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Exploring Expect

Cover of Exploring Expect by Don Libes Published by O'Reilly Media, Inc.
  1. Exploring Expect
    1. SPECIAL OFFER: Upgrade this ebook with O’Reilly
    2. A Note Regarding Supplemental Files
    3. Preface
      1. Expect—Why another tool?
      2. Tcl—A Little History
      3. Acknowledgments
      4. We’d Like to Hear From You
    4. How To Read This Book
      1. Notational Conventions
      2. Exercises
    5. 1. Intro—What Is Expect?
      1. Ouch, Those Programs Are Painful!
      2. A Very Brief Overview
      3. A First Script—dialback
      4. Total Automation
      5. Differing Behavior When Running Non-Interactively
      6. Partial Automation
      7. Dangerous, Unfriendly, Or Otherwise Unlikable User Interfaces
      8. Graphical Applications
      9. A Little More About Tcl
      10. Job Control
      11. Background Processes
      12. Using Expect With Other Programs
      13. Using Expect On UNIX
      14. Using Expect On Other Operating Systems
      15. Using Expect In Real Applications
      16. Using Expect In Commercial Applications—Legalese
      17. Obtaining Expect and the Examples
      18. Expect And Tcl Resources
      19. Exercises
    6. 2. Tcl—Introduction And Overview
      1. Everything Is A String
      2. Quoting Conventions
      3. Expressions
      4. Braces—Deferring Evaluation
      5. Control Structures
      6. More On Expressions
      7. Lists
      8. More Ways To Manipulate Strings
      9. Arrays
      10. Indirect References
      11. Handling Errors
      12. Evaluating Lists As Commands
      13. Passing By Reference
      14. Working With Files
      15. File I/O
      16. Executing UNIX Commands
      17. Environment Variables
      18. Handling Unknown Commands
      19. Libraries
      20. Is There More To Tcl?
      21. Exercises
    7. 3. Getting Started With Expect
      1. The send Command
      2. The expect Command
      3. Anchoring
      4. What Happens When Input Does Not Match
      5. Pattern-Action Pairs
      6. Example—Timed Reads In The Shell
      7. The spawn Command
      8. The interact Command
      9. Example—Anonymous ftp
      10. Exercises
    8. 4. Glob Patterns And Other Basics
      1. The * Wildcard
      2. More Glob Patterns
      3. Backslashes
      4. Handling Timeout
      5. Handling End Of File (eof)
      6. Hints On The spawn Command
      7. Back To Eof
      8. The close Command
      9. Programs That Ignore Eof
      10. The wait Command
      11. Exercises
    9. 5. Regular Expressions
      1. Regular Expressions—A Quick Start
      2. Identifying Regular Expressions And Glob Patterns
      3. Using Parentheses To Override Precedence
      4. Using Parentheses For Feedback
      5. More On The timed–read Script
      6. Pattern Matching Strategy
      7. Nested Parentheses
      8. Always Count Parentheses Even Inside Of Alternatives
      9. Example—The Return Value From A Remote Shell
      10. Matching Customized Prompts
      11. Example—A Smart Remote Login Script
      12. What Else Gets Stored In expect_out
      13. More On Anchoring
      14. Exercises
    10. 6. Patterns, Actions, And Limits
      1. Matching Anything But
      2. Really Complex Patterns
      3. Really Simple Patterns
      4. Matching One Line And Only One Line
      5. Tcl’s string match Command
      6. Tcl’s regexp Command
      7. Tcl’s regsub Command
      8. Ignoring Case
      9. All Those Other String Functions Are Handy, Too
      10. Actions That Affect Control Flow
      11. Example—rogue
      12. Character Graphics
      13. More Actions That Affect Control Flow
      14. Matching Multiple Times
      15. Recognizing Prompts (Yet Again)
      16. Speed Is On Your Side
      17. Controlling The Limits Of Pattern Matching Input
      18. The full_buffer Keyword
      19. Double Buffering
      20. Perpetual Buffering
      21. The Politics Of Patterns
      22. Expecting A Null Character
      23. Parity
      24. Length Limits
      25. Comments In expect Commands
      26. Restrictions On expect Arguments
      27. eval—Good, Bad, And Ugly
      28. Exercises
