Pseudo-terminal will not be allocated because stdin is not a terminal

I am trying to write a shell script that creates some directories on a remote server and then uses scp to copy files from my local machine onto the remote. Here’s what I have so far:

ssh -t [email protected]<<EOT
DEP_ROOT='/home/matthewr/releases'
datestamp=$(date +%Y%m%d%H%M%S)
REL_DIR=$DEP_ROOT"/"$datestamp
if [ ! -d "$DEP_ROOT" ]; then
    echo "creating the root directory"
    mkdir $DEP_ROOT
fi
mkdir $REL_DIR
exit
EOT

scp ./dir1 [email protected]:$REL_DIR
scp ./dir2 [email protected]:$REL_DIR

Whenever I run it I get this message:

Pseudo-terminal will not be allocated because stdin is not a terminal.

And the script just hangs forever.

My public key is trusted on the server and I can run all the commands outside of the script just fine. Any ideas?

10 Answers

Try ssh -t -t(or ssh -tt for short) to force pseudo-tty allocation even if stdin isn’t a terminal.

See also: Terminating SSH session executed by bash script

From ssh manpage:

-T      Disable pseudo-tty allocation.

-t      Force pseudo-tty allocation.  This can be used to execute arbitrary 
        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.

Also with option -T from manual

Disable pseudo-tty allocation

Per zanco’s answer, you’re not providing a remote command to ssh, given how the shell parses the command line. To solve this problem, change the syntax of your ssh command invocation so that the remote command is comprised of a syntactically correct, multi-line string.

There are a variety of syntaxes that can be used. For example, since commands can be piped into bash and sh, and probably other shells too, the simplest solution is to just combine ssh shell invocation with heredocs:

ssh [email protected] /bin/bash <<'EOT'
echo "These commands will be run on: $( uname -a )"
echo "They are executed by: $( whoami )"
EOT

Note that executing the above without /bin/bash will result in the warning Pseudo-terminal will not be allocated because stdin is not a terminal. Also note that EOT is surrounded by single-quotes, so that bash recognizes the heredoc as a nowdoc, turning off local variable interpolation so that the command text will be passed as-is to ssh.

If you are a fan of pipes, you can rewrite the above as follows:

cat <<'EOT' | ssh [email protected] /bin/bash
echo "These commands will be run on: $( uname -a )"
echo "They are executed by: $( whoami )"
EOT

The same caveat about /bin/bash applies to the above.

Another valid approach is to pass the multi-line remote command as a single string, using multiple layers of bash variable interpolation as follows:

ssh [email protected] "$( cat <<'EOT'
echo "These commands will be run on: $( uname -a )"
echo "They are executed by: $( whoami )"
EOT
)"

The solution above fixes this problem in the following manner:

  1. ssh [email protected] is parsed by bash, and is interpreted to be the ssh command, followed by an argument [email protected] to be passed to the ssh command

  2. " begins an interpolated string, which when completed, will comprise an argument to be passed to the ssh command, which in this case will be interpreted by ssh to be the remote command to execute as [email protected]

  3. $( begins a command to be executed, with the output being captured by the surrounding interpolated string

  4. cat is a command to output the contents of whatever file follows. The output of cat will be passed back into the capturing interpolated string

  5. << begins a bash heredoc

  6. 'EOT' specifies that the name of the heredoc is EOT. The single quotes ' surrounding EOT specifies that the heredoc should be parsed as a nowdoc, which is a special form of heredoc in which the contents do not get interpolated by bash, but rather passed on in literal format

  7. Any content that is encountered between <<'EOT' and <newline>EOT<newline> will be appended to the nowdoc output

  8. EOT terminates the nowdoc, resulting in a nowdoc temporary file being created and passed back to the calling cat command. cat outputs the nowdoc and passes the output back to the capturing interpolated string

  9. ) concludes the command to be executed

  10. " concludes the capturing interpolated string. The contents of the interpolated string will be passed back to ssh as a single command line argument, which ssh will interpret as the remote command to execute as [email protected]

If you need to avoid using external tools like cat, and don’t mind having two statements instead of one, use the read built-in with a heredoc to generate the SSH command:

IFS='' read -r -d '' SSH_COMMAND <<'EOT'
echo "These commands will be run on: $( uname -a )"
echo "They are executed by: $( whoami )"
EOT

ssh [email protected] "${SSH_COMMAND}"

I’m adding this answer because it solved a related problem that I was having with the same error message.

Problem: I had installed cygwin under Windows and was getting this error: Pseudo-terminal will not be allocated because stdin is not a terminal

Resolution: It turns out that I had not installed the openssh client program and utilities. Because of that cygwin was using the Windows implementation of ssh, not the cygwin version. The solution was to install the openssh cygwin package.

