When using Expect to run commands on a remote Un*x system, we can trigger off of carefully-constructed events, to give us greater insight into the state of the remote machine:

event: process ended

Instead of running some_command, we run some_command; echo 's'ome_command_ended. If we ever see the string "some_command_ended" sent back, then we know for sure that that command ended.

This is PARTICULARLY useful when we're running sudo/ssh/telnet from a remote host (ie. we're doing multi-hop connections). If we ever see the trigger string, then we know that the telnet/ssh command finished, and that we have returned to the previous machine in the connection-chain.

event: shell started

If we just entered the password, and we think the shell will be starting soon, we can send a command like echo 's'hell_started_detected. If we ever see the string "shell_started_detected" sent back, then we know the shell started successfully.

The usual procedure is to wait for the machine's prompt. This technique allows us to detect the shell-start event in a prompt-independent way. (however, it does require us to know what the machine's shell is)

event: before process starts

Goal: make sure we can distinguish between a command's output, and the command-line prompt text.

To do this, instead of some_command, we run echo 's'ome_command_beforestart; some_command; echo 's'ome_command_ended. As soon as we see the text "some_command_beforestart", we know that all text after that will be the output of 'some_command'.






detection strings

We should use long and randomly-generated trigger-strings to ensure that there is no possibility of accidental conflict with normal data.

shell behavior

Here, we're depending on the shell to convert a string like 's'ome_command_started to "some_command_started". It's possible we can't always rely on the remote shell to do this, if the remote shell is something very different from Bash/Csh. (eg. Cisco IOS)

Alternatives include:

see also

Other pages that attempt to improve the reliability of Expect sessions include: