How to run a program in a screen, redirect all output to a file and detach

Question

Running a command in a screen and detaching is quite simple.

screen -S test -d -m echo "output of command that runs forever"

However I would also like to pipe all the output to a file for logging, how can run the following in a screen and detach.

echo "output of command that runs forever" &> output.log

Edit:

Just to clarify, I need this for a script so simple starting a screen and doing running the command and detaching is not an option.

Answer 1

You can wrap your command in an extra bash (or your actual shell) call and make redirection there:

$ screen -dm bash -c 'echo hello > ./out'
$ cat ./out
hello

From the man screen:

-d -m Start screen in "detached" mode. This creates a new session but doesn't attach to it. This is useful for system startup scripts.

Naming a session

It may be suitable to name your background jobs to disambiguate them in the screen -ls and to attach if neccessary:

$ screen -S mysession -dm sleep 20
$ screen -ls
8431.mysession  (10/04/2021 12:47:58 PM)    (Detached)

# attach
$ screen -r mysession

How to pass arguments

Parameterization may be a hassle though. The arguments being passed are fed into bash starting with $0 -- note the ignore_me_arg:

# note the outer-most quotes to be single -- we don't want them to
# be expanded on the caller's side, but on the callee's one

$ screen -dm bash -c 'echo $1 > ./out' ignore_me_arg hello
$ cat ./out
hello

As a final example, let's say you have a program super_script that does some stuff and prints all its arguments to stdout (as echo would do). You want to call it inside screen, make a redirection, and do all that jazz with different sets of arguments. Then this should do:

$ screen -dm bash -c 'super_script "$@"  > ./out.1' arg0 arg1
$ screen -dm bash -c 'super_script "$@"  > ./out.2' arg0 arg1 arg2
... wait for screen's to finish
$ cat out.1
arg1
$ cat out.2
arg1
arg2

Answer 2

screen -dmS workspace; screen -S workspace -X stuff $'ps aux > output-x\n'

I first create a detached session with the -d switch, I called my session workspace. I then send my command to the same session with -X stuff, I am using $'', but you could also use double quotes, but have to do a control M instead of a \n, which I don't like so I normally use the method I described above.

After this piece of code runs, you will find the output-x with the list of processes, and also if you do a:

screen -ls

you will see the session has been detached.

Since you said you are going to be running a script. You might want to have your script search for a detached session (I am using workspace), and if it exists send commands to that pre-existing session, instead of making a new session every time "screen -dmS sessionName" is ran, example is below:

    #!/bin/bash
    if ! ( screen -ls | grep workspace > /dev/null); then
       screen -dmS workspace;
    fi
    screen -S workspace -X stuff $'ps aux > output-x\n'

Answer 3

You can use the disown or nohup

usage: How to monitor WGET download progress after closing SSH session