The warning message Pseudo-terminal will not be allocated because stdin is not a terminal. is due to the fact that no command is specified for ssh while stdin is redirected from a here document. Due to the lack of a specified command as an argument ssh first expects an interactive login session (which would require the allocation of a pty on the remote host) but then has to realize that its local stdin is no tty/pty. Redirecting ssh‘s stdin from a here document normally requires a command (such as /bin/sh) to be specified as an argument to ssh – and in such a case no pty will be allocated on the remote host by default.

Since there are no commands to be executed via ssh that require the presence of a tty/pty (such as vim or top) the -t switch to ssh is superfluous. Just use ssh -T [email protected] <<EOT ... or ssh [email protected] /bin/bash <<EOT ... and the warning will go away.

If <<EOF is not escaped or single-quoted (i. e. <<EOT or <<'EOT') variables inside the here document will be expanded by the local shell before it is executing ssh .... The effect is that the variables inside the here document will remain empty because they are defined only in the remote shell.

So, if $REL_DIR should be both accessible by the local shell and defined in the remote shell, $REL_DIR has to be defined outside the here document before the ssh command (version 1 below); or, if <<EOT or <<'EOT' is used, the output of the ssh command can be assigned to REL_DIR if the only output of the ssh command to stdout is genererated by echo "$REL_DIR" inside the escaped/single-quoted here document (version 2 below).

A third option would be to store the here document in a variable and then pass this variable as a command argument to ssh -t [email protected] "$heredoc" (version 3 below).

And, last but not least, it would be no bad idea to check if the directories on the remote host were created successfully (see: check if file exists on remote host with ssh).

# version 1

unset DEP_ROOT REL_DIR
DEP_ROOT='/tmp'
datestamp=$(date +%Y%m%d%H%M%S)
REL_DIR="${DEP_ROOT}/${datestamp}"

ssh localhost /bin/bash <<EOF
if [ ! -d "$DEP_ROOT" ] && [ ! -e "$DEP_ROOT" ]; then
   echo "creating the root directory" 1>&2
   mkdir "$DEP_ROOT"
fi
mkdir "$REL_DIR"
#echo "$REL_DIR"
exit
EOF

scp -r ./dir1 [email protected]:"$REL_DIR"
scp -r ./dir2 [email protected]:"$REL_DIR"


# version 2

REL_DIR="$(
ssh localhost /bin/bash <<EOF
DEP_ROOT='/tmp'
datestamp=$(date +%Y%m%d%H%M%S)
REL_DIR="${DEP_ROOT}/${datestamp}"
if [ ! -d "$DEP_ROOT" ] && [ ! -e "$DEP_ROOT" ]; then
   echo "creating the root directory" 1>&2
   mkdir "$DEP_ROOT"
fi
mkdir "$REL_DIR"
echo "$REL_DIR"
exit
EOF
)"

scp -r ./dir1 [email protected]:"$REL_DIR"
scp -r ./dir2 [email protected]:"$REL_DIR"


# version 3

heredoc="$(cat <<'EOF'
# -onlcr: prevent the terminal from converting bare line feeds to carriage return/line feed pairs
stty -echo -onlcr
DEP_ROOT='/tmp'
datestamp="$(date +%Y%m%d%H%M%S)"
REL_DIR="${DEP_ROOT}/${datestamp}"
if [ ! -d "$DEP_ROOT" ] && [ ! -e "$DEP_ROOT" ]; then
   echo "creating the root directory" 1>&2
   mkdir "$DEP_ROOT"
fi
mkdir "$REL_DIR"
echo "$REL_DIR"
stty echo onlcr
exit
EOF
)"

REL_DIR="$(ssh -t localhost "$heredoc")"

scp -r ./dir1 [email protected]:"$REL_DIR"
scp -r ./dir2 [email protected]:"$REL_DIR"

All relevant information is in the existing answers, but let me attempt a pragmatic summary:

tl;dr:

  • DO pass the commands to run using a command-line argument:
    ssh [email protected] '...'

    • '...' strings can span multiple lines, so you can keep your code readable even without the use of a here-document:
      ssh [email protected] ' ... '
  • Do NOT pass the commands via stdin, as is the case when you use a here-document:
    ssh [email protected] <<'EOF' # Do NOT do this ... EOF

Passing the commands as an argument works as-is, and:

  • the problem with the pseudo-terminal will not even arise.
  • you won’t need an exit statement at the end of your commands, because the session will automatically exit after the commands have been processed.

In short: passing commands via stdin is a mechanism that is at odds with ssh‘s design and causes problems that must then be worked around.
Read on, if you want to know more.


