CIT 383: Administrative Scripting

1. Commands CIT 383: Administrative Scripting CIT 383: Administrative Scripting

2. CIT 383: Administrative Scripting Topics • System • Exec • Command Quotes • Popen • Expect

3. CIT 383: Administrative Scripting System Executes command string in a subshell system(“tar cjf ruby.tar.bz2 *.rb”) system(“cut –d: -f1 /etc/passwd | sort”) All shell features are available Globbing (*/*.c) Tilde expansion (~jsmith) I/O redirection Pipes

4. CIT 383: Administrative Scripting System with Multiple Arguments Multiple arguments have different behavior First argument is name of command. Later arguments are command line arguments. None are interpreted by shell. Examples system(“echo *”) prints all files in directory system(“echo”, “*”) prints a * system(“tar”, “c”, “f”, “ruby.tar”, “rubyfiles/”)

5. CIT 383: Administrative Scripting System Security Archiving user specified files files = gets system(“tar cf ruby.tar #{files}”) What if the user enters “*; rm –rf/”? tar cjf ruby.tar.bz2 * rm –rf / Use multiple argument form to avoid this bug. files = gets system(“tar”, “c”, “f”, “ruby.tar”, files)

6. CIT 383: Administrative Scripting Exec Replaces current process by running command. exec(“ls –l”) # program never reaches this point Single argument form invokes shell exec(“echo *”) Multiple argument form does not exec(“echo”, “*”)

7. CIT 383: Administrative Scripting Command Quotes Ruby will run commands in backquotes os = `uname` os = %x|uname| Return value is output of command as String. Command quotes invoke a subshell: files = `echo *` sortedfiles = `echo * | sort`

8. CIT 383: Administrative Scripting Popen Pipe Open IO.popen(command_string, mode) Opens command like a file r: read from command’s STDOUT. w: write to command’s STDIN. Similar to command quotes in read mode: uname_fh = IO.popen(‘uname –a’, ‘r’) unixname = uname_fh.readlines

9. CIT 383: Administrative Scripting Popen Popen offers more control than command quotes. Use less memory (read a line at a time.) Obtain partial output immediately. Examples vmfh = popen(“vmstat 5 5”) # Throw away header lines then print vmfh.gets vmfh.gets vmfh.each do |vmline| puts vmline end

10. CIT 383: Administrative Scripting Expect Automation tool for interactive processes. fsck ftp minicom passwd telnet Methods spawn: start an external command expect: wait for command to output pattern send: send string to command as input

11. CIT 383: Administrative Scripting Expect PTY.spawn(‘telnet zork.nku.edu’) do |r_f,w_f,pid| r_f.expect(/^Username.*: /) do w_f.print ‘jsmith’ end r_f.expect('Password:') do w_f.print password end r_f.expect(‘$ ‘) do w_f.print “passwd #{password} spameggs“ end end

12. CIT 383: Administrative Scripting References • Michael Fitzgerald, Learning Ruby, O’Reilly, 2008. • David Flanagan and Yukihiro Matsumoto, The Ruby Programming Language, O’Reilly, 2008. • Hal Fulton, The Ruby Way, 2nd edition, Addison-Wesley, 2007. • Dave Thomas with Chad Fowler and Andy Hunt, Programming Ruby, 2nd edition, Pragmatic Programmers, 2005.