Deprecate $_ in favor of (status current-command)

Closes #813.

CHANGELOG.md

@@ -4,6 +4,7 @@ This section is for changes merged to the `major` branch that are not also merge
 ## Deprecations
 - The `IFS` variable is deprecated and will be removed in fish 4.0 (#4156).
 - The `function --on-process-exit` event will be removed in future (#4700). Use the `fish_exit` event instead.
+- `$_` is deprecated and will removed in the future (#813). Use `status current-command` in a subshell instead.
 
 ## Notable non-backward compatible changes
 - `.` command no longer exists -- use `source` (#4294).

doc_src/index.hdr.in

@@ -1297,14 +1297,14 @@ To customize the syntax highlighting, you can set the environment variables list
 
 \subsection title Programmable title
 
-When using most virtual terminals, it is possible to set the message displayed in the titlebar of the terminal window. This can be done automatically in fish by defining the `fish_title` function. The `fish_title` function is executed before and after a new command is executed or put into the foreground and the output is used as a titlebar message. The $_ environment variable will always contain the name of the job to be put into the foreground (Or 'fish' if control is returning to the shell) when the `fish_prompt` function is called. The first argument to fish_title will contain the most recently executed foreground command as a string, starting with fish 2.2.
+When using most virtual terminals, it is possible to set the message displayed in the titlebar of the terminal window. This can be done automatically in fish by defining the `fish_title` function. The `fish_title` function is executed before and after a new command is executed or put into the foreground and the output is used as a titlebar message. The `status current-command` builtin will always return the name of the job to be put into the foreground (or 'fish' if control is returning to the shell) when the `fish_prompt` function is called. The first argument to fish_title will contain the most recently executed foreground command as a string, starting with fish 2.2.
 
 Examples:
 The default `fish` title is
 
 \fish
 function fish_title
-    echo $_ ' '
+    echo (status current-command) ' '
     pwd
 end
 \endfish

src/builtin_fg.cpp

@@ -102,7 +102,8 @@ int builtin_fg(parser_t &parser, io_streams_t &streams, wchar_t **argv) {
     }
 
     const wcstring ft = tok_first(j->command());
-    if (!ft.empty()) env_set_one(L"current_cmd", ENV_EXPORT, ft);
+    //For compatibility with fish 2.0's $_, now replaced with `status current-command`
+    if (!ft.empty()) env_set_one(L"_", ENV_EXPORT, ft);
     reader_write_title(j->command());
 
     job_promote(j);

src/builtin_status.cpp

@@ -25,6 +25,7 @@ enum status_cmd_t {
     STATUS_IS_FULL_JOB_CTRL,
     STATUS_IS_INTERACTIVE_JOB_CTRL,
     STATUS_IS_NO_JOB_CTRL,
+    STATUS_CURRENT_CMD,
     STATUS_FILENAME,
     STATUS_FUNCTION,
     STATUS_LINE_NUMBER,
@@ -35,6 +36,7 @@ enum status_cmd_t {
 
 // Must be sorted by string, not enum or random.
 const enum_map<status_cmd_t> status_enum_map[] = {
+    {STATUS_CURRENT_CMD, L"current-command"},
     {STATUS_FILENAME, L"current-filename"},
     {STATUS_FUNCTION, L"current-function"},
     {STATUS_LINE_NUMBER, L"current-line-number"},
@@ -374,6 +376,12 @@ int builtin_status(parser_t &parser, io_streams_t &streams, wchar_t **argv) {
             streams.out.append(parser.stack_trace());
             break;
         }
+        case STATUS_CURRENT_CMD: {
+            CHECK_FOR_UNEXPECTED_STATUS_ARGS(opts.status_cmd)
+            streams.out.append(program_name);
+            streams.out.push_back(L'\n');
+            break;
+         }
     }
 
     return retval;

src/env.cpp

@@ -322,7 +322,7 @@ bool string_set_contains(const T &set, const wchar_t *val) {
 
 /// Check if a variable may not be set using the set command.
 static bool is_read_only(const wchar_t *val) {
-    const string_set_t env_read_only = {L"PWD", L"SHLVL", L"history", L"status", L"version", L"fish_pid", L"hostname", L"current_cmd"};
+    const string_set_t env_read_only = {L"PWD", L"SHLVL", L"history", L"status", L"version", L"fish_pid", L"hostname", L"_"};
     return string_set_contains(env_read_only, val);
 }
 

src/reader.cpp

@@ -99,7 +99,7 @@
 #define MODE_PROMPT_FUNCTION_NAME L"fish_mode_prompt"
 
 /// The default title for the reader. This is used by reader_readline.
-#define DEFAULT_TITLE L"echo $_ \" \"; __fish_pwd"
+#define DEFAULT_TITLE L"echo (status current-command) \" \"; __fish_pwd"
 
 /// The maximum number of characters to read from the keyboard without repainting. Note that this
 /// readahead will only occur if new characters are available for reading, fish will never block for
@@ -1620,7 +1620,8 @@ static void reader_interactive_init() {
 
     invalidate_termsize();
 
-    env_set_one(L"current_cmd", ENV_GLOBAL, L"fish");
+    //For compatibility with fish 2.0's $_, now replaced with `status current-command`
+    env_set_one(L"_", ENV_GLOBAL, L"fish");
 }
 
 /// Destroy data for interactive use.
@@ -1897,7 +1898,8 @@ void reader_run_command(parser_t &parser, const wcstring &cmd) {
 
     wcstring ft = tok_first(cmd);
 
-    if (!ft.empty()) env_set_one(L"current_cmd", ENV_GLOBAL, ft);
+    //For compatibility with fish 2.0's $_, now replaced with `status current-command`
+    if (!ft.empty()) env_set_one(L"_", ENV_GLOBAL, ft);
 
     reader_write_title(cmd);
 
@@ -1913,7 +1915,8 @@ void reader_run_command(parser_t &parser, const wcstring &cmd) {
 
     term_steal();
 
-    env_set_one(L"current_cmd", ENV_GLOBAL, program_name);
+    //For compatibility with fish 2.0's $_, now replaced with `status current-command`
+    env_set_one(L"_", ENV_GLOBAL, program_name);
 
 #ifdef HAVE__PROC_SELF_STAT
     proc_update_jiffies();