doc: Add "Build Utilities" section.

* doc/guix.texi (Build Utilities): New section.
(Defining Packages): Refer to it.
(Build Systems): Likewise.
* doc/guix-cookbook.texi (Extended example): Likewise.

doc/guix-cookbook.texi

@@ -1143,6 +1143,9 @@ then restore the previous working directory.
 A ``@command{sed}-like'' function.
 @end table
 
+@xref{Build Utilities,,, guix, GNU Guix Reference Manual}, for more
+information on these utilities.
+
 @subsubsection Module prefix
 
 The license in our last example needs a prefix: this is because of how the

doc/guix.texi

@@ -252,6 +252,7 @@ Programming Interface
 * Package Modules::             Packages from the programmer's viewpoint.
 * Defining Packages::           Defining new packages.
 * Build Systems::               Specifying how packages are built.
+* Build Utilities::             Helpers for your package definitions and more.
 * The Store::                   Manipulating the package store.
 * Derivations::                 Low-level interface to package derivations.
 * The Store Monad::             Purely functional interface to the store.
@@ -6085,6 +6086,7 @@ package definitions.
 * Package Modules::             Packages from the programmer's viewpoint.
 * Defining Packages::           Defining new packages.
 * Build Systems::               Specifying how packages are built.
+* Build Utilities::             Helpers for your package definitions and more.
 * The Store::                   Manipulating the package store.
 * Derivations::                 Low-level interface to package derivations.
 * The Store Monad::             Purely functional interface to the store.
@@ -6241,6 +6243,10 @@ represents the familiar GNU Build System, where packages may be
 configured, built, and installed with the usual @code{./configure &&
 make && make check && make install} command sequence.
 
+When you start packaging non-trivial software, you may need tools to
+manipulate those build phases, manipulate files, and so on.  @xref{Build
+Utilities}, for more on this.
+
 @item
 The @code{arguments} field specifies options for the build system
 (@pxref{Build Systems}).  Here it is interpreted by
@@ -6816,7 +6822,8 @@ The list of phases used for a particular package can be changed with the
 @end example
 
 means that all the phases described above will be used, except the
-@code{configure} phase.
+@code{configure} phase.  @xref{Build Utilities}, for more info on
+@code{modify-phases} and build phases in general.
 
 In addition, this build system ensures that the ``standard'' environment
 for GNU packages is available.  This includes tools such as GCC, libc,
@@ -7646,6 +7653,273 @@ with @code{build-expression->derivation} (@pxref{Derivations,
 @code{build-expression->derivation}}).
 @end defvr
 
