expect学习

1. Expect

• a program to control interactive applications
  • it maybe possible to control interactive applications from other scripting languages as well
    • python, perl
• expect programming is very easy compared to perl!
  • crackers use expect to try and gain access to machines via ssh, ftp, telnet
    • ask AJC Blyth

2. Expect example uses

• as system administrators we may have need for automating interactive activities
  • testing remote access works (telnet, ftp)
  • down loading ftp files
  • changing password
  • running security checks
• we could build a script which test that our server is safe
  • from guest accounts
  • no telnet access is allowed
  • no ssh from lab machines is allowed
    • but ssh from privileged machines is working

3. Expect and examples

• automatically creating passwords
  • can be done via C
    • takes a lot of time to write such programs
    • will it work with NIS, shadow passwords or Kerboros?
• with expect we run the user program passwd and send keyboard input

4. Password Trial

• advice first run the program that you want to connect to expect by hand
  • note the output and build expect around it
  • so let us change a password for bob

• passwd bob
  Enter new UNIX password:
  Retype new UNIX password:
  passwd: password updated successfully

  • passwords were entered, but obviously the passwd program did not echo back our input! 
    

5. Trial results

• we note that the program prompted us for a password twice
  • both times it ended its sentence with password:
  • as we are lazy we can wait until we see password: and ignore anything that was before
    • why is this good practice?
    • what must we watch out for?

• #!/usr/bin/expect
  
  
  spawn passwd [lindex $argv 0]
  set password [lindex $argv 1]
  expect "password:"
  send "$password\r"
  expect "password:"
  send "$password\r"
  expect eof

6. Testing the example

• ./exp1.exp bob 123
  spawn passwd bob
  Enter new UNIX password:
  Retype new UNIX password:
  passwd: password updated successfully

• here we change user bob password to 123 
  

7. Script explanation

• #!/usr/bin/expect
  
  
  spawn passwd [lindex $argv 0]
  set password [lindex $argv 1]
  expect "password:"
  send "$password\r"
  expect "password:"
  send "$password\r"
  expect eof

• #!/usr/bin/expect script is interpreted via expect located in directory /usr/bin
• spawn passwd [lindex $argv 0]
  • run the program passwd bob and connect expect to this program
  • note that [lindex $argv 0] resolves to bob
    • actually argument one (hmm..) 
      

8. Script explanation

• set password [lindex $argv 1]
  • defines a variable password and sets it to 123
• expect "password:"
  • waits for the program passwd to issue password: before continuing
• send "$password\r"
  • sends the users password to passwd followed by a carriage return
• note the script repeats the last two commands, why?
• finally expect eof wait for passwd to finish

9. Anchoring

• you might want to match text at the beginning or end of a line, this is via
  • ^ for the beginning of a line
  • $ for the end of a line
• also note that * means any number of characters

10. Pattern action pairs

• #!/usr/bin/expect
  
  set timeout 15
  
  
  expect "hi"  { send "You said hi\n" } \
       "hello"  { send "Hello to you\n" } \
       "bye"    { send "Goodbye\n" } \
       timeout  { send "I’m fed up\nbye\n" }

• if we run the script as shown below and type nothing we get:

• I’m fed up
  bye

• note that different actions can be associated with different input
  • note also that the default timeout time is set at 10 seconds to disable the timeout facility
  • set timeout -1 
    

11. Autoftp and expect

• so far we have built the front end to autoftp
  • scans the input file for URLs
  • handles arguments
• we will use expect to control ftp, we will build this up this utility
• firstly we will ftp manually

12. Ftp session

• fred@merlin:$ ftp guenevere
  Connected to guenevere.
  220 guenevere FTP server (Version wu-2.6.0(1))
  Name (guenevere:fred): anonymous
  331 Guest login ok, send your complete e-mail
  331 address as password.
  Password:nobody@nowhere.com
  
  
  230-Welcome, archive user anonymous@merlin !
  230-
  230-The local time is: Mon Feb 12 15:18:14 2001
  230-
  230 Guest login ok, access restrictions apply.
  Remote system type is UNIX.
  Using binary mode to transfer files.

