[util] Add swift_build_support.shell.capture.

 - This is an analog to `call`, which returns the captured output of the
   command.

Author: ddunbar

utils/swift_build_support/swift_build_support/shell.py

@@ -60,6 +60,13 @@ def _print_command(dry_run, command, env=None, prompt="+ "):
 
 
 def call(command, stderr=None, env=None, dry_run=None, print_command=True):
+    """
+    call(command, ...) -> str
+
+    Execute the given command.
+
+    This function will raise an exception on any command failure.
+    """
     dry_run = _coerce_dry_run(dry_run)
     if dry_run or print_command:
         _print_command(dry_run, command, env=env)
@@ -81,6 +88,35 @@ def call(command, stderr=None, env=None, dry_run=None, print_command=True):
             "': " + e.strerror)
 
 
+def capture(command, stderr=None, env=None, dry_run=None, print_command=True):
+    """
+    capture(command, ...) -> str
+
+    Execute the given command and return the standard output.
+
+    This function will raise an exception on any command failure.
+    """
+    dry_run = _coerce_dry_run(dry_run)
+    if dry_run or print_command:
+        _print_command(dry_run, command, env=env)
+    if dry_run:
+        return
+    _env = None
+    if env is not None:
+        _env = dict(os.environ)
+        _env.update(env)
+    try:
+        return subprocess.check_output(command, env=_env, stderr=stderr)
+    except subprocess.CalledProcessError as e:
+        diagnostics.fatal(
+            "command terminated with a non-zero exit status " +
+            str(e.returncode) + ", aborting")
+    except OSError as e:
+        diagnostics.fatal(
+            "could not execute '" + quote_command(command) +
+            "': " + e.strerror)
+
+
 @contextmanager
 def pushd(path, dry_run=None, print_command=True):
     dry_run = _coerce_dry_run(dry_run)

utils/swift_build_support/tests/test_shell.py

@@ -63,6 +63,12 @@ def test_call(self):
 + cp {foo_file} {bar_file}
 '''.format(foo_file=foo_file, bar_file=bar_file))
 
+    def test_capture(self):
+        self.assertEqual(shell.capture(["echo", "hi"]), "hi\n")
+
+        with self.assertRaises(SystemExit):
+            shell.capture(["false"])
+
     def test_rmtree(self):
         shell.dry_run = False
         path = os.path.join(self.tmpdir, 'foo', 'bar')