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Every once in a while I will do something like

ssh user@host sudo thing

and I am reminded that ssh doesn't allocate a pseudo-tty by default. Why doesn't it? What benefits would I be losing if I aliased ssh to ssh -t?

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> I am reminded that ssh doesn't allocate a psuedo-tty What happens? It would enrich the question to understand what the problem is. –  Air Sep 24 at 21:30
    
@Air There isn't a problem I was trying to fix. There was a choice in how ssh is implemented that I was trying to understand. The question is very clear and the answer by Andrew B addresses the question nicely. The answer can be summed up like this: running ssh -t always is bad because it can cause some commands to break in weird ways. Whereas, running a command that needs a PTY without one results in a clear error message that you need a terminal. –  Chas. Owens Sep 24 at 22:18

3 Answers 3

up vote 16 down vote accepted

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)

STDIN

  • 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 $?

STDOUT

  • 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.

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3  
One almost gets the impression that I've worked on a team that has done this... –  Andrew B May 6 at 17:14

From man ssh:

 -t      Force pseudo-tty allocation.  This can be used to execute arbi-
         trary screen-based programs on a remote machine, which can be
         very useful, e.g. when implementing menu services.  Multiple -t
         options force tty allocation, even if ssh has no local tty.

This allows you to get a "shell" of sorts to the remote server. For servers that do not grant shell access but allow SSH (i.e, Github is a known example for SFTP access), using this flag will cause the server to reject your connection.

The shell also has all your environmental variables (like $PATH) so executing scripts generally needs a tty to work.

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This does not answer the question. I already know how to allocate a pseudo-tty. I want to know why I shouldn't just always allocate one. –  Chas. Owens May 6 at 14:59
1  
@Chas.Owens Because, as I pointed out in the answer some SSH servers do not allow tty access and it'll drop the connection if you request one from the server. –  Nathan C May 6 at 15:10
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I think you may be getting some of your terminology confused. It's not a shell simply because a PTY is associated with it. There are generally three types of shell: non-interactive, interactive, and login. login is an additional characteristic of the other two shell types. The permutations of these three determine which files are sourced on login, which in turn influences how the environment will be initialized. (variables, as you were mentioning) –  Andrew B May 6 at 16:29
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@AndrewB You're right...I learned something from this question too. :) –  Nathan C May 7 at 0:09

Think about backward-compatibility.

The 2 primary modes of ssh are interactive-login with a tty, and specified-command without a tty, because those were the exact capabilities of rlogin and rsh respectively. ssh needed to provide a superset of rlogin/rsh features to be successful as a replacement.

So the defaults were decided before ssh was born. Combinations like "I wanna specify a command and get a tty" had to be accessed with new options. Be glad that at least we have that option now, unlike when we were using rsh. We didn't trade away any useful features to get encrypted connections. We got bonus features!

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