How to check if screen session is running?

Question

Is there any way to check if a screen session is running in bash?
For example:

if [screen is running]
  then
    screen -r          #if session is running then resume the session
  else
    screen "command"   #else start a new session
fi

Answer 1

Take advantage of the PPID (Parent PID) environment variable, and start with

$ ps -fp$PPID
UID        PID  PPID  C STIME TTY          TIME CMD
w3       19305 19304  0 00:00 ?        00:00:00 SCREEN
+w3@aardvark:~(0)$ 

or,

ps -fp$PPID | head -n 2 | tail -n 1 | egrep -q SCREEN
screen_is_running=$((1 - ${PIPESTATUS[-1]}))
# screen_is_running == 1 for yes, 0 for No, -1 for egrep error

Of course, this won't work if you've spawned, exec'ed, nohup'd or something, and made your $PPID not SCREEN.

If that's the case, you could build something with pgrep, pstree, egrep that could follow the $PPID chain back (stop when $PPID is 1).

Answer 2

By reading man screen you find the COMMAND LINE OPTIONS:

COMMAND-LINE OPTIONS
       Screen has the following command-line options:

    ...snip...

       -d -r   Reattach a session and if necessary detach it first.

       -d -R   Reattach a session and if necessary detach or even create it first.

       -d -RR  Reattach a session and if necessary detach or create it. Use the first session if more than one session is available.

       -D -r   Reattach a session. If necessary detach and logout remotely first.

       -D -R   Attach  here  and now. In detail this means: If a session is running, then reattach. If necessary detach and logout remotely first.  If it was not running create it and notify
               the user. This is the author's favorite.

       -D -RR  Attach here and now. Whatever that means, just do it.

            Note: It is always a good idea to check the status of your sessions by means of "screen -list".

Surely one of these would do what you want without the variable.