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The last example is something I do fairly regularly. I often want to gather information from each system named in a list of machines. This example shows how to get the list of node names by using a command call within back-ticks (` `). A command string enclosed within back-ticks denotes not the given string, but the string obtained by evaluating the command string and replacing it with what is returned.

$ for node in `cat some_nodelist_file` > do > if [ "$node" = "cheese" ] > then > continue > else > ssh $node uname -a > ssh $node uptime > fi > done

In this case, `cat some_nodelist_file` would be replaced with an actual list of nodes originally contained in that file. The file is assumed to contain the list of nodes that are space-delimited or that appear individually on each line of the file, or a combination of both. This loop does not iterate through the file line-by-line.1The for loop iterates through each of the nodes. I ve also included an if/then statement that will skip any node named "cheese" to provide an example that includes a conditional. A final trick for using these kinds of scripts efficiently is command-line recall, which is discussed in 15. If you make a mistake while typing (not unlikely when you re working with many lines at once), you can return to the previous (mistyped) command in your history and then edit the command sequence using vi-style command-line editing. You can also recall a previously entered miniscript for easy modification and reuse.

he expect utility s name suggests precisely what it does: expect some output from an interactive program, and send the program some input in response. expect has much more functionality than I cover in this chapter, but this chapter provides a good example of how it can be used. To find more complete information, you can consult the expect manual page. You may find that when you try to automate a task, the utilities or tools you are using don t lend themselves well to scripting. In the past, use of the format or fdisk command (along with many others) was difficult to automate. Today we have versions of these utilities, such as sfdisk, that are much easier to use within a script. A more modern use of expect might include logging into specialized hardware to gather information or to customize settings, as is required when administering network routers, switches, and firewalls. This chapter presents a pair of scripts for automating the control of a serial terminal server. This is a type of network-accessible hardware that looks very much like a network hub or a switch with multiple RJ45 ports. Each physical port can be connected to serial devices, such as serial consoles. Once consoles are attached to the terminal server, you can telnet to a specific network port on the terminal server and establish a connection with the attached console. The first example in this chapter is a shell script that processes user-provided command-line switches that specify what commands to send to the terminal server. The second script, which is called by the first, is an expect script that performs all the manual labor. expect is an extension of the Tcl scripting language. expect was designed to communicate with an interactive program, and it works well with ssh, telnet, ftp, and other interactive utilities.

The first script obtains the user input necessary to connect to the desired terminal server(s) and perform the intended tasks. It displays usage instructions and allows the user to specify a specific terminal server or to provide a file containing node names if there are multiple terminal servers with which the user wants to communicate in the same way and at the same time. First we need to define a few variables:

#!/bin/sh NODE="" CMDS="" NODEFILE="" AUTO="" USAGE="There is a problem with the command, type $0 -h for syntax"

The variables are initialized to null strings, except for the USAGE variable, which contains a message that is displayed whenever the script finds a problem with the command-line call the user provided. The script gets the information it needs from the user on the command line, so we check that switches have been passed.