+@node Build Utilities
+@section Build Utilities
+
+As soon as you start writing non-trivial package definitions
+(@pxref{Defining Packages}) or other build actions
+(@pxref{G-Expressions}), you will likely start looking for helpers for
+``shell-like'' actions---creating directories, copying and deleting
+files recursively, manipulating build phases, and so on.  The
+@code{(guix build utils)} module provides such utility procedures.
+
+When writing package definitions, most build systems load @code{(guix
+build utils)} (@pxref{Build Systems}).  Thus, when writing custom build
+phases or similar, you can usually assume those procedures are in scope.
+
+When writing g-expressions, you can import @code{(guix build utils)} on
+the ``build side'' using @code{with-imported-modules} and then put it in
+scope with the @code{use-modules} form (@pxref{Using Guile Modules,,,
+guile, GNU Guile Reference Manual}):
+
+@lisp
+(with-imported-modules '((guix build utils))  ;import it
+  (computed-file "empty-tree"
+                 #~(begin
+                     ;; Put it in scope.
+                     (use-modules (guix build utils))
+
+                     ;; Happily use its 'mkdir-p' procedure.
+                     (mkdir-p (string-append #$output "/a/b/c")))))
+@end lisp
+
+The remainder of this section is the reference for most of the utility
+procedures provided by @code{(guix build utils)}.
+
+@c TODO Document what's missing.
+
+@subsection Dealing with Store File Names
+
+This section of procedures deals with store file names.
+
+@deffn {Scheme Procedure} %store-directory
+Return the directory name of the store.
+@end deffn
+
+@deffn {Scheme Procedure} store-file-name? @var{file}
+Return true if @var{file} is in the store.
+@end deffn
+
+@deffn {Scheme Procedure} strip-store-file-name @var{file}
+Strip the @file{/gnu/store} and hash from @var{file}, a store file name.
+The result is typically a @code{"PACKAGE-VERSION"} string.
+@end deffn
+
+@deffn {Scheme Procedure} package-name->name+version @var{name}
+Given @var{name}, a package name like @code{"foo-0.9.1b"}, return two
+values: @code{"foo"} and @code{"0.9.1b"}.  When the version part is
+unavailable, @var{name} and @code{#f} are returned.  The first hyphen
+followed by a digit is considered to introduce the version part.
+@end deffn
+
+@subsection File Types
+
+The procedures below deal with files and file types.
+
+@deffn {Scheme Procedure} directory-exists? @var{dir}
+Return @code{#t} if @var{dir} exists and is a directory.
+@end deffn
+
+@deffn {Scheme Procedure} executable-file? @var{file}
+Return @code{#t} if @var{file} exists and is executable.
+@end deffn
+
+@deffn {Scheme Procedure} symbolic-link? @var{file}
+Return @code{#t} if @var{file} is a symbolic link (aka. a ``symlink'').
+@end deffn
+
+@deffn {Scheme Procedure} elf-file? @var{file}
+@deffnx {Scheme Procedure} ar-file? @var{file}
+@deffnx {Scheme Procedure} gzip-file? @var{file}
+Return @code{#t} if @var{file} is, respectively, an ELF file, an
+@code{ar} archive (such as a @file{.a} static library), or a gzip file.
+@end deffn
+
+@deffn {Scheme Procedure} reset-gzip-timestamp @var{file} [#:keep-mtime? #t]
+If @var{file} is a gzip file, reset its embedded timestamp (as with
+@command{gzip --no-name}) and return true.  Otherwise return @code{#f}.
+When @var{keep-mtime?} is true, preserve @var{file}'s modification time.
+@end deffn
+
+@subsection File Manipulation
+
+The following procedures and macros help create, modify, and delete
+files.  They provide functionality comparable to common shell utilities
+such as @command{mkdir -p}, @command{cp -r}, @command{rm -r}, and
+@command{sed}.  They complement Guile's extensive, but low-level, file
+system interface (@pxref{POSIX,,, guile, GNU Guile Reference Manual}).
+
+@deffn {Scheme Syntax} with-directory-excursion @var{directory} @var{body}@dots{}
+Run @var{body} with @var{directory} as the process's current directory.
+
+Essentially, this macro changes the current directory to @var{directory}
+before evaluating @var{body}, using @code{chdir} (@pxref{Processes,,,
+guile, GNU Guile Reference Manual}).  It changes back to the initial
+directory when the dynamic extent of @var{body} is left, be it @i{via}
+normal procedure return or @i{via} a non-local exit such as an
+exception.
+@end deffn
+
+@deffn {Scheme Procedure} mkdir-p @var{dir}
+Create directory @var{dir} and all its ancestors.
+@end deffn
+
+@deffn {Scheme Procedure} install-file @var{file} @var{directory}
+Create @var{directory} if it does not exist and copy @var{file} in there
+under the same name.
+@end deffn
+
+@deffn {Scheme Procedure} make-file-writable @var{file}
+Make @var{file} writable for its owner.
+@end deffn
+
+@deffn {Scheme Procedure} copy-recursively @var{source} @var{destination} @
+  [#:log (current-output-port)] [#:follow-symlinks? #f] [#:keep-mtime? #f]
+Copy @var{source} directory to @var{destination}.  Follow symlinks if
+@var{follow-symlinks?}  is true; otherwise, just preserve them.  When
+@var{keep-mtime?} is true, keep the modification time of the files in
+@var{source} on those of @var{destination}.  Write verbose output to the
+@var{log} port.
+@end deffn
+
+@deffn {Scheme Procedure} delete-file-recursively @var{dir} @
+  [#:follow-mounts? #f]
+Delete @var{dir} recursively, like @command{rm -rf}, without following
+symlinks.  Don't follow mount points either, unless @var{follow-mounts?}
+is true.  Report but ignore errors.
