Pass command as first argument to shell (#2061)

README.md

@@ -1347,6 +1347,18 @@ file.
   interpret `command` is the same shell that is used to evaluate recipe lines,
   and can be changed with `set shell := […]`.
 
+  `command` is passed as the first argument, so if the command is `'echo $@'`,
+  the full command line, with the default shell command `shell -cu` and `args`
+  `'foo'` and `'bar'` will be:
+
+  ```
+  'shell' '-cu' 'echo $@' 'echo $@' 'foo' 'bar'
+  ```
+
+  This is so that `$@` works as expected, and `$1` refers to the first
+  argument. `$@` does not include the first positional argument, which is
+  expected to be the name of the program being run.
+
 ```just
 # arguments can be variables
 file := '/sys/class/power_supply/BAT0/status'
@@ -1362,9 +1374,10 @@ full := shell('echo $1', 'foo')
 ```
 
 ```just
-# using python as the shell
+# Using python as the shell. Since `python -c` sets `sys.argv[0]` to `'-c'`,
+# the first "real" positional argument will be `sys.argv[2]`.
 set shell := ["python3", "-c"]
-olleh := shell('import sys; print(sys.argv[1][::-1]))', 'hello')
+olleh := shell('import sys; print(sys.argv[2][::-1]))', 'hello')
 ```
 
 #### Environment Variables

src/evaluator.rs

@@ -226,7 +226,7 @@ impl<'src, 'run> Evaluator<'src, 'run> {
       })
   }
 
-  pub(crate) fn run_command(&self, command: &str, args: &[String]) -> Result<String, OutputError> {
+  pub(crate) fn run_command(&self, command: &str, args: &[&str]) -> Result<String, OutputError> {
     let mut cmd = self.settings.shell_command(self.config);
     cmd.arg(command);
     cmd.args(args);

src/function.rs

@@ -460,8 +460,12 @@ fn sha256_file(evaluator: &Evaluator, path: &str) -> Result<String, String> {
 }
 
 fn shell(evaluator: &Evaluator, command: &str, args: &[String]) -> Result<String, String> {
+  let args = iter::once(command)
+    .chain(args.iter().map(String::as_str))
+    .collect::<Vec<&str>>();
+
   evaluator
-    .run_command(command, args)
+    .run_command(command, &args)
     .map_err(|output_error| output_error.to_string())
 }
 

tests/functions.rs

@@ -779,7 +779,17 @@ fn shell_no_argument() {
 
 #[test]
 fn shell_minimal() {
-  assert_eval_eq("shell('echo $0 $1', 'justice', 'legs')", "justice legs");
+  assert_eval_eq("shell('echo $1 $2', 'justice', 'legs')", "justice legs");
+}
+
+#[test]
+fn shell_args() {
+  assert_eval_eq("shell('echo $@', 'justice', 'legs')", "justice legs");
+}
+
+#[test]
+fn shell_first_arg() {
+  assert_eval_eq("shell('echo $0')", "echo $0");
 }
 
 #[test]