    11. 7. Debugging Patterns And Controlling Output
      1. Pattern Debugging
      2. Enabling Internal Diagnostics
      3. Logging Internal Diagnostics
      4. Disabling Normal Program Output
      5. The log_user Command
      6. Example—su2
      7. Recording All Expect Output
      8. Sending Messages To The Log
      9. About File Names
      10. Log And Diagnostic State
      11. Exercises
    12. 8. Handling A Process And A User
      1. The send_user Command
      2. The send_error Command
      3. The expect_user Command
      4. Dealing With Programs That Reprompt
      5. Dealing With Programs That Miss Input
      6. Sleeping
      7. Line Versus Character-Oriented And Other Terminal Modes
      8. Echoing
      9. Prompting For A Password On Behalf Of A Program
      10. Security And Insecurity
      11. Resetting The Terminal Upon Exit
      12. More On The stty Command
      13. The system Command
      14. Redirecting The Standard Input Or Output
      15. The expect_tty Command
      16. The send_tty Command
      17. Exercises
    13. 9. The Expect Program
      1. Expect—Just Another Program
      2. Invoking Scripts Without Saying “expect”
      3. Rewriting The #! Line
      4. The .exp Extension
      5. The—And Other Flags
      6. The —c Flag
      7. The -f Flag
      8. Writing The #! Line
      9. The −i Flag
      10. The -n And -N Flags
      11. The -d Flag
      12. The -D Flag
      13. The -b Flag
      14. The - Flag
      15. The interpreter Command
      16. Exercises
    14. 10. Handling Multiple Processes
      1. The spawn_id Variable
      2. Example—chess Versus chess
      3. Example—Automating The write Command
      4. How exp_continue Affects spawn_id
      5. The Value Of spawn_id Affects Many Commands
      6. Symbolic Spawn Ids
      7. Job Control
      8. Procedures Introduce New Scopes
      9. How Expect Writes Variables In Different Scopes
      10. Predefined Spawn Ids
      11. Exercises
    15. 11. Handling Multiple Processes Simultaneously
      1. Implicit Versus Explicit Spawn Ids
      2. Waiting From Multiple Processes Simultaneously
      3. Example—Answerback
      4. Which Pattern Goes With Which Spawn Id
      5. Which Spawn Id Matched
      6. Spawn Id Lists
      7. Example—Connecting Together Two Users To An Application
      8. Example—Timing All Commands
      9. Matching Any Spawn Id Already Listed
      10. The expect_before And expect_after Commands
      11. Indirect Spawn Ids
      12. Exercises
    16. 12. Send
      1. Implicit Versus Explicit Spawn Ids
      2. Sending To Multiple Processes
      3. Sending Without Echoing
      4. Sending To Programs In Cooked Mode
      5. Sending Slowly
      6. Sending Humanly
      7. Sending Nulls
      8. Sending Breaks
      9. Sending Strings That Look Like Flags
      10. Sending Character Graphics
      11. Comparing send To puts
      12. Exercises
    17. 13. Spawn
      1. The Search Path
      2. Philosophy--Processes Are Smart
      3. Treating Files As Spawned Processes
      4. Opening Ttys
      5. Bugs And Workarounds
      6. Process Pipelines And Ptys
      7. Automating xterm
      8. Checking For Errors From spawn
      9. spawn -noecho
      10. Example—unbuffer
      11. Obtaining Console Output
      12. Setting Pty Modes From spawn
      13. Hung Ptys
      14. Restrictions On Spawning Multiple Processes
      15. Getting The Process Id From A Spawn Id
      16. Using File I/O Commands On Spawned Processes
      17. Exercises
    18. 14. Signals
      1. Signals
      2. Signals In Spawned Processes
      3. Notes On Specific Signals
      4. When And Where Signals Are Evaluated
      5. Overriding The Original Return Value
      6. Using A Different Interpreter To Process Signals
      7. Exit Handling
      8. Exercises
    19. 15. Interact
      1. The interact Command
      2. Simple Patterns
      3. Exact Matching
      4. Matching Patterns From The Spawned Process
      5. Regular Expressions
      6. What Happens To Things That Do Not Match
      7. More Detail On Matching
      8. Echoing
      9. Avoiding Echoing
      10. Giving Feedback Without -echo
