Update.

test/subprocess/popen.wren

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+// retoor <retoor@molodetz.nl>
+
+import "subprocess" for Popen
+
+var p1 = Popen.new(["echo", "test"])
+System.print(p1.pid > 0) // expect: true
+var out1 = p1.stdout.read()
+System.print(out1.trim()) // expect: test
+System.print(p1.wait()) // expect: 0
+
+var p2 = Popen.new(["cat"])
+p2.stdin.write("hello")
+p2.stdin.close()
+var out2 = p2.stdout.read()
+System.print(out2) // expect: hello
+System.print(p2.wait()) // expect: 0

test/subprocess/popen_basic.wren

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+// retoor <retoor@molodetz.nl>
+
+import "subprocess" for Popen
+
+var p = Popen.new(["echo", "hello"])
+var output = p.stdout.read()
+System.print(output.trim()) // expect: hello
+var code = p.wait()
+System.print(code) // expect: 0

test/subprocess/popen_pty.wren

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+// retoor <retoor@molodetz.nl>
+
+import "subprocess" for Popen
+
+var p = Popen.new(["sh"], true)
+System.print(p.pid > 0) // expect: true
+p.stdin.write("echo pty_test\n")
+p.stdin.write("exit\n")
+var out = p.stdout.read()
+System.print(out.contains("pty_test")) // expect: true
+System.print(p.wait()) // expect: 0