In my previous post, I talked about how to automate the server login process with the help of Expect programming language. This post continues my learning process.

Difference between send and send_user command?

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There are two similar commands in Expect, send and send_user. send and send_user have different purposes. send is used to provide input for the spawned processes, while send_user is used to send some message to via terminal to the users.

Difference between \n and \r in Expect?

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According to my test, when you use send to send characters to the spawned process, it makes no difference to use either \r (carriage return) or \n (line feed). They both works as if the user has press the Enter key.

However, they work differently when you want use send_user to display something to the user. \n will give a new line while \r will just move the cursor to the beginning of the line (i.e., your message will be erased). To verify, execute the following command

expect {
    "prompt1" {send_user "answer1\n"}
    "prompt2" {send_user "answer2\r"}
}

If you execute the above script with expect, when you input prompt1, you will get the output answer1 and a new line is started; when you input prompt2, you get nothing in the screen. Actually, answer2 is shown and is then erased by \r.

Check if a file exists

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Sometimes, we may want to check if a file exists and take some actions based on the condition. We use file exists <name> to check that:

if {[file exists .ssh/known_hosts]} {
    spawn rm .ssh/known_hosts
}

Match optional prompts

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Sometimes, there are optional prompts which may occur depending on some conditions. For example, when we first log into a new server, we are warned that the authenticity of the server can not be established and if we want to continue connecting. After that, we may not see this prompt anymore. We can use the exp_continue command to achieve that.

expect {
    "Are you sure*" {
        send "yes\r"
        exp_continue
    }
    "*password: " {
        send "pass\r"
    }
}

The exp_continue command will continue the expect block until it matches another pattern or timeouts.

Pattern matching

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Double quotes and curly braces in pattern matching

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In expect script, both double quotes and curly braces can be used to group argument. They are quite different.

Inside double quotes, variable substitution and command substitution and backslash substitution take place:

• variable substitution: $var will be replaced with actual value of variable var.
• command substitution: [some_char] is considered as command invoking, and he result of executing command some_char replaces [some_char]. Also see this guide page.
• backslash substitution: characters after \ is replaced with the character verbatim. There are several exceptions to this rule. For example, \n, \t, \uHHHH etc. have special meanings. For a complete list, see this page (the part about backslash substitution).

Inside double braces, command substitution, variable expansion and backslash substitution does not take place. For example:

expect {
    # match $var literally
    {$var} {send_user "yes\n"}
    # match character \ and $
    {\$} {send_user "yes\n"}
    # match character \ and n
    {\n} {send_user "yes\n"}
}

Glob match patterns

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For simple matching, glob patterns can be used, and it works similarly to the bash glob functionality. * is used to represent any number of characters, and ? is used to represent a single character. For more info about glob patterns, see doc here.

Some patterns and their meanings are listed below:

expect{
    # match strings containing abc
    expect *abc*
    # match literal [a]
    {\[a]} {send_user "yes\n"}
    # match literal [a]
    {\[a\]} {send_user "yes\n"}
    # match strings containing ab followed by another character
    {ab?} {send_user "yes\n"}
    # match `abc` or `abd`
    {ab[cd]} {send_user "yes\n"}
}

Regex matching

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If you want to match more complex patterns, you can also use the regex provided by the Tcl language¹. To use regex, you will use the -re option for expect command. For example, to match either ab or cd, you can use the following command:

expect -re ab|cd {send_user "hello\n"}

As discussed before, since patterns inside the double quotes undergo several substitutions, you should take extra care when you want to match patterns using regex. For example, to match consecutive digits, you should use expect -re "\\d+" or expect -re {\d+}. If you use expect -re "\d+", you will match consecutive characterd since backslash will translate \d inside double quotes.

 expect {
    # the following two patterns both match two consecutive alphabet chars.
    -re "\[a-z\]{2}" {send_user "yes\n"}
    -re {[a-z]{2}} {send_user "yes\n"}
    # match a lenth 2 string which contains only alpha-numerical chars.
    -re {^[a-z0-9]{2}\n$} {send_user "yes\n"}
    timeout {send_user "time out\n"}
 }

Matching bracket literally

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One extremely complex and bewildering pattern is to match square bracket literally. It is difficult because square bracket have special meanings in both command substitution and in regex or glob pattern.

If you use double quotes, in order to match [a] literally, you have to use triple backslash to escape it:

expect -re "\\\[a\\\]" {send_user "yes\n"}
# or you can use
expect "\\\[a\\\]" {send_user "yes\n"}

First backslash is to prevent command substitution. Second backslash and third backslash is to prevent backslash substitution so that regex engine can see one backslash. So the final pattern regex engine sees is actually \[a\], which prevents a from being interpreted as a regex range. Thus, it is matched literally. This page also explains why three backslashes are needed in order to match literal square bracket if we use double quotes.

To prevent such complex patterns, it is highly encouraged to use double braces to simplify the pattern. The above pattern can be simplified as the following:

expect {
    {\[a\]} {send_user "yes\n"}
}

References

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• Expect check if file exists.
• Expect: match optional prompt.
• Difference between \r and \n in expect.
• Expect regex
• Expect glob and other patterns
• Tcl regex syntax references.
• Grouping with double quotes.