Exploring Expect

Total Automation

Earlier I mentioned some programs that cannot be automated with the shell. It is difficult to imagine why you might even want to embed some of these programs in shell scripts. Certainly the original authors of the programs did not conceive of this need. As an example, consider passwd.

passwd is the command to change a password. The passwd program does not take the new password from the command line. Instead, it interactively prompts for it—twice. Here is what it looks like when run by a system administrator. (When run by users, the interaction is slightly more complex because they are prompted for their old passwords as well.)

# passwd libes
Changing password for libes on thunder.
New password:
Retype new password:

This is fine for a single password. But suppose you have accounts of your own on a number of unrelated computers and you would like them all to have the same password. Or suppose you are a system administrator establishing 1000 accounts at the beginning of each semester. All of a sudden, an automated passwd makes a lot of sense. Here is an Expect script to do just that—automate passwd so that it can be called from a shell script.

spawn passwd [lindex $argv 0]
set password [lindex $argv 1]
expect "password:"
send "$password\r"
expect "password:"
send "$password\r"
expect eof

The first line starts the passwd program with the username passed as an argument. The next line saves the password in a variable for convenience. As in shell scripts, variables do not have ...