bash linux - write and read from stdin and stdout

Question

I have an application which I can run from the commonand line and it acts like a CLI for me.

However its part of a suite of applications and now I want them all to run on boot up and so they are being started as a service (systemd).

So all my apps includeing my CLI are running great. However I can't see the output of the CLI and I can't send commands to it.

I was using the stdout to print the CLI screen - I guess I can work around this by outputting to a file - but it would be nice to get access to its stdout. The bigger issue is that I can't write to its stdin.

I read this: writing-to-stdin-of-background-process and tried to echo -e "command\n" > /proc/.../0, but nothing happened - I checked the log of my CLI (gets written to a file) and it did not get the input.

I then did a ls -l on /proc/<pid>/fd/ and I notice that 0 --> /dev/null meaning stdin is linked to /dev/null.

So, how can I get access to its stdin?

Bonus question - is there a way to operate (in the same bash shell) such that I re-direct my stdin to the stdin of this process AND redirect the process stdout to my shell?

I don't know if it works, but one idea is to create fifo using mkfifo /tmp/hoo. And in the systemd file set: ExecStart=/bin/sh -c '/your/cli/command < /tmp/hoo'. Now if you echo -e "command\n" > /tmp/hoo" it should be piped to /your/cli/command — ymonad
– ymonad, Commented Apr 11, 2019 at 8:23
@ymonad I am just testing this on the command line first. When I run ./myapp < /tmp/hoo (after making the fifo) it does not seem to run the app until I do my first echo command to it. That's quite strange! - but it does start to work after that — code_fodder
– code_fodder, Commented Apr 11, 2019 at 8:41
maybe your cli command is not intended to access via stdin. did you checked that echo command | /your/command works? — ymonad
– ymonad, Commented Apr 11, 2019 at 9:19
@ymonad - yes that worked... Your first idea did work, but the app did not seem to start until the received the first command - and then it ran normally — code_fodder
– code_fodder, Commented Apr 11, 2019 at 9:37

Kubator · Accepted Answer · 2019-04-11 08:43:51Z

2

Find the terminal (pty) that You want to redirect. Example for pty0 redirection can be done via:

exec < /dev/pty0  #stdin
exec > /dev/pty0  #stdout

answered Apr 11, 2019 at 8:43

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2 Comments

code_fodder Over a year ago

How do you find the /dev/pty?

Kubator Over a year ago

Similar You did for background process /proc/<pid>/fd/. So go to active terminal , then do echo $$, that will gives You pid of current shell and if shell is interactive it must have set pty in /proc/<pid>/fd/

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