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What I'm trying to do is execute an ssh command that prints out the /etc/shadow file. Obviously that file requires root or sudo permission, and I don't have root creds, therefore I have to use sudo.

Here is the command I've worked out:

expect -c 'spawn -noecho ssh -q -t -S /tmp/t2.ssh dummy "sudo cat /etc/shadow";expect "assword"; send "password99\r";interact'

It outputs exactly what I need, but it also outputs the prompt for the sudo password on the very first line:

[sudo] password for student99:
root:$x$xxx:18029:0:99999:7:::
daemon:*:17575:0:99999:7:::
bin:*:17575:0:99999:7:::
sys:*:17575:0:99999:7:::
sync:*:17575:0:99999:7:::

Without using other programs (like grep, awk, tail, etc) is there a way to modify the expect command so that it prints only the output of the cat command and not the [sudo] password for student99: prompt?

Solution:
Thanks to @larsks, my final working command is: 

expect -c 'spawn -noecho ssh -q -t -S /tmp/t2.ssh dummy "sudo cat /etc/shadow";log_user 0;expect "assword"; send "password99\r";interact'
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Reset to default
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You can disable output logging by setting log_user 0. From the docs:

log_user -info|0|1
    By default, the send/expect dialogue is logged to stdout (and a logfile if
    open). The logging to stdout is disabled by the command "log_user 0" and
    reenabled by "log_user 1". Logging to the logfile is unchanged.

    The -info flag causes log_user to return a description of the most recent
    non-info arguments given.
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  • This is it! Thank you! My final command is: expect -c 'spawn -noecho ssh -q -t -S /tmp/t2.ssh dummy "sudo cat /etc/shadow";log_user 0;expect "assword"; send "password99\r";interact'
    – beechfuzz
    Commented May 22, 2019 at 2:27

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