w "$test_directory"; rm -rf "$test_directory"' EXIT
 
    (simple-service 'add-environment-variable
                    home-environment-variables-service-type
-                   '(("TODAY" . "26 messidor")))
+                   `(("TODAY" . "26 messidor")
+                     ("LITERAL" . ,(literal-string "${abc}"))))
 
    (simple-service 'home-bash-service-extension-test
                    home-bash-service-type
@@ -149,6 +150,7 @@ EOF
     grep -q "the content of ~/.config/test.conf" "${HOME}/.config/test.conf"
     grep '^export PS1="\$GUIX_ENVIRONMENT λ "$' "${HOME}/.bash_profile"
     ( . "${HOME}/.guix-home/setup-environment"; test "$TODAY" = "26 messidor" )
+    ( . "${HOME}/.guix-home/setup-environment"; test "$LITERAL" = '${abc}' )
 
     # This one should still be here.
     grep "stay around" "$HOME/.config/random-file"