Difference between revisions of "Expect (command)"

Latest revision as of 03:05, 13 April 2008

Expect is a Linux automation and testing tool, written by Don Libes, for interactive applications such as telnet, ftp, passwd, fsck, rlogin, tip, ssh, and others. With Tk, interactive applications can be wrapped in X11 GUIs.

Basics

Expect is an extension of Tcl with two significant commands added: send, and expect. These allow a script to control command-line tools originally designed to interact only with a human. For example, programs which interactively prompt for a password cannot be automated with shell scripts or other languages but can be trivially automated with an Expect script.

Expect has regular expression pattern matching and general program capabilities, allowing simple scripts to intelligently control programs such as telnet, ftp, and ssh, all of which lack a programming language, macros, or any other program mechanism. The result is that Expect scripts provide old tools with significantly new power, flexibility, and reliability.

Examples

• A simple example is a script that automates a telnet session:

# Assume $remote_server, $my_user_id, $my_password, and $my_command
# were read in earlier in the script.
# Open a telnet session to a remote server, and wait for a username prompt.
spawn telnet $remote_server
expect "username:"
# Send the username, and then wait for a password prompt.
send "$my_user_id\r"
expect "password:"
# Send the password, and then wait for a shell prompt.
send "$my_password\r"
expect "%"
# Send the pre-built command, and then wait for another shell prompt.
send "$my_command\r"
expect "%"
# Capture the results of the command into a variable. This can be displayed, or written to disk.
set results $expect_out(buffer)
# Exit the telnet session, and wait for a special end-of-file character.
send "exit\r"
expect eof

• Another example is a script that automates ftp:

# Open an ftp session to a remote server, and wait for a username prompt.
spawn ftp $remote_server
expect "username:"
# Send the username, and then wait for a password prompt.
send "$my_user_id\r"
expect "password:"
# Send the password, and then wait for an ftp prompt.
send "$my_password\r"
expect "ftp>"
# Switch to binary mode, and then wait for an ftp prompt.
send "bin\r"
expect "ftp>"
# Turn off prompting.
send "prompt\r"
expect "ftp>"
# Get all the files
send "mget *\r"
expect "ftp>"
# Exit the ftp session, and wait for a special end-of-file character.
send "bye\r"
expect eof

Usage

Expect serves as a "glue" to link existing utilities together. The general idea is to try to figure out how to make Expect utilize the system's existing tools rather than figure out how to solve a problem inside of Expect.

A key usage of Expect involves commercial software products. Many of these products provide some type of command-line interface, but these usually lack the power needed to write scripts. They were built to service the users administering the product, but the company often doesn't spend the resources to implement a fully robust scripting language. An Expect script can spawn a shell, look up environmental variables, perform some Linux commands to retrieve more information, and then enter into the product's command-line interface armed with the necessary information to achieve the user's goal. After looking up information inside the product's command-line interface, the script can make an intelligent decision about what action to take, if any.

Every time an Expect operation is completed, the results are stored in a local variable called $expect_out. This allows the script to both harvest information to feedback to the user, and it also allows conditional behavior of what to send next based on the circumstances.

A common use of Expect is to set up a testing suite, whether it be for programs, utilities or embedded systems. DejaGnu is a testing suite written using Expect for use in testing. It has been used extensively for testing gcc and is very well suited to testing remote targets such as embedded development.

Opinion

Pros

Expect can be run at regular intervals through the use of cron to encapsulate system administration tasks. The reason this works is because Expect merely uses system administration tools already located on the host computer. No extra tools need to be learned. If the programmer has already learned Tcl, then migrating to Expect is a relatively easy transition. The same programming structures and syntax exist, but with additional features built in.

There is large support in the industry for using Expect for many in-house administration tasks. It is widely used by companies such as Silicon Graphics, IBM, HP, Sun, Xerox, Amdahl, Tektronix, AT&T, ComputerVision and the World Bank to run in-house automated testing for development projects, file transfers, account administration, and network testing.

Expect has been ported to Python and Perl languages in various add-on module projects. Subroutines generally are an interpretation of the original version - with equivalent functionality. Once you understand the concepts, you can move to other languages as needed.

Cons

Expect inherits the same syntax convention as Tcl, which may seem unfamiliar if used to other script languages. Compared to languages such as bash, csh, and Perl, Expect has a different twist, and is sometimes challenging to remember when a variable must be prefixed with a "$", and when it is not. There are versions of expect available for Perl and Python for those familiar with their syntax.

Another limitation is the difficulty in porting Expect scripts between platforms. For example, an Expect script that was written to use several Unix-based tools, might not be suitable if migrated to a Windows platform. If possible, the programmer must find counterpart command-line applications that provide the same information, and this will probably require changing the send/expect's, which can be a major part of the script. This is not an issue if you load tcl, perl or python on the machines in question, and use those languages native posix interfaces for accessing files (open/close) and so on, as well as using standard posix utilities (telnet, ftp etc.) for remote interaction.

Also, Expect automates command-line tools, not GUI-based tools.

References

• Libes, Don (1995). Exploring Expect: A Tcl-Based Tool for Automating Interactive Programs. O'Reilly & Associates, Inc. ISBN 1-56592-090-2.

See also

• Pexpect — expect for Python

External links

• Official homepage
• Expect Wiki
• wikipedia:Expect