examples: Process examples (#8598)

examples/process/process_stdin_trick.v

@@ -0,0 +1,83 @@
+module main
+
+import os
+
+// this is a example script to show you stdin can be used and keep a process open
+
+fn exec(cmd string) (string, int) {
+	mut cmd2 := cmd
+	mut out := ''
+	mut line := ''
+	mut rc := 0
+	mut p := os.new_process('/bin/bash')
+
+	// there are methods missing to know if stderr/stdout has data as such its better to redirect bot on same FD
+	// not so nice trick to run bash in bash and redirect stderr, maybe someone has a better solution
+	p.set_args(['-c', 'bash 2>&1'])
+	p.set_redirect_stdio()
+	p.run()
+
+	if !cmd2.ends_with('\n') {
+		cmd2 += '\n'
+	}
+
+	p.stdin_write(cmd2)
+	p.stdin_write('\necho **OK**\n')
+
+	for {
+		if !p.is_alive() {
+			break
+		}
+		line = p.stdout_read()
+		println(line)
+		// line_err = p.stderr_read() //IF WE CALL STDERR_READ will block
+		// we need a mechanism which allows us to check if stderr/stdout has data or it should never block
+		// is not a good way, need to use a string buffer, is slow like this
+		out += line
+		if out.ends_with('**OK**\n') {
+			out = out[0..(out.len - 7)]
+			break
+		}
+	}
+
+	// println("read from stdout, should not block")
+	// is not really needed but good test to see behaviour
+	// out += p.stdout_read()
+	// println("read done")
+
+	// println(cmd.stderr_read())
+
+	if p.code > 0 {
+		rc = 1
+		println('ERROR:')
+		println(cmd2)
+		print(out)
+	}
+	// documentation says we need to call p.wait(), but this does not seem to work, will be process stop or become zombie?
+	// p.wait()
+
+	return out, rc
+}
+
+fn main() {
+	mut out := ''
+	mut rc := 0
+
+	// the following does not work, not sure why not
+	// out,rc = exec("find /tmp/ && echo '******'")
+
+	out, rc = exec("find /tmp/ ; echo '******'")
+	println(out)
+	assert out.ends_with('******\n')
+
+	out, rc = exec('echo to stdout')
+	assert out.contains('to stdout')
+
+	out, rc = exec('echo to stderr 1>&2')
+	assert out.contains('to stderr')
+
+	out, rc = exec('ls /sssss')
+	assert rc > 0 // THIS STILL GIVES AN ERROR !
+
+	println('test ok stderr & stdout is indeed redirected')
+}