It allows you to spawn a process so that you can replace your terminal interaction with an automated script. You then send any necessary text to that process as if typed from the keyboard , and you tell your Perl program what to expect back I've written a complete [source code here] for the delegates on this week's course, and taken the example of "pinging" half a dozen different hosts on the Internet for which we are responsible or have an interest in; If I run the script, I get a report from each of the computers as six pings are run, results are grabbed, analysed and tabulated.
Notes: a Just like Tcl's Expect, multiple processes can be controlled at the same time; in the example just published, I've only used a single process at a time to help you learn.
In Perl, you don't have that global variable, but you do have methods such as before. It's quite hard to find examples of this Thank you for visiting us. We do not provide sponsorships or invitations to our courses to support UK Visa applications. If you wish to book any of our courses and are a national of India, please check with your local British Consulate and make any necessary Visa arrangements before you book.
More You can Add a comment or ranking to this page. Like this? Related short articles. Python examples - updates under way. When to check an object type - Python isinstance example. One line scripts - Awk, Perl and Ruby. Ruby testing with RSpec - a new example. Separating detailed data code from the main application - Ruby example.
Recursion in Python - the classic example. Unique word locator - Python dict example. Object and Static methods - what is the difference; example in Python 3. What are callbacks? Why use them? Lets create an expect script that supports two parameters from the end user. These parameters will be username and password. Expect should then take care of changing that users password. This time we are using two variables inside our expect script.
The first parameter will be username and second will be password. Use of variables in expect is done in pretty much the same way as in bash. Lets now execute our script as shown below. You might be thinking that its really painful to construct an expect script, because it needs to be provided with the exact same new line characters and return characters and other input formats etc.
I agree. Its not easy to get an expect script working in the first attempt. To make our life easier, there is another helping program called as " autoexpect". Basically execute an interactive program using autoexpect command and it will create an expect script for that interactive program. So you need to install it first. In Red hat based systems autoexpect must already be installed along with expect. Using autoexpect is quite simple and straightforward.
You simply execute the interactive program by prepending autoexpect to it. For example, the script that we wrote previously can be provided as an argument to autoexpect as shown below. This will then create a file called script. This is shown below. Basically autoexpect will capture the behavior of the interactive program that is passed as argument and then create a expect script that does the same thing automatically.
You need to basically change the content of script. Apart from all these, expect also supports general programming capabilities like if else statements, case statements etc. These conditional statements supported by expect is slightly different from the standard bash shell conditionals. Actually expect uses majority of the features provided by TCL language. TCL supports many different types of programming methods.
Its simple and powerful. Let's modify our password changing script and include a for loop to address it for multiple users. Remember, this for loop is not similar to bash for loop. This is TCL language for loop which is used by expect , and as our script starts with a shebang of expect, this tcl statement works perfectly fine.
This is because everything in the file will be interpreted by expect program. Tcl provides control flow e. To know exactly all the command supported by Expect check his man page and one of tcl. Expect reads cmdfile for a list of commands to execute. Expect may also be invoked implicitly on systems which support the! Create a new file, for example ssh.
Normally timeouts are nonnegative, but the special case of -1 signifies that expect should wait forever. It could be useful to change automatically without interaction the password for a given username, we can do this with expect NOTE: this script must be run as root.
Oh…non sai da quanto tempo attendevo un articolo su questo argomento. Expect will constantly loop through the multiple given patterns until a match is found. Expect will match the first pattern it finds in the order you specified them. When it finds a match it will execute any commands given and keep following any more nested commands until it hits the last command.
It will never return to the calling block. It then moves onto the next part of the script. If at any time there are no matches it will timeout. If the pattern keyword match word "timeout" is used you can have it perform an action when the timeout happens.
If an eof is returned it will exit the spawned process and move on. If you use "eof" as a pattern keyword then you can have it also perform an action if an eof happens. You can also use the pattern keyword called "default" that can perform an action if either eof or timeout are reached.
We will see how to use this to make great error messages later. Before we get to actual Expect script examples let me lay out how I design my scripts and what I have learned works best for me. The first example will be logging into a linux box with ssh to see if the given accounts password works. By doing this we are also checking other things like if your machine is up and if you can login to it and get a prompt back.
So in actuality this script does a bunch of things. Let me describe what is going on in this script line by line. Line 1 executes the start of the script with the path to the expect binary. Line 3,4,5 set variables for username, password, and hostname which are taken from the command line for when the script is run. If we remove this line you will see the whole login process.
Line 8 is the first if statement. It checks to see if any arguments have been given to the script and if not it will print out how to use the script. If you do this with every script you will always be able to remind yourself of how to use it. Line 13 prints a banner with the hosts name. Line 15 starts the ssh process and turns on quiet mode and turns off host key checking only turn off key checking on a trusted network.
It uses the variables we give it on the command line to ssh to the host. Line 17 is our first expect statement. First we set our timeout message in case our expect statement can not find a match for what its looking for. According to our timeout we set this will occur in 9 seconds.
This means it will loop through the ssh output for 9 seconds and if it can not find a match it will print this error an then exit as we have told it to do. We also set our EOF value here. If our ssh session does not connect or gets disconnected EOF will be returned and our error message will print. This will then exit like we tell it to do.
This is a regular expression that looks for 0 or more characters with the letters assword after it. Many password login prompts for ssh show up as "Password: " so this should match that. If it does match then it moves to the next statement. If it does not match we will hit the timeout, then our error message will print and the script will exit.
Line 23 will actually send the password we specified on the command line to ssh. We know we can send the password now because we verified we had a matching password prompt. Line 25 starts our next expect statement. We set the timeout error message to say that the login must be incorrect. Because the matching loop has started again looking for output.
This time we are looking for a command prompt. If we can not match a command prompt in the output then that means we never got one. That means our password must have failed. We have to escape this because it is a special character for regular expressions which means end of line match. Your prompts are likely to vary so change your matching expression to meet your needs. Line 30 sends output to the screen to tell the person the we got a correct prompt back.
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