fish-shell · krobelus · Nov 25, 2019 · Oct 23, 2019
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -16,6 +16,7 @@
 - Brace expansion now only takes place if the braces include a "," or a variable expansion, meaning common commands such as `git reset HEAD@{0}` do not require escaping (#5869).
 - New redirections `&>` and `&|` may be used to redirect or pipe stdout, and also redirect stderr to stdout (#6192).
 - `switch` now allows arguments that expand to nothing, like empty variables (#5677).
+- The `VAR=val cmd` syntax can now be used to run a command in a modified environment (#6287).
 
 ### Scripting improvements
 - `string split0` now returns 0 if it split something (#5701).

diff --git a/sphinx_doc_src/cmds/set.rst b/sphinx_doc_src/cmds/set.rst
@@ -119,6 +119,12 @@ Examples
         echo "Python is at $python_path"
     end
 
+    # Like other shells, fish 3.1 supports this syntax for passing a variable to just one command:
+    # Run fish with a temporary home directory.
+    HOME=(mktemp -d) fish
+    # Which is essentially the same as:
+    begin; set -lx HOME (mktemp -d); fish; end
+
 Notes
 -----
 

diff --git a/sphinx_doc_src/faq.rst b/sphinx_doc_src/faq.rst
@@ -8,6 +8,10 @@ Use the :ref:`set <cmd-set>` command::
     set -x key value
     set -e key
 
+Since fish 3.1 you can set an environment variable for just one command using the ``key=value some command`` syntax, like in other shells.  The two lines below behave identically - unlike other shells, fish will output ``value`` both times::
+
+    key=value echo $key
+    begin; set -lx key value; echo $key; end
 
 How do I run a command every login? What's fish's equivalent to .bashrc?
 ------------------------------------------------------------------------
@@ -95,22 +99,6 @@ If you are just interested in success or failure, you can run the command direct
 
 See the documentation for :ref:`test <cmd-test>` and :ref:`if <cmd-if>` for more information.
 
-How do I set an environment variable for just one command?
-----------------------------------------------------------
-``SOME_VAR=1 command`` produces an error: ``Unknown command "SOME_VAR=1"``.
-
-Use the ``env`` command.
-
-``env SOME_VAR=1 command``
-
-You can also declare a local variable in a block::
-
-    begin
-        set -lx SOME_VAR 1
-        command
-    end
-
-
 How do I check whether a variable is defined?
 ---------------------------------------------
 

diff --git a/src/complete.cpp b/src/complete.cpp
@@ -1511,6 +1511,11 @@ void completer_t::perform() {
     }
 
     if (cmd_tok.location_in_or_at_end_of_source_range(cursor_pos)) {
+        maybe_t<size_t> equal_sign_pos = variable_assignment_equals_pos(current_token);
+        if (equal_sign_pos) {
+            complete_param_expand(current_token.substr(*equal_sign_pos + 1), true /* do_file */);
+            return;
+        }
         // Complete command filename.
         complete_cmd(current_token);
         complete_abbr(current_token);
@@ -1570,9 +1575,30 @@ void completer_t::perform() {
                 if (wants_transient) {
                     parser->libdata().transient_commandlines.push_back(cmdline);
                 }
-                // Now invoke any custom completions for this command.
-                if (!complete_param(cmd, previous_argument_unescape, current_argument_unescape,
-                                    !had_ddash)) {
+                bool is_variable_assignment = bool(variable_assignment_equals_pos(cmd));
+                if (is_variable_assignment && parser) {
+                    // To avoid issues like #2705 we complete commands starting with variable
+                    // assignments by recursively calling complete for the command suffix
+                    // without the first variable assignment token.
+                    wcstring unaliased_cmd;
+                    if (parser->libdata().transient_commandlines.empty()) {
+                        unaliased_cmd = cmdline;
+                    } else {
+                        unaliased_cmd = parser->libdata().transient_commandlines.back();
+                    }
+                    tokenizer_t tok(unaliased_cmd.c_str(), TOK_ACCEPT_UNFINISHED);
+                    maybe_t<tok_t> cmd_tok = tok.next();
+                    assert(cmd_tok);
+                    unaliased_cmd = unaliased_cmd.replace(0, cmd_tok->offset + cmd_tok->length, L"");
+                    parser->libdata().transient_commandlines.push_back(unaliased_cmd);
+                    cleanup_t remove_transient([&] { parser->libdata().transient_commandlines.pop_back(); });
+                    std::vector<completion_t> comp;
+                    complete(unaliased_cmd, &comp,
+                             completion_request_t::fuzzy_match, parser->vars(), parser->shared());
+                    this->completions.insert(completions.end(), comp.begin(), comp.end());
+                    do_file = false;
+                } else if (!complete_param(cmd, previous_argument_unescape, current_argument_unescape,
+                                    !had_ddash)) { // Invoke any custom completions for this command.
                     do_file = false;
                 }
                 if (wants_transient) {

