Merge branch 'iw/trace-argv-on-alias'

The alias-expanded command lines are logged to the trace output.

* iw/trace-argv-on-alias:
  run-command: show prepared command
  Documentation: alias: add notes on shell expansion
  Documentation: alias: rework notes into points

Documentation/config/alias.txt

@@ -21,8 +21,23 @@ If the alias expansion is prefixed with an exclamation point,
 it will be treated as a shell command.  For example, defining
 `alias.new = !gitk --all --not ORIG_HEAD`, the invocation
 `git new` is equivalent to running the shell command
-`gitk --all --not ORIG_HEAD`.  Note that shell commands will be
-executed from the top-level directory of a repository, which may
-not necessarily be the current directory.
-`GIT_PREFIX` is set as returned by running `git rev-parse --show-prefix`
-from the original current directory. See linkgit:git-rev-parse[1].
+`gitk --all --not ORIG_HEAD`.  Note:
++
+* Shell commands will be executed from the top-level directory of a
+  repository, which may not necessarily be the current directory.
+* `GIT_PREFIX` is set as returned by running `git rev-parse --show-prefix`
+  from the original current directory. See linkgit:git-rev-parse[1].
+* Shell command aliases always receive any extra arguments provided to
+  the Git command-line as positional arguments.
+** Care should be taken if your shell alias is a "one-liner" script
+   with multiple commands (e.g. in a pipeline), references multiple
+   arguments, or is otherwise not able to handle positional arguments
+   added at the end.  For example: `alias.cmd = "!echo $1 | grep $2"`
+   called as `git cmd 1 2` will be executed as 'echo $1 | grep $2
+   1 2', which is not what you want.
+** A convenient way to deal with this is to write your script
+   operations in an inline function that is then called with any
+   arguments from the command-line.  For example `alias.cmd = "!c() {
+   echo $1 | grep $2 ; }; c" will correctly execute the prior example.
+** Setting `GIT_TRACE=1` can help you debug the command being run for
+   your alias.

run-command.c

@@ -746,6 +746,8 @@ fail_pipe:
 		goto end_of_spawn;
 	}
 
+	trace_argv_printf(&argv.v[1], "trace: start_command:");
+
 	if (pipe(notify_pipe))
 		notify_pipe[0] = notify_pipe[1] = -1;
 
@@ -913,6 +915,7 @@ end_of_spawn:
 	else if (cmd->use_shell)
 		cmd->args.v = prepare_shell_cmd(&nargv, sargv);
 
+	trace_argv_printf(cmd->args.v, "trace: start_command:");
 	cmd->pid = mingw_spawnvpe(cmd->args.v[0], cmd->args.v,
 				  (char**) cmd->env.v,
 				  cmd->dir, fhin, fhout, fherr);

t/t0014-alias.sh

@@ -44,4 +44,15 @@ test_expect_success 'run-command formats empty args properly' '
     test_cmp expect actual
 '
 
+test_expect_success 'tracing a shell alias with arguments shows trace of prepared command' '
+	cat >expect <<-EOF &&
+	trace: start_command: SHELL -c ${SQ}echo \$* "\$@"${SQ} ${SQ}echo \$*${SQ} arg
+	EOF
+	git config alias.echo "!echo \$*" &&
+	env GIT_TRACE=1 git echo arg 2>output &&
+	# redact platform differences
+	sed -n -e "s/^\(trace: start_command:\) .* -c /\1 SHELL -c /p" output >actual &&
+	test_cmp expect actual
+'
+
 test_done