SSH Terminal allocation

The primary difference is the concept of interactivity. It's similar to running commands locally inside of a script, vs. typing them out yourself. It's different in that a remote command must choose a default, and non-interactive is safest. (and usually most honest)

If a PTY is allocated, applications can detect this and know that it's safe to prompt the user for additional input without breaking things. There are many programs that will skip the step of prompting the user for input if there is no terminal present, and that's a good thing. It would cause scripts to hang unnecessarily otherwise.
Your input will be sent to the remote server for the duration of the command. This includes control sequences. While a Ctrl-c break would normally cause a loop on the ssh command to break immediately, your control sequences will instead be sent to the remote server. This results in a need to "hammer" the keystroke to ensure that it arrives when control leaves the ssh command, but before the next ssh command begins.
I would caution against using ssh -t in unattended scripts, such as crons. A non-interactive shell asking a remote command to behave interactively for input is asking for all kinds of trouble.

You can also test for the presence of a terminal in your own shell scripts. To test STDIN with newer versions of bash:

# fd 0 is STDIN
[ -t 0 ]; echo $?
When aliasing ssh to ssh -t, you can expect to get an extra carriage return in your line ends. It may not be visible to you, but it's there; it will show up as ^M when piped to cat -e. You must then expend the additional effort of ensuring that this control code does not get assigned to your variables, particularly if you're going to insert that output into a database.
There is also the risk that programs will assume they can render output that is not friendly for file redirection. Normally if you were to redirect STDOUT to a file, the program would recognize that your STDOUT is not a terminal and omit any color codes. If the STDOUT redirection is from the output of the ssh client and the there is a PTY associated with the remote end of the client, the remote programs cannot make such a distinction and you will end up with terminal garbage in your output file. Redirecting output to a file on the remote end of the connection should still work as expected.
Here is the same bash test as earlier, but for STDOUT:

# fd 1 is STDOUT
[ -t 1 ]; echo $?
While it's possible to work around these issues, you're inevitably going to forget to design scripts around them. All of us do at some point. Your team members may also not realize/remember that this alias is in place, which will in turn create problems for you when they write scripts that use your alias.

Aliasing ssh to ssh -t is very much a case where you'll be violating the design principle of least surprise; people will be encountering problems they do not expect and may not understand what is causing them.

SSH escape characters and transfer of binary files
One advantage that hasn’t been mentioned in the other answers is that when operating without a pseudo-terminal, the SSH escape characters such as ~C are not supported; this makes it safe for programs to transfer binary files which may contain these sequences.

Proof of concept
Copy a binary file using a pseudo-terminal:

$ ssh -t anthony@remote_host 'cat /usr/bin/free' > ~/free
Connection to remote_host closed.
Copy a binary file without using a pseudo-terminal:

$ ssh anthony@remote_host 'cat /usr/bin/free' > ~/free2
The two files aren’t the same:

$ diff ~/free*
Binary files /home/anthony/free and /home/anthony/free2 differ
The one which was copied with a pseudo-terminal is corrupted:

$ chmod +x ~/free*
$ ./free
Segmentation fault
while the other isn’t:

$ ./free2
total used free shared buffers cached
Mem: 2065496 1980876 84620 0 48264 1502444
-/+ buffers/cache: 430168 1635328
Swap: 4128760 112 4128648
Transferring files over SSH
This is particularly important for programs such as scp or rsync which use SSH for data transfer. This detailed description of how the SCP protocol works explains how the SCP protocol consists of a mixture of textual protocol messages and binary file data.

OpenSSH helps protects you from yourself
It’s worth noting that even if the -t flag is used, the OpenSSH ssh client will refuse to allocate a pseudo-terminal if it detects that its stdin stream is not a terminal:

$ echo testing | ssh -t anthony@remote_host 'echo $TERM'
Pseudo-terminal will not be allocated because stdin is not a terminal.
You can still force the OpenSSH client to allocate a pseudo-terminal with -tt:

$ echo testing | ssh -tt anthony@remote_host 'echo $TERM'
In either case, it (sensibly) doesn’t care if stdout or stderr are redirected:

$ ssh -t anthony@remote_host 'echo $TERM' >| ssh_output
Connection to remote_host closed.


2019-04-16 22:03:29


Add a Comment

Login or Register to post a Comment.