tmux-output

I made this because of a question on unix.stackexchange.com, which touched upon this problem. I tought that it may be convenient to have a "quick" way to send commands to a shell-like program and get the result synchronously. If the shell has a prompt string or symbol, waiting for the symbol would make more sense than what this is doing here. This is meant for "symbol-idependent detection" of when a command is done.

Currently, this is not really stable or reliable This is a proof-of-concept. Suggestions and or PRs are welcome.

How to use

You can start the a tmux session like that:

~# tmux new-session -d -s mysession "python3"

How to run a command (and get the output):

~# ./trun.sh -s mysession -c "the command"

It should return the desired output. You may have to adjust the -H and -T parameters. In some cases, like the python3 interpreter, you may have to use -e "C-m C-m (double enter) for multiline commands.

Theory of operation

Send command tmux send-keys -t $session_name "$command" C-m will send a command to the program running in the tmux session.

Detect when the command is done To detect when the command is done, these checks are performed in a loop:

• Check if process is in interruptable sleep
• Check if process is running
• If both previous conditions apply, check if stacktrace "looks stuck"
• If all of the statements are true, the command should be completed. (It could still get stuck at something else...)

About the stack trace To detect at which point the program is stuck at a relevant IO Operation we can run a stack trace and look for that. Also see: How do I tell if a command is running or waiting for user input?(askubuntu).

That may be a bit tricky because it may be temporarily stuck at another read or something like that before the desired call. It may also have to involve some guessing based on how long it is stuck and at which file descriptor.

While testing this, I've encountered read(10, (zsh) and select(1, [0], NULL, NULL, NULL (python3 cli interpreter). There may be few other possibilities, which is why I didn't limit the RegEx in the script to just that. It is just looking for an open parenthesis now.

Capturing the output

• Prior to sending the command, create a temporary file and pipe the tmux output into it: tmux pipe-pane -t $session_name -o "cat > $tmpfile".

• Send the command and wait for completion

• Stop piping into the file: tmux pipe-pane -t $session_name

• Output the file contents (and crop as needed)

Tests

I tested this with python3 and zsh:

Python

~# tmux new-session -d -s pytest "python3"
~# ./trun.sh -s pytest -c 'for i in range(1000000): print("test")' -e "C-m C-m"
test
[redacted]
test

ZSH

~# tmux new-session -d -s zshtest "/bin/zsh"
~# ./trun.sh -s zshtest -c 'ls' -T 1
README.md trun.sh