Split nix-env and nix-store documentation per-subcommand

Documentation on "classic" commands with many sub-commands are
notoriously hard to discover due to lack of overview and anchor links.
Additionally the information on common options and environment variables
is not accessible offline in man pages, and therefore often overlooked
by readers.

With this change, each sub-command of nix-store and nix-env gets its
own page in the manual (listed in the table of contents), and each own
man page.

Also, man pages for each subcommand now (again) list common options
and environment variables. While this makes each page quite long and
some common parameters don't apply, this should still make it easier
to navigate as that additional information was not accessible on the
command line at all.

It is now possible to run 'nix-store --<subcommand> --help` to display
help pages for the given subcommand.

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

1 files changed, 64 insertions, 21 deletions

diff --git a/src/nix-store/nix-store.cc b/src/nix-store/nix-store.cc
index 54479489f..a62cb874f 100644
--- a/src/nix-store/nix-store.cc
+++ b/src/nix-store/nix-store.cc
@@ -1024,62 +1024,104 @@ static int main_nix_store(int argc, char * * argv)
         Strings opFlags, opArgs;
         Operation op = 0;
         bool readFromStdIn = false;
+        std::string opName;
+        bool showHelp = false;
 
         parseCmdLine(argc, argv, [&](Strings::iterator & arg, const Strings::iterator & end) {
             Operation oldOp = op;
 
             if (*arg == "--help")
-                showManPage("nix-store");
+                showHelp = true;
             else if (*arg == "--version")
                 op = opVersion;
-            else if (*arg == "--realise" || *arg == "--realize" || *arg == "-r")
+            else if (*arg == "--realise" || *arg == "--realize" || *arg == "-r") {
                 op = opRealise;
-            else if (*arg == "--add" || *arg == "-A")
+                opName = "-realise";
+            }
+            else if (*arg == "--add" || *arg == "-A"){
                 op = opAdd;
-            else if (*arg == "--add-fixed")
+                opName = "-add";
+            }
+            else if (*arg == "--add-fixed") {
                 op = opAddFixed;
+                opName = arg->substr(1);
+            }
             else if (*arg == "--print-fixed-path")
                 op = opPrintFixedPath;
-            else if (*arg == "--delete")
+            else if (*arg == "--delete") {
                 op = opDelete;
-            else if (*arg == "--query" || *arg == "-q")
+                opName = arg->substr(1);
+            }
+            else if (*arg == "--query" || *arg == "-q") {
                 op = opQuery;
-            else if (*arg == "--print-env")
+                opName = "-query";
+            }
+            else if (*arg == "--print-env") {
                 op = opPrintEnv;
-            else if (*arg == "--read-log" || *arg == "-l")
+                opName = arg->substr(1);
+            }
+            else if (*arg == "--read-log" || *arg == "-l") {
                 op = opReadLog;
-            else if (*arg == "--dump-db")
+                opName = "-read-log";
+            }
+            else if (*arg == "--dump-db") {
                 op = opDumpDB;
-            else if (*arg == "--load-db")
+                opName = arg->substr(1);
+            }
+            else if (*arg == "--load-db") {
                 op = opLoadDB;
+                opName = arg->substr(1);
+            }
             else if (*arg == "--register-validity")
                 op = opRegisterValidity;
             else if (*arg == "--check-validity")
                 op = opCheckValidity;
-            else if (*arg == "--gc")
+            else if (*arg == "--gc") {
                 op = opGC;
-            else if (*arg == "--dump")
+                opName = arg->substr(1);
+            }
+            else if (*arg == "--dump") {
                 op = opDump;
-            else if (*arg == "--restore")
+                opName = arg->substr(1);
+            }
+            else if (*arg == "--restore") {
                 op = opRestore;
-            else if (*arg == "--export")
+                opName = arg->substr(1);
+            }
+            else if (*arg == "--export") {
                 op = opExport;
-            else if (*arg == "--import")
+                opName = arg->substr(1);
+            }
+            else if (*arg == "--import") {
                 op = opImport;
+                opName = arg->substr(1);
+            }
             else if (*arg == "--init")
                 op = opInit;
-            else if (*arg == "--verify")
+            else if (*arg == "--verify") {
                 op = opVerify;
-            else if (*arg == "--verify-path")
+                opName = arg->substr(1);
+            }
+            else if (*arg == "--verify-path") {
                 op = opVerifyPath;
-            else if (*arg == "--repair-path")
+                opName = arg->substr(1);
+            }
+            else if (*arg == "--repair-path") {
                 op = opRepairPath;
-            else if (*arg == "--optimise" || *arg == "--optimize")
+                opName = arg->substr(1);
+            }
+            else if (*arg == "--optimise" || *arg == "--optimize") {
                 op = opOptimise;
-            else if (*arg == "--serve")
+                opName = "-optimise";
+            }
+            else if (*arg == "--serve") {
                 op = opServe;
-            else if (*arg == "--generate-binary-cache-key")
+                opName = arg->substr(1);
+            }
+            else if (*arg == "--generate-binary-cache-key") {
                 op = opGenerateBinaryCacheKey;
+                opName = arg->substr(1);
+            }
             else if (*arg == "--add-root")
                 gcRoot = absPath(getArg(*arg, arg, end));
             else if (*arg == "--stdin" && !isatty(STDIN_FILENO))
@@ -1109,6 +1151,7 @@ static int main_nix_store(int argc, char * * argv)
             return true;
         });
 
+        if (showHelp) showManPage("nix-store" + opName);
         if (!op) throw UsageError("no operation specified");
 
         if (op != opDump && op != opRestore) /* !!! hack */