Exploring Expect

Partial Automation

Expect’s interact command turns control of a process over to you so that you can type directly to the process instead of through send commands.

Consider fsck, the UNIX program I mentioned earlier, which checks file system consistency. fsck provides almost no way of answering questions in advance. About all you can say is “answer everything yes” or “answer everything no”.

The following fragment shows how a script can automatically answer some questions differently than others. The script begins by spawning fsck and then in a loop answering yes to one type of question and no to another. The \\ prevents the next character from being interpreted as a wildcard. In this example, the asterisk is a wild card but the question mark is not and matches a literal question mark.

while 1 {
    expect {
        eof                     {break}
        "UNREF FILE*CLEAR\\?"   {send "y\r"}
        "BAD INODE*FIX\\?"      {send "n\r"}
        "\\? "                  {interact +}
    }
}

The last question mark is a catch-all. If the script sees a question it does not understand, it executes the interact command, which passes control back to you. Your keystrokes go directly to fsck. When done, you can exit or return control to the script, here triggered by pressing the plus key. If you return control to the script, automated processing continues where it left off.

Without Expect, fsck can be run non-interactively only with very reduced functionality. It is barely programmable and yet it is the most critical of system administration tools. Many other tools have similarly ...