cmd/--prefix: add --installed flag

This changes the output for uninstalled formulae from Cellar prefix to empty string, so:
```sh
$ brew --prefix abcde python@3.9 tcl-tk
/usr/local/Cellar/abcde/2.9.3_1
/usr/local/opt/python@3.9
/usr/local/Cellar/tcl-tk/8.6.10
$ brew --prefix --installed abcde python@3.9 tcl-tk

/usr/local/opt/python@3.9

$
```

Library/Homebrew/cmd/--prefix.rb

@@ -23,6 +23,9 @@ module Homebrew
       EOS
       switch "--unbrewed",
              description: "List files in Homebrew's prefix not installed by Homebrew."
+      switch "--installed",
+             description: "Outputs nothing and returns a failing status code if <formula> is not installed."
+      conflicts "--unbrewed", "--installed"
 
       named_args :formula
     end
@@ -31,6 +34,8 @@ module Homebrew
   def __prefix
     args = __prefix_args.parse
 
+    raise UsageError, "`--installed` requires a formula argument." if args.installed? && args.no_named?
+
     if args.unbrewed?
       raise UsageError, "`--unbrewed` does not take a formula argument." unless args.no_named?
 
@@ -38,9 +43,27 @@ module Homebrew
     elsif args.no_named?
       puts HOMEBREW_PREFIX
     else
-      puts args.named.to_resolved_formulae.map { |f|
-        f.opt_prefix.exist? ? f.opt_prefix : f.latest_installed_prefix
-      }
+      formulae = args.named.to_resolved_formulae
+      prefixes = formulae.map do |f|
+        if f.opt_prefix.exist?
+          f.opt_prefix
+        elsif args.installed?
+          nil
+        else
+          f.latest_installed_prefix
+        end
+      end.compact
+      puts prefixes
+      if args.installed?
+        missing_formulae = formulae.reject(&:optlinked?)
+                                   .map(&:name)
+        return if missing_formulae.blank?
+
+        raise NotAKegError, <<~EOS
+          The following formulae are not installed:
+          #{missing_formulae.join(" ")}
+        EOS
+      end
     end
   end
 

Library/Homebrew/test/cmd/--prefix_spec.rb

@@ -14,4 +14,18 @@ describe "brew --prefix", :integration_test do
       .and not_to_output.to_stderr
       .and be_a_success
   end
+
+  it "errors if the given Formula doesn't exist" do
+    expect { brew "--prefix", "--installed", "nonexistent" }
+      .to output(/No available formula/).to_stderr
+      .and not_to_output.to_stdout
+      .and be_a_failure
+  end
+
+  it "prints a warning with `--installed` if the given Formula is not installed" do
+    expect { brew "--prefix", "--installed", testball }
+      .to not_to_output.to_stdout
+      .and output(/testball/).to_stderr
+      .and be_a_failure
+  end
 end

completions/bash/brew

@@ -202,6 +202,7 @@ _brew___prefix() {
       __brewcomp "
       --debug
       --help
+      --installed
       --quiet
       --unbrewed
       --verbose

completions/fish/brew.fish

@@ -248,6 +248,7 @@ __fish_brew_complete_arg '--env' -a '(__fish_brew_suggest_formulae_all)'
 __fish_brew_complete_cmd '--prefix' 'Display Homebrew\'s install path'
 __fish_brew_complete_arg '--prefix' -l debug -d 'Display any debugging information'
 __fish_brew_complete_arg '--prefix' -l help -d 'Show this message'
+__fish_brew_complete_arg '--prefix' -l installed -d 'Outputs nothing and returns a failing status code if formula is not installed'
 __fish_brew_complete_arg '--prefix' -l quiet -d 'Make some output more quiet'
 __fish_brew_complete_arg '--prefix' -l unbrewed -d 'List files in Homebrew\'s prefix not installed by Homebrew'
 __fish_brew_complete_arg '--prefix' -l verbose -d 'Make some output more verbose'

completions/zsh/_brew

@@ -312,8 +312,9 @@ _brew___prefix() {
   _arguments \
     '--debug[Display any debugging information]' \
     '--help[Show this message]' \
+    '(--unbrewed)--installed[Outputs nothing and returns a failing status code if formula is not installed]' \
     '--quiet[Make some output more quiet]' \
-    '--unbrewed[List files in Homebrew'\''s prefix not installed by Homebrew]' \
+    '(--installed)--unbrewed[List files in Homebrew'\''s prefix not installed by Homebrew]' \
     '--verbose[Make some output more verbose]' \
     '::formula:__brew_formulae'
 }

docs/Manpage.md

@@ -723,7 +723,7 @@ the list is formatted for export to `bash`(1) unless `--plain` is passed.
 * `--plain`:
   Generate plain output even when piped.
 
-### `--prefix` [*`--unbrewed`*] [*`formula`* ...]
+### `--prefix` [*`--unbrewed`*] [*`--installed`*] [*`formula`* ...]
 
 Display Homebrew's install path. *Default:*
 
@@ -736,6 +736,8 @@ is or would be installed.
 
 * `--unbrewed`:
   List files in Homebrew's prefix not installed by Homebrew.
+* `--installed`:
+  Outputs nothing and returns a failing status code if *`formula`* is not installed.
 
 ### `--repository`, `--repo` [*`tap`* ...]
 

manpages/brew.1

@@ -978,7 +978,7 @@ Generate a list of environment variables for the specified shell, or \fB\-\-shel
 \fB\-\-plain\fR
 Generate plain output even when piped\.
 .
-.SS "\fB\-\-prefix\fR [\fI\-\-unbrewed\fR] [\fIformula\fR \.\.\.]"
+.SS "\fB\-\-prefix\fR [\fI\-\-unbrewed\fR] [\fI\-\-installed\fR] [\fIformula\fR \.\.\.]"
 Display Homebrew\'s install path\. \fIDefault:\fR
 .
 .IP "\(bu" 4
@@ -999,6 +999,10 @@ If \fIformula\fR is provided, display the location in the Cellar where \fIformul
 \fB\-\-unbrewed\fR
 List files in Homebrew\'s prefix not installed by Homebrew\.
 .
+.TP
+\fB\-\-installed\fR
+Outputs nothing and returns a failing status code if \fIformula\fR is not installed\.
+.
 .SS "\fB\-\-repository\fR, \fB\-\-repo\fR [\fItap\fR \.\.\.]"
 Display where Homebrew\'s git repository is located\.
 .