Add a -to-file <filename> switch to the echo command

As discussed in #2836

doc/pages/commands.asciidoc

@@ -206,6 +206,10 @@ of the file onto the filesystem
     *-debug*:::
         print the given text to the *\*debug** buffer
 
+    *-to-file* <filename>:::
+        write the given text to the given file on the host
+        filesystem.
+
 *set-face* <scope> <name> <facespec>::
     *alias* face +
     define a face in *scope*

src/commands.cc

@@ -1200,6 +1200,7 @@ const CommandDesc echo_cmd = {
     "echo <params>...: display given parameters in the status line",
     ParameterDesc{
         { { "markup", { false, "parse markup" } },
+          { "to-file", { true, "echo contents to given filename" } },
           { "debug", { false, "write to debug buffer instead of status line" } } },
         ParameterDesc::Flags::SwitchesOnlyAtStart
     },
@@ -1208,7 +1209,11 @@ const CommandDesc echo_cmd = {
     CommandCompleter{},
     [](const ParametersParser& parser, Context& context, const ShellContext&)
     {
-        String message = fix_atom_text(join(parser, ' ', false));
+        String message = join(parser, ' ', false);
+        if (auto filename = parser.get_switch("to-file"))
+            return write_to_file(*filename, message);
+
+        message = fix_atom_text(message);
         if (parser.get_switch("debug"))
             write_to_debug_buffer(message);
         else if (parser.get_switch("markup"))

src/file.cc

@@ -257,6 +257,15 @@ void write(int fd, StringView data)
     }
 }
 
+void write_to_file(StringView filename, StringView data)
+{
+    const int fd = open(filename.zstr(), O_CREAT | O_WRONLY | O_TRUNC, 0644);
+    if (fd == -1)
+        throw file_access_error(filename, strerror(errno));
+    auto close_fd = on_scope_end([fd]{ close(fd); });
+    write(fd, data);
+}
+
 struct BufferedWriter
 {
     BufferedWriter(int fd) : fd{fd} {}

src/file.hh

@@ -39,6 +39,7 @@ bool fd_writable(int fd);
 String read_fd(int fd, bool text = false);
 String read_file(StringView filename, bool text = false);
 void write(int fd, StringView data);
+void write_to_file(StringView filename, StringView data);
 
 struct MappedFile
 {

test/compose/echo-to-file/cmd

@@ -0,0 +1 @@
+:echo -to-file data %{foo bar}<ret>!cat data<ret>

test/compose/echo-to-file/in

@@ -0,0 +1 @@
+

test/compose/echo-to-file/out

@@ -0,0 +1 @@
+foo bar