How do I know the name of a tmux session?

Have a look here, using this post I understood that the one thing which distinguishes the tmux clients is the their tty but session names are tmux server property and is the same when we look at the session name from inside or outside a tmux client. Next, we may use tmux info | head which displays the the pid and tty of the client and the pid of the server.

tmux info | grep $$ | grep -q `ps -p $$ -o tty=`

The following command return value is 0 if we are inside a tmux client or non zero if we are running outside a tmux client. If you are running it from a script, make sure you use ppid instead of $$ since the tty is associated with the shell process not the script you are running.

One easy way but less robust to know if we are inside or outside tmux client is by looking at the TERM environment variable. It appears that outside the client it says 'xterm' but inside it says 'screen'. However, I find the first way much better.

Here's a perl script; making it a shell script or shell script function is an exercise left to the reader.

#!/opt/local/bin/perl

use strict;
use warnings;

if (exists $ENV{'TMUX'}) {
    # session ID is the last element
    # /tmp/1s/ffkjhk76sdgn/T/tmux-771/default,16772,8
    my($id) = $ENV{'TMUX'};
    $id =~ s{^.*,(\d+)$}{$1};

    # get session name
    my($cmd) = q(tmux ls -F "#{session_name}: #{session_id}");
    open(IN, "$cmd |") || die qq([ERROR] Cannot open pipe from "$cmd" - $!\n);

    while (<IN>) {
        chomp;

        if (m{(^.*):\s+\$$id$}) {
            print "Session name: <$1>\n";
            exit 0;
        }
    }

    print "Unable to determine TMUX session name\n";
    exit 1;
}
else {
    print "Not in a TMUX session\n";

    exit 1;
}

This produces the following output in a TMUX session:

<~> $ tmuxsessionname
Session name: <1>

<~> $ tmux rename-session "foobar"

<~> $ tmuxsessionname
Session name: <foobar>

Outside of a TMUX session it would produce the following:

<~> $ tmuxsessionname
Not in a TMUX session

You can ditch the print statements and just use the exit code if you want to use this in a shell script or something.