HOUSTON -- If these last few weeks have been an audition for next season for rookie Casey Coleman, he's certainly made the most of his opportunity.
Coleman (4-2) allowed six hits and struck out two, improving to 3-0 with a 2.25 ERA in his last three starts. He also had an RBI double for Chicago, which has won four of five.
"He's done excellent," Cubs interim manager Mike Quade said. "It's ultimately about performance and I don't know how many people knew who Casey Coleman was eight or nine weeks ago and now he's got a special place in my heart because they know who he is now. So that's good."
Andrew Cashner allowed one hit and struck out one in the eighth before Carlos Marmol pitched a perfect ninth to extend his career high with his 38th save in 43 opportunities. He has converted 17 straight chances and has held opponents scoreless in his last 19 outings.
Chicago's relievers now have pitched 23 1/3 consecutive scoreless innings.
Bud Norris (9-10) pitched six innings for Houston, yielding two runs and three hits. But the right-hander also issued a career-high seven walks as the Astros lost for the fifth time in six games.
"Seven walks really was tough and both runs scored were off walks," Houston manager Brad Mills said. "He did battle through some of them. It's tough anytime that batters are getting on and they're not really having to swing the bat. But at the same time, he did work out of some of those situations."
Norris couldn't put his finger on exactly why he struggled.
"It's a little frustrating, but you stay out there and [pitching coach Brad] Arnsberg kept saying to stick with it and keep the game plan going and I finally got a couple of changeup ground balls and got out of some situations," Norris said.
The 23-year-old Coleman, who was promoted from Triple-A Iowa on Aug. 2, said this time with the Cubs has been important to his development.
"It is awesome," he said. "It builds a lot of confidence. It's huge for the offseason, going into spring training. It just sets you up for a new fresh start next year, knowing that you can pitch here and gaining a lot of confidence in you, your teammates and coaches."
Coleman is the first Cubs rookie to pitch seven or more scoreless innings with at least one RBI since Kyle Farnsworth pitched nine scoreless innings and drove in a run in August 1999 against the Dodgers, according to STATS LLC.
Scales went 1 for 1 and walked three times. Quade said his play on Friday is an example of the growth of his young players.
"They've done a really nice job being selective at the plate and ... that will allow you to get the most out of your ability," he said. "I've been really pleased with that and that's a tribute to all the guys in the minor leagues."
Jason Michaels hit his second double with one out in the seventh but Coleman retired the next two batters to preserve the shutout.
The Astros also had a strong opportunity in the fifth, when they loaded the bases with one out. Coleman then got Norris to ground into a double play.
Hunter Pence hit a two-out infield single in the Houston first and advanced to second when third baseman Scales' throw to first flew over the head of Micah Hoffpauir. A wild pitch sent Pence to third, but Carlos Lee grounded out.
Quade said Cashner and Marmol will not be available Saturday. ... Houston LHP Wandy Rodriguez tweaked his back Thursday but was feeling better Friday and the Astros expect him to be ready to start the season finale Sunday. ... Friday was the first time since 2005 that the Cubs have played a regular-season road game in October. The last time they did it was also in Houston. ... Mills said he doesn't expect to see CF Michael Bourn play again this season. Bourn has missed the last 11 games with a strained right oblique muscle.