      11. Telling The User About New Features
      12. Sending Characters While Pattern Matching
      13. The continue And break Actions
      14. The return Action
      15. The Default Action
      16. Detecting End-Of-File
      17. Matching A Null Character
      18. Timing Out
      19. More On Terminal Modes (Or The -reset Flag)
      20. Example—Preventing Bad Commands
      21. Exercises
    20. 16. Interacting With Multiple Processes
      1. Connecting To A Process Other Than The Currently Spawned Process
      2. Connecting To A Process Instead Of The User
      3. Example—rz And sz Over rlogin
      4. Redirecting Input And Output
      5. Default Input And Output
      6. Controlling Multiple Processes—kibitz
      7. Combining Spawn Ids In A Single -input Or -output
      8. Which Spawn Id Matched
      9. Indirect Spawn Ids
      10. An Extended Example—xkibitz
      11. Exercises
    21. 17. Background Processing
      1. Putting Expect In The Background
      2. Running Expect Without A Controlling Terminal
      3. Disconnecting The Controlling Terminal
      4. The fork Command
      5. The disconnect Command
      6. Reconnecting
      7. Using kibitz From Other Expect Scripts
      8. Mailing From Expect
      9. A Manager For Disconnected Processes—dislocate
      10. Expect As A Daemon
      11. Example—Automating Gopher and Mosaic telnet Connections
      12. Exercises
    22. 18. Debugging Scripts
      1. Tracing
      2. Logging
      3. Command Tracing
      4. Variable Tracing
      5. Example—Logging By Tracing
      6. UNIX System Call Tracing
      7. Tk And tkinspect
      8. Traditional Debugging
      9. Debugger Command Overview And Philosophy
      10. Stepping Over Procedure Calls
      11. Stepping Into Procedure Calls
      12. Where Am I
      13. The Current Scope
      14. Moving Up And Down The Stack
      15. Returning From A Procedure
      16. Continuing Execution
      17. Defining Breakpoints
      18. Help
      19. Changing Program Behavior
      20. Changing Debugger Behavior
      21. Exercises
    23. 19. Expect + Tk = Expectk
      1. Tk—A Brief Technical Overview
      2. Expectk
      3. The send Command
      4. An Extended Example—tkpasswd
      5. The expect Command And The Tk Event Loop
      6. The expect_background Command
      7. Multiple Spawn Ids In expect_background
      8. Background Actions
      9. Example—A Dumb Terminal Emulator
      10. Example—A Smarter Terminal Emulator
      11. Using The Terminal Emulator For Testing And Automation
      12. Exercises
    24. 20. Extended Examples
      1. Encrypting A Directory
      2. File Transfer Over telnet
      3. You Have Unread News—tknewsbiff
      4. Exercises
    25. 21. Expect, C, And C++
      1. Overview
      2. Linking
      3. Include Files
      4. Ptys And Processes
      5. Allocating Your Own Pty
      6. Closing The Connection To The Spawned Process
      7. Expect Commands
      8. Regular Expression Patterns
      9. Exact Matching
      10. Matching A Null
      11. What Characters Matched
      12. When The Number Of Patterns Is Not Known In Advance
      13. Expecting From Streams
      14. Running In The Background
      15. Handling Multiple Inputs And More On Timeouts
      16. Output And Debugging Miscellany
      17. Pty Trapping
      18. Exercises
    26. 22. Expect As Just Another Tcl Extension
      1. Adding Expect To Another Tcl-based Program
      2. Differences Between Expect And The Expect Extension In Another Program
      3. Adding Extensions To Expect
      4. Adding Extensions To Expectk
      5. Creating Script-less Expect Programs
      6. Functions And Variables In The Expect Extension
      7. Exercises
    27. 23. Miscellaneous
      1. Random Numbers
      2. Example—Generating Random Passwords
      3. The Expect Library
      4. Expect Versions
      5. Timestamps
      6. The time Command
      7. Exercises
    28. A. Appendix—Commands and Variables
      1. Commands And Flags
      2. Variables
    29. Index
    30. About the Author
    31. SPECIAL OFFER: Upgrade this ebook with O’Reilly
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Backslashes