diff --git a/src/exec.cpp b/src/exec.cpp
@@ -961,6 +961,17 @@ static bool exec_process_in_job(parser_t &parser, process_t *p, std::shared_ptr<
         parser.libdata().exec_count++;
     }
 
+    const block_t *block = nullptr;
+    cleanup_t pop_block([&]() {
+        if (block) parser.pop_block(block);
+    });
+    if (!p->variable_assignments.empty()) {
+        block = parser.push_block(block_t::variable_assignment_block());
+    }
+    for (const auto &assignment : p->variable_assignments) {
+        parser.vars().set(assignment.variable_name, ENV_LOCAL | ENV_EXPORT, assignment.values);
+    }
+
     // Execute the process.
     p->check_generations_before_launch();
     switch (p->type) {

diff --git a/src/fish_tests.cpp b/src/fish_tests.cpp
@@ -4626,6 +4626,7 @@ static void test_highlighting() {
     });
 
     highlight_tests.push_back({
+        {L"HOME=.", highlight_role_t::param},
         {L"false", highlight_role_t::command},
         {L"|&", highlight_role_t::error},
         {L"true", highlight_role_t::command},

diff --git a/src/highlight.cpp b/src/highlight.cpp
@@ -1200,6 +1200,12 @@ highlighter_t::color_array_t highlighter_t::highlight() {
                 this->color_node(node, highlight_role_t::operat);
                 break;
 
+            case symbol_variable_assignment: {
+                tnode_t<g::variable_assignment> variable_assignment = {&parse_tree, &node};
+                this->color_argument(variable_assignment.child<0>());
+                break;
+            }
+
             case parse_token_type_pipe:
             case parse_token_type_background:
             case parse_token_type_end:
@@ -1223,6 +1229,8 @@ highlighter_t::color_array_t highlighter_t::highlight() {
                 if (!this->io_ok) {
                     // We cannot check if the command is invalid, so just assume it's valid.
                     is_valid_cmd = true;
+                } else if (variable_assignment_equals_pos(*cmd)) {
+                    is_valid_cmd = true;
                 } else {
                     wcstring expanded_cmd;
                     // Check to see if the command is valid.

diff --git a/src/parse_constants.h b/src/parse_constants.h
@@ -40,6 +40,8 @@ enum parse_token_type_t : uint8_t {
     symbol_not_statement,
     symbol_decorated_statement,
     symbol_plain_statement,
+    symbol_variable_assignment,
+    symbol_variable_assignments,
     symbol_arguments_or_redirections_list,
     symbol_andor_job_list,
     symbol_argument_list,
@@ -277,11 +279,6 @@ void parse_error_offset_source_start(parse_error_list_t *errors, size_t amt);
 /// Error issued on $.
 #define ERROR_NO_VAR_NAME _(L"Expected a variable name after this $.")
 
-/// Error on foo=bar.
-#define ERROR_BAD_EQUALS_IN_COMMAND5                                                        \
-    _(L"Unsupported use of '='. To run '%ls' with a modified environment, please use 'env " \
-      L"%ls=%ls %ls%ls'")
-
 /// Error message for Posix-style assignment: foo=bar.
 #define ERROR_BAD_COMMAND_ASSIGN_ERR_MSG \
     _(L"Unsupported use of '='. In fish, please use 'set %ls %ls'.")