+@end deffn
+
+@deffn {Scheme Syntax} substitute* @var{clause}@dots{}
+Substitute @var{regexp} in @var{file} by the string returned by
+@var{body}.  @var{body} is evaluated with each @var{match-var} bound to
+the corresponding positional regexp sub-expression.  For example:
+
+@lisp
+(substitute* file
+  (("hello")
+   "good morning\n")
+  (("foo([a-z]+)bar(.*)$" all letters end)
+   (string-append "baz" letter end)))
+@end lisp
+
+Here, anytime a line of @var{file} contains @code{hello}, it is replaced
+by @code{good morning}.  Anytime a line of @var{file} matches the second
+regexp, @var{all} is bound to the complete match, @var{letters} is bound
+to the first sub-expression, and @var{end} is bound to the last one.
+
+When one of the @var{match-var} is @code{_}, no variable is bound to the
+corresponding match substring.
+
+Alternatively, @var{file} may be a list of file names, in which case
+they are all subject to the substitutions.
+
+Be careful about using @code{$} to match the end of a line; by itself it
+won't match the terminating newline of a line.
+@end deffn
+
+@subsection File Search
+
+@cindex file, searching
+This section documents procedures to search and filter files.
+
+@deffn {Scheme Procedure} file-name-predicate @var{regexp}
+Return a predicate that returns true when passed a file name whose base
+name matches @var{regexp}.
+@end deffn
+
+@deffn {Scheme Procedure} find-files @var{dir} [@var{pred}] @
+  [#:stat lstat] [#:directories? #f] [#:fail-on-error? #f]
+Return the lexicographically sorted list of files under @var{dir} for
+which @var{pred} returns true.  @var{pred} is passed two arguments: the
+absolute file name, and its stat buffer; the default predicate always
+returns true.  @var{pred} can also be a regular expression, in which
+case it is equivalent to @code{(file-name-predicate @var{pred})}.
+@var{stat} is used to obtain file information; using @code{lstat} means
+that symlinks are not followed.  If @var{directories?} is true, then
+directories will also be included.  If @var{fail-on-error?} is true,
+raise an exception upon error.
+@end deffn
+
+Here are a few examples where we assume that the current directory is
+the root of the Guix source tree:
+
+@lisp
+;; List all the regular files in the current directory.
+(find-files ".")
+@result{} ("./.dir-locals.el" "./.gitignore" @dots{})
+
+;; List all the .scm files under gnu/services.
+(find-files "gnu/services" "\\.scm$")
+@result{} ("gnu/services/admin.scm" "gnu/services/audio.scm" @dots{})
+
+;; List ar files in the current directory.
+(find-files "." (lambda (file stat) (ar-file? file)))
+@result{} ("./libformat.a" "./libstore.a" @dots{})
+@end lisp
+
+@deffn {Scheme Procedure} which @var{program}
+Return the complete file name for @var{program} as found in
+@code{$PATH}, or @code{#f} if @var{program} could not be found.
+@end deffn
+
+@subsection Build Phases
+
+@cindex build phases
+The @code{(guix build utils)} also contains tools to manipulate
+@dfn{build phases} as found in @code{gnu-build-system} and in fact most
+build systems (@pxref{Build Systems}).  Build phases are represented as
+association lists or ``alists'' (@pxref{Association Lists,,, guile, GNU
+Guile Reference Manual}) where each key is a symbol for the name of the
+phase, and the associated value is a procedure that accepts an arbitrary
+number of arguments.
+
+Guile core and the @code{(srfi srfi-1)} module both provide tools to
+manipulate alists.  The @code{(guix build utils)} module complements
+those with tools written with build phases in mind.
+
+@cindex build phases, modifying
+@deffn {Scheme Syntax} modify-phases @var{phases} @var{clause}@dots{}
+Modify @var{phases} sequentially as per each @var{clause}, which may
+have one of the following forms:
+
+@lisp
+(delete @var{old-phase-name})
+(replace @var{old-phase-name} @var{new-phase})
+(add-before @var{old-phase-name} @var{new-phase-name} @var{new-phase})
+(add-after @var{old-phase-name} @var{new-phase-name} @var{new-phase})
+@end lisp
+
+Where every @var{phase-name} above is an expression evaluating to a
+symbol, and @var{new-phase} an expression evaluating to a procedure.
+@end deffn
+
+The example below is taken from the definition of the @code{grep}
+package.  It adds a phase to run after the @code{install} phase, called
+@code{fix-egrep-and-fgrep}.  That phase is a procedure (@code{lambda*}
+is for anonymous procedures) that takes a @code{#:outputs} keyword
+argument and ignores extra keyword arguments (@pxref{Optional
+Arguments,,, guile, GNU Guile Reference Manual}, for more on
+@code{lambda*} and optional and keyword arguments.)  The phase uses
+@code{substitute*} to modify the installed @file{egrep} and @file{fgrep}
+scripts so that they refer to @code{grep} by its absolute file name:
+
+@lisp
+(modify-phases %standard-phases
+  (add-after 'install 'fix-egrep-and-fgrep
+    ;; Patch 'egrep' and 'fgrep' to execute 'grep' via its
+    ;; absolute file name instead of searching for it in $PATH.
+    (lambda* (#:key outputs #:allow-other-keys)
+      (let* ((out (assoc-ref outputs "out"))
+             (bin (string-append out "/bin")))
+        (substitute* (list (string-append bin "/egrep")
+                           (string-append bin "/fgrep"))
+          (("^exec grep")
+           (string-append "exec " bin "/grep")))
+        #t))))
+@end lisp
+
+@c TODO: Add more examples.
+
 @node The Store
 @section The Store