• ftp> dir
  200 PORT command successful.
  150 Opening ASCII mode data connection for /bin/ls.
  total 20
  d--x--x--x    2 0 0 4096 Aug 10  2000 bin
  d--x--x--x    2 0 0 4096 Aug 10  2000 etc
  d--x--x--x    2 0 0 4096 Aug 10  2000 lib
  dr-xr-xr-x    4 0 0 4096 Sep 28 22:17 pub
  -rw-r--r--    1 0 0  346 Aug 10  2000 welcome.msg
  226 Transfer complete.
  ftp> quit
  221-You have transferred 0 bytes in 0 files.
  221-Total traffic for this session was 1182
  221-bytes in 1 transfers.
  221-Thank you for using the FTP service on
  221-guenevere. Goodbye.

13. Simple semi automated ftp script

• this script will log us into a site and then interact with the user

• #!/usr/bin/expect
  
  
  set site [lindex $argv 0]
  spawn ftp $site
  expect "Name"
  send "anonymous\r"
  expect "Password:"
  send "nobody@nowhere.com\r"
  interact

• note the interact statement
  • connects the keyboard to the ftp program
• most ftp servers do not check for a valid email address! 
  

14. Expect control constructs

• expect uses extends the Tcl language
• expect provides: if then else, while, set, interact, expect, for, switch, incr, return, proc

• if {$count < 0} {
     set total 1
  }

• if {$count < 5} {
     puts "count is less than five"
  } else {
     puts "count is not less than five"
  }

• you must place the statement begin brace on the same line as the if or else
  • if in doubt follow the templates in these notes 
    

15. More complex if

• if {$count < 0} {
     puts "count is less than zero"
  } elseif {$count > 0 {
     puts "count is greater than zero"
  } else {
     puts "count is equal to zero"
  }

16. While statement

• #!/usr/bin/expect
  
  
  set count 10
  while {$count > 0} {
     puts "the value of count is $count"
     set count [expr $count-1]
  }

• note the effect of the braces in {$count > 0}
  • they defer evaluation of $count
  • if you remove the braces then $count > 0 is internally replaced via: 10 > 0 
    

17. for command

• has the syntax

  for start expression next {

  }

• #!/usr/bin/expect
  
  
  for {set count 10} {$count > 0} {incr count -1} {
     puts "the value of count is $count"
  }

18. Expressions

• 0 is false, 1 is true
• boolean operators || (or), && (and), ! (not)
• comparison operators <=, ==, != etc
• the brackets [ ] give this expression a higher precedence
• so in the example

• set count [expr $count-1]

• the [expr $count-1] is evaluated before the set! 
  

19. proc and return

• #!/usr/bin/expect
  
  proc mycompare {a b} {
      if {$a < $b} {
          puts "$a is less than $b"
          return -1
      } elseif {$a > $b} {
          puts "$a is greater than $b"
          return 1
      } else {
          puts "$a is equal to $b"
          return 0
      }
  }
  
  
  set value [mycompare 1 4]
  puts "comparison returned $value"

• ./exp6.exp
  1 is less than 4
  comparison returned -1

20. Autoftp Tutorial

• #!/usr/bin/expect
  # this program is called exp7.exp
  proc connect {} {
     expect {
         "Name*:" {
             send "anonymous\r"
             expect {
                 "Password:" {
                     send "nobody@nowhere.com\r"
                     expect "login ok*ftp>"
                     return 0
                 }
             }
         }
     }
     # timed out
     return 1
  }

• set site [lindex $argv 0]
  spawn ftp $site
  while {[connect]} {
      send "quit\r"
      expect eof
      spawn ftp $site
  }
  send "binary\r"
  send "cd  [lindex $argv 1]\r"
  send "get [lindex $argv 2]\r"
  send "quit\r"
  expect eof

21. Laboratory session

• work on your assignment
• try out the following expect example
  • you will need to add your password at the position yourpassword

• #!/usr/bin/expect
  
  log_user 0
  spawn pwd
  expect -re "(^.*/.*)$"
  set localdir $expect_out(1,string)
  expect eof
  
  
  log_user 0
  spawn ssh moppsy.comp.glam.ac.uk
  expect "assword: "
  send "yourpassword\r"
  expect "\\$"
  send "cd $localdir"
  interact

  22.参考链接

  • http://floppsie.comp.glam.ac.uk/Glamorgan/gaius/scripting/5.html
    
  • http://w2home.blogspot.com/2007/11/expect-tutorial.html
    
  • http://oreilly.com/catalog/expect/chapter/ch03.html
    
  • http://blog.ihipop.info/2010/12/1949.html
    
  • http://www.tldp.org/HOWTO/TclTk-HOWTO.html