Tcl makes various substitutions when you have backslashes, dollar signs, and brackets in command arguments. You should be familiar with these from Chapter 2 (p. 23). In this section, I am going to focus on backslashes.

Backslash translations are done by Tcl only when processing command arguments. For example, \n is translated to a linefeed, \[ is translated to a "[“, and \\ is translated to a "\“. Sequences that have no special translation are replaced by the character without the backslash. For example, \z is translated to a "z“.

While pattern matching, Expect uses these translated values. For example:

expect "\n" ;# matches \n (linefeed character)
expect "\r" ;# matches \r (return character)
expect "\z" ;# matches z  (literal z)
expect "\{" ;# matches {  (literal left brace)

If any backslashes remain after Tcl’s translation, the pattern matcher (i.e., pattern matching algorithm) then uses these remaining backslashes to force the following character into its literal equivalent. For example, the string "\\*" is translated by Tcl to "\*“. The pattern matcher then interprets the "\*" as a request to match a literal "*“.

expect "*"    ;# matches * and ? and X and abc
expect "\\*"  ;# matches * but not ? or X or abc

Similarly, backslashes prevent a ? from acting like a wildcard.

expect "?"    ;# matches * and ? and X but not abc
expect "\\?"  ;# matches ? but not * or X or abc

So that you can see the consistency here, I have written out some more examples. Do not try to memorize these. Just remember two rules:

  1. Tcl translates backslash sequences.

  2. The pattern matcher treats backslashed characters as literals.

These rules are executed in order and only once per command.

For example, in the second command below, Tcl translates the "\n" to a linefeed. The pattern matcher gets the linefeed and therefore tries to match a linefeed. In the third command, Tcl translates the "\\" to "\" so that the pattern matches sees the two characters "\n“. By the second rule above, the pattern matcher interprets this as a literal n. In the fourth command, Tcl translates "\\" to "\" and "\n" to a linefeed. By the second rule, the pattern matcher strips off the backslash and matches a literal linefeed.

In summary, \n is replaced with a linefeed by Tcl but a literal n by the pattern matcher. Any character special to Tcl but not to the pattern matcher behaves similarly.

expect "n" ;# matches n
expect "\n" ;# matches \n (linefeed character)
expect "\\n" ;# matches n
expect "\\\n" ;# matches \n
expect "\\\\n" ;# matches sequence of \ and n
expect "\\\\\n" ;# matches sequence of \ and \n
expect "\\\\\\n" ;# matches sequence of \ and n
expect "\\\\\\\n" ;# matches sequence of \ and \n
expect "\\\\\\\\n" ;# matches sequence of \, \, and n

In the next set of examples, \* is replaced with a literal * by Tcl and by the pattern matcher. Any character special to the pattern matcher but not Tcl behaves similarly.

expect "*" ;# matches anything
expect "\*" ;# matches anything
expect "\\*" ;# matches *
expect "\\\*" ;# matches *
expect "\\\\*" ;# matches \ followed by anything
expect "\\\\\*" ;# matches \ followed by anything
expect "\\\\\\*" ;# matches \ followed by *

The "[" is special to both Tcl and the pattern matcher so it is particularly messy. To match a literal "[“, you have to backslash once from Tcl and then again so that it is not treated as a range during pattern matching. The first backslash, of course, has to be backslashed to prevent it from turning the next backslash into a literal backslash!

expect "\\\["   ;# matches literal [

This is quite a headache. In fact, if the rest of the pattern is sufficiently specific, you may prefer to improve readability by just using using a ? and accepting any character rather than explicitly forcing a check for the "[“.

The next set of examples shows the behavior of "[" as a pattern preceded by differing numbers of backslashes. If the "[" is not prefixed by a backslash, Tcl interprets whatever follows as a command. For these examples, imagine that there is a procedure named XY that returns the string "n*“.

expect "[XY]" ;# matches n followed by anything
expect "\[XY]" ;# matches X or Y
expect "\\[XY]" ;# matches n followed by anything
expect "\\\[XY]" ;# matches [XY]
expect "\\\\[XY]" ;# matches \ followed by n followed ...
expect "\\\\\[XY]" ;# matches sequence of \ and X or Y

The \\[XY] case deserves close scrutiny. Tcl interprets the first backslash to mean that the second is a literal character. Tcl then interprets [XY] as the result of the XY command. The pattern matcher ultimately sees the four character string "\n*w“. The pattern matcher interprets this in the usual way. The backslash indicates that the n is to be matched literally (which it would even without the backslash since the n is not special to the pattern matcher). Then as many characters as possible are matched so that a w can also be matched.

By now, you may be wondering why I write all patterns in double quotes in preference to using braces. It is true that braces shorten some of the patterns I have shown here. However, braces do not allow patterns to be specified from variables, nor do they allow backslashed characters such as newlines. But such patterns occur so frequently that you have to be familiar with using double quotes anyway. Constantly thinking about whether to use braces or double quotes is unproductive. Learn how to use double quotes and do not think further about using braces for patterns. If you know Tcl very well and skippedChapter 2 (p. 23), it may be helpful for you to now go back and read the beginning of it as well as the discussion of eval on page 55.

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