How to pipe commands to any terminal

commandfifopipeterminaltty

I often use more than one terminal (or terminal emulator) at a time; and while in X I can copy-paste commands, besides not being very practical, it obviously does not work on the real TTY. The first idea that occurs to me is something alike:

command > /dev/sometty

Unfortunately, the command is run before it is piped, and no tricks like echo `command` will work, no matter how many weird bash characters ($, `, ", etc.) are there. So /dev/sometty simply gets some text.

The problem is that, sometimes, it is even worth for I to pipe those commands into a file, make it executable, etc., or shortly: making a script and running it from the appropriate terminal. But that is a lot of work. I've been thinking of making a script to make these files, for an instruction:

termpipe -e "command1\ncommand2\"command 1 argument\\n(s)\"" -t /dev/sometty

it would do something like:

  1. pipe the commands to, say, /tmp/termpipe-20140208-153029-1.sh
  2. make the file executable
  3. run the file in the appropriate terminal
  4. delete the script when it is done executing

AFAIK, the problem is in 3.: this doesn't solve any problem, as I'd need another termpipe instance to run the first on the appropriate terminal. And one for that one. And another for that, ad infinitum. So this cannot work. Or can it?…

The solution could be to use a named pipe per terminal, and when each starts, a script would tell the pipe to forward anything it receives to the terminal, and then to execute it (like a sort of daemon).

I think this might work but I don't know how to set up the initial script. How can I? How can I tell the FIFO to give piped commands for the respective terminal to run? I don't know much Bash, so I'd appreciate full explanations.

Best Answer

Following this, one can very well make that last plan of yours work. For the command to-be-sent not to be processed by the shell, it has to be in the form of a string when reaches the pipe (thus echo "command", not echo `command`). Then it has to be read by a background process (alike a daemon, but not necessarily) started in the appropriate terminal. It should be evaluated by the same process.

But it is boiler-platey to have a script per pipe. So let's generalize making a script as term-pipe-r.sh (don't forget to chmod +x it!):

#!/bin/bash

pipe=$1                     # the pipe name is the first argument
trap 'rm -f "$pipe"' EXIT     # ignore exit and delete messages until the end

if [[ ! -p $pipe ]]; then   # if the pipe doesn't exist, create it
    mkfifo $pipe
fi

while true                  # cycle eternally..
do
    if read line <$ pipe; then
        if [[ "$line" == 'close the term-pipe pipe' ]]; then
            break
            # if the pipe closing message is received, break the while cycle
        fi

        echo                # a line break should be used because of the prompt 
        eval $line          # run the line: as this script should be started
    fi                          # in the target terminal, 
done                            # the line will be run there.

echo "<pipe closing message>"   # custom message on the end of the script

So say you want /dev/tty3 to receive commands: just go there, do

./term-pipe-r.sh tty3pipe &     # $1 will be tty3pipe (in a new process)

And to send commands, from any terminal (even from itself):

echo "command" > tty3pipe

or to run a file there:

cat some-script.sh > tty3pipe

Note this piping ignores files like .bashrc, and the aliases in there, such as alias ls='ls --color'. Hope this helps someone out there.

Edit (note - advantage of non-daemon):

Above I talked about the pipe reader not being a daemon necessarily, but in fact, I checked the differences, and it turns out it is way better to be a mere background process in this case. Because this way, when you close the terminal, an EXIT signal (SIGHUP, SIGTERM, or whatever) is received by the script as well, and the pipe is deleted then (see the line starting with trap in the script) automatically, avoiding a useless process and file (and maybe others if there were such redirecting to the useless pipe).

Edit (automation):

Still, it is boring to have to run a script you (I, at least) probably want most of the time. So, let's automatize it! It should start in any terminal, and one thing all of them read is .bashrc. Plus, it sucks to have to use ./term-pipe-r.sh. So, one may do:

cd /bin # go to /bin, where Bash gets command names
ln -s /directory/of/term-pipe-r.sh tpr  # call it tpr (terminal pipe reader)

Now to run it you'd only need tpr tty3pipe & in /dev/tty3 whenever you'd want. But why do that when you can have it done automatically? So this should be added to .bashrc. But wait: how will it know the pipe name? It can base the name on the TTY (which can be know with the tty command), using simple REGEX's in sed (and some tricks). What you should add to ~/.bashrc will then be:

pipe="$(sed 's/\/dev\///' <<< `tty` | sed 's/\///')pipe"
                # ^^^- take out '/dev/' and other '/', add 'pipe'
tpr $pipe &     # start our script with the appropriate pipe name
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