Exploring Expect

Example—A Smart Remote Login Script

In Chapter 3 (p. 83), I showed a script called aftp. This automated the initialization for an anonymous ftp session and then passed control from the script to the user. The same idea can be applied to many other programs.

Imagine the following scenario. You try to create a file only to find out that your computer has no permission to change the file system because it is mounted read-only from another computer, the file server.

% rm libc.a
rm: libc.a not removed: Read-only file system

All you have to do is log in to the server and then repeat the command:

% rlogin server

You are logged in to the server. Please be careful.

% rm libc.a
rm: libc.a: No such file or directory

Oops. You are not in the right directory. rlogin does not propagate your current working directory. Now you have to find out what directory you were in, and enter the appropriate cd command. If the directory is long enough or if you do this kind of thing frequently, the procedure can become a nuisance.

Here is a script to automatically rlogin in such a way that you are automatically placed in the same directory on the remote host.

set cwd [pwd]
spawn rlogin $argv
expect "% "
send "cd $cwd\r"
expect "% "
interact

The scripts starts by running pwd, which returns the current directory. The result is saved in the variable cwd. Then rlogin is spawned. When the prompt arrives, the script sends the cd command and then waits for another prompt. Finally, interact is executed and control is returned ...