Script Programming, Linux expect script passwd: 365 programming project day twenty two

What the heck is a Script?

Is writing a Script still programming?

A script is a written set of instructions for a platform (usually an Operating System) that makes the computer (or hardware) do something.

Sounds a lot like a Program!

Well... A computer software program is usually source code (written by a human) that is then "compiled" into "machine code" which a machine can understand - except that there's also a linker and loader to figure out exactly what places in memory (and for what piece of hardware) the Program will run - finally turning it into Object code and then an Executable (usually called binary because it's in 1's and 0's directly in the language of the hardware).

A script gives hardware a set of commands indirectly through another piece of software (it's environment/platform, e.g. DOS, Python, Windows OS, GNU Linux OS, etc.).

The software then executes those commands (which often then tell an Operating System which then tells Hardware to to do something - like clearing the screen or adding some numbers).

So maybe html is even less Programming like than a script! Since it only tells a browser (a specific application) how to format/display data!?!?!

Ah, enough semanticarating!

What happens if you're running a script and somewhere it requires user input?

"Expect" is a piece of software that "waits" for input from the linux operating system command line...

apt-get install expect

to install it on linux (ubuntu)

yum install expect

to install it on linux centos/redhat

touch test-script.sh

create the file

chmod +x test-script.sh

make the file an executable in linux

nano test-script.sh

edit the file to enter our script commands, alternatively use vim test-script.sh

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#!/usr/bin/expect
set timeout 1
send "passwd USER\n"
expect "Password: "
send "password\n"
sleep 1
interact

• First we tell the operating system what program (environment platform) will actually be able to read and execute the instructions (normally linux uses BASH /bin/bash but in this case it's EXPECT).
• Setting the timeout means that we will wait at least and at most 1 second before a non responding command is skipped to execute the next command.
• Send tells the operating system something (e.g. we want to run the change USER's password program)
• The expect command tells the script to wait for the operating system to prompt the user to type in a password.
• Once the operating system responds (with exactly what we "expected") the script continues by sending (just as if a user typed it in) some predetermined password picked by the script writer.
• Finally we tell our script to sleep for one second after all of it's hard work.
• Oh, and it's very important to "interact", which returns the command prompt back to the user/operating system.

Whew... that's just the tip of the iceberg of how "expect" greatly extends the functionality of Linux scripts!

Later I hope to demonstrate some VBScript (for windows) and maybe even a script on how to run a "non interactive FTP session" (as most FTP programs require "interaction" aka human at the keyboard)...