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Editorial Team
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Asked: 2026-05-18T11:59:58+00:00

I am trying to send input to a interactive command running via screen. Here

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I am trying to send input to a interactive command running via screen. Here is my initial command

screen -L -c ./customrc -S psql -d -m /opt/PostgreSQL/9.0/bin/psql

The above command will run interactive psql in screen detach mode. The customrc is used to define a log file for the output (which I will read from another process by polling)

I am using following two commands to send input to psql running in screen

screen -S psql -X readreg p psqlcommands.sql
screen -S psql -X paste p

The problem is that the above commands do not work unless I reattach screen at least once. Once I have attached screen and detached, the above commands work as expected. I have to launch these commands via background java process hence the interactive shell (bash) is not available. My goal is to run psql in interactive mode and pass input to it and capture its output via a log file.

So far I have tried to run screen via xterm (or konsole or gnome-terminal) in attach mode, use readreg/paste and then detach, but I realise that xterm will not be available in my production environment. I’ve also tried sending output to /proc//fd/0 but I am unable to emulate ‘ENTER’ from keyboard (I have to attach and press in order for the output to be accepted by psql). I think pipes and fifo may help but I am unable to figure out how to proceed with them using screen and psql.

I appreciate any further hints or workarounds.

Thank you,

Usman.

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1 Answer

  1. Editorial Team

    FINAL ANSWER: It is a bug/feature in ‘GNU screen’ that it needs a DISPLAY atleast once for ‘paste’ command to work. Following are possible workarounds this problem:

    Finally figured out how to utilise psql with pipes and screen. Here is the solution:

    mkfifo psql.pipe
screen -L -c ./customrc -S psql -d -m bash -i -c "while (true); do cat psql.pipe; done | /opt/PostgreSQL/9.0/bin/psql -a"

    After that, I can cat my commands to the pipe:

    cat ./mycommands.sql > psql.pipe

    To quit from screen and terminating psql, I used

    screen -S psql -X quit

    EDIT: (finally) figured out the solution for my problem without using screen command. Meet ‘empty‘ utility. 

    empty -f -i psql.in -o psql.o -p psql.pid <psqlpath>

    This allows psql to run in full interactive mode as opposed to the original solution that I used (in which psql does not run in interactive mode).

    Thanks.

    Usman

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