Optional background information:

ssh‘s mechanism for accepting commands to execute on the target server is a command-line argument: the final operand (non-option argument) accepts a string containing one or more shell commands.

  • By default, these commands run unattended, in an non-interactive shell, without the use of a (pseudo) terminal (option -T is implied), and the session automatically ends when the last command finishes processing.

  • In the event that your commands require user interaction, such as responding to an interactive prompt, you can explicitly request the creation of a pty (pseudo-tty), a pseudo terminal, that enables interacting with the remote session, using the -t option; e.g.:

    • ssh -t [email protected] 'read -p "Enter something: "; echo "Entered: [$REPLY]"'

    • Note that the interactive read prompt only works correctly with a pty, so the -t option is needed.

    • Using a pty has a notable side effect: stdout and stderr are combined and both reported via stdout; in other words: you lose the distinction between regular and error output; e.g.:

In the absence of this argument, ssh creates an interactive shell – including when you send commands via stdin, which is where the trouble begins:

  • For an interactive shell, ssh normally allocates a pty (pseudo-terminal) by default, except if its stdin is not connected to a (real) terminal.

    • Sending commands via stdin means that ssh‘s stdin is no longer connected to a terminal, so no pty is created, and ssh warns you accordingly:
      Pseudo-terminal will not be allocated because stdin is not a terminal.

    • Even the -t option, whose express purpose is to request creation of a pty, is not enough in this case: you’ll get the same warning.

      • Somewhat curiously, you must then double the -t option to force creation of a pty: ssh -t -t ... or ssh -tt ... shows that you really, really mean it.

      • Perhaps the rationale for requiring this very deliberate step is that things may not work as expected. For instance, on macOS 10.12, the apparent equivalent of the above command, providing the commands via stdin and using -tt, does not work properly; the session gets stuck after responding to the read prompt:
        ssh -tt [email protected] <<<'read -p "Enter something: "; echo "Entered: [$REPLY]"'


In the unlikely event that the commands you want to pass as an argument make the command line too long for your system (if its length approaches getconf ARG_MAX – see this article), consider copying the code to the remote system in the form of a script first (using, e.g., scp), and then send a command to execute that script.

In a pinch, use -T, and provide the commands via stdin, with a trailing exit command, but note that if you also need interactive features, using -tt in lieu of -T may not work.

I don’t know where the hang comes from, but redirecting (or piping) commands into an interactive ssh is in general a recipe for problems. It is more robust to use the command-to-run-as-a-last-argument style and pass the script on the ssh command line:

ssh [email protected] 'DEP_ROOT="/home/matthewr/releases"
datestamp=$(date +%Y%m%d%H%M%S)
REL_DIR=$DEP_ROOT"/"$datestamp
if [ ! -d "$DEP_ROOT" ]; then
    echo "creating the root directory"
    mkdir $DEP_ROOT
fi
mkdir $REL_DIR'

(All in one giant '-delimited multiline command-line argument).

The pseudo-terminal message is because of your -t which asks ssh to try to make the environment it runs on the remote machine look like an actual terminal to the programs that run there. Your ssh client is refusing to do that because its own standard input is not a terminal, so it has no way to pass the special terminal APIs onwards from the remote machine to your actual terminal at the local end.

What were you trying to achieve with -t anyway?

After reading a lot of these answers I thought I would share my resulting solution. All I added is /bin/bash before the heredoc and it doesn’t give the error anymore.

Use this:

ssh [email protected] /bin/bash <<'ENDSSH'
   hostname
ENDSSH

Instead of this (gives error):

ssh [email protected] <<'ENDSSH'
   hostname
ENDSSH

Or use this:

ssh [email protected] /bin/bash < run-command.sh

Instead of this (gives error):

ssh [email protected] < run-command.sh

EXTRA:

If you still want a remote interactive prompt e.g. if the script you’re running remotely prompts you for a password or other information, because the previous solutions won’t allow you to type into the prompts.

ssh -t [email protected] "$(<run-command.sh)"

And if you also want to log the entire session in a file logfile.log:

ssh -t [email protected] "$(<run-command.sh)" | tee -a logfile.log

I was having the same error under Windows using emacs 24.5.1 to connect to some company servers through /ssh:[email protected] What solved my problem was setting the “tramp-default-method” variable to “plink” and whenever I connect to a server I ommit the ssh protocol. You need to have PuTTY’s plink.exe installed for this to work.

Solution

  1. M-x customize-variable (and then hit Enter)
  2. tramp-default-method (and then hit Enter again)
  3. On the text field put plink and then Apply and Save the buffer
  4. Whenever I try to access a remote server I now use C-x-f /[email protected]: and then input the password. The connection is now correctly made under Emacs on Windows to my remote server.

ssh -t [email protected] yourscript.pl

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