$TERM becomes unknown when sudoing via ssh

Recently, I found that $TERM becomes unknown when I told ssh to sudo on a remote server. In short,

$ ssh localhost sudo /bin/bash -c '"/bin/echo \$TERM"'

prints unknown while

$ ssh localhost /bin/bash -c '"/bin/echo \$TERM"'

prints dumb.

I thought it was strange because sudo should inherit TERM environment variable even when env_reset is enabled.

At first, I thought the ssh server set TERM environment variable to dumb, but it was not. Actually, TERM environment variable was even not set. So

$ ssh localhost env | grep TERM

prints nothing. I looked into the source code of bash and found that the fact is that bash defines TERM shell variable to dumb when it is not set. So, if you use /bin/sh (on Ubuntu, it’s dash),

$ ssh localhost /bin/sh -c '"/bin/echo \$TERM"'

prints an empty string.

Although bash defines TERM shell variable, it doesn’t export it as an environment variable.

Now, I looked into the source code of sudo and found that it sets TERM environment variable to unknown if it has not been set. That’s why $TERM becomes unknown in the first example.

As you know, if you pass -t to ssh, it allocates a pseudo terminal and let the server know its name. So $TERM becomes xterm or something based on what terminal you use on the client.