Category: Command Line Interface

Someone has recently asked an interesting question - can you disable the reload command? Although I would strongly discourage you from doing that (after all, every router I've ever worked on since a venerable MGS running IOS 10.0 had to be reloaded every now and then), here's what you can do:

• define an alias for the reload command that does something else. For example, alias exec reload show ip interface brief. While this would remind a careless operator, it would still not prevent someone using an abbreviation like relo to reload the device.
• Use TACACS+ command accounting and disable the reload command on the TACACS+ server. The benefit of this approach is that you can do it on user-by-user basis ... but of course you need TACACS+ server, RADIUS will not do.
• Disable the reload command with the Embedded Event Manager applet.

Every introductory Cisco course tells you that you can stop any IOS command (for example, ping or traceroute) with the Ctrl/^ (also written as ^^ or Ctrl-Shift-6) escape character. What they usually forget to tell you is how to do that on non-US-ASCII keyboards or with telnet programs that do not want to recognize weird control characters.

The trick is simple - if you cannot generate ^^ (ASCII code 30), change the escape character. You can change it for the current session with the terminal escape-character char exec-level command or permanently with the escape-character char line configuration command. For example, to set the escape character for the current session to ctrl-C, use terminal escape-character 3 command.

With IOS file system (IFS) introduced in IOS release 11.3AA (integrated in 12.0), IOS got the more command that displays any local or remote file that you could specify with IFS. The obvious use of the more command would be display of startup configuration (more nvram:startup-config), but you could also display built-in Tcl EEM policies (for example, more system:lib/tcl/http.tcl) or remote router configurations (for example, more tftp://host/cfg-file). But that's not all, you could even troubleshoot web servers and display HTML generated by the web server (for example, more http://192.168.0.2/index.html).

The IOS command line interface has long included unix-style pipes that you could use to limit the output generated by the show commmands. Initially, the only available filters were begin (include everything after the first regular expression match), end (stop the output at the RE match) or include (include only matching lines).

IOS release 12.3(2)T (integrated in 12.4) brought us a few new filters, among them the section filter that includes or excludes whole sections (start of section being defined by a line with no leading space). You can use this filter to focus on parts of your router configuration. For example, to display only the routing protocols configuration, use show running | section include router command.

If you want to disable the Cisco IOS more ... prompt (for example, when listing router's configuration with the show running command), set the terminal screen length to zero with the terminal length 0 exec-mode command. To change the terminal lenght permanently, use the length lines line configuration command, for example:

line console
 length 0
line vty 0 4
 length 0

I always hate it when Cisco IOS asks me for things I've already supplied in a command line, the most notable case being the copy command. For example, if you supply the complete source and destination file name in the command line, IOS still insists on asking you all the same questions (at least filling in the parameters I've supplied in the command line):

Cisco IOS supports the Unix tar format with the archive command. For example, to inspect the contents of the Secure Device Manager (SDM) that is present in Flash memory on most routers, use the archive tar /table flash:sdm.tar command.

You can also use the archive tar /xtract command to extract a tar file (local or external) into a directory (yet again local or external). For example, with the command archive tar /xtract flash:sdm.tar tftp://10.0.0.10 you'd extract the SDM tar archive to a TFTP server.