When attempting to run tmux as a non-root user, you may encounter the following error:

create session failed: : No such file or directory

The most effective way to diagnose this issue is using strace:

strace -f -e trace=file tmux

This typically reveals permission problems with pseudo-terminal devices:

open("/dev/ptmx", O_RDWR)         = -1 EACCES (Permission denied)
open("/dev/ptyp0", O_RDWR)        = -1 EACCES (Permission denied)
...
open("/dev/ptyq0", O_RDWR)        = -1 ENOENT (No such file or directory)

The issue stems from incorrect permissions on the pseudo-terminal devices in /dev. These devices are crucial for terminal multiplexers like tmux to function properly.

There are several approaches to resolve this:

1. Check and Fix Device File Permissions

ls -l /dev/ptmx /dev/pts/
sudo chmod 666 /dev/ptmx
sudo chmod g+rw /dev/pts/*

2. Verify the pts Group

Ensure your user is in the correct group:

groups
sudo usermod -a -G tty $(whoami)
sudo usermod -a -G pts $(whoami)

3. Check udev Rules (System-specific)

For systems using udev, verify the rules:

cat /etc/udev/rules.d/* | grep pty

To make the changes permanent, create a udev rule:

echo 'KERNEL=="ptmx", MODE="0666"' | sudo tee /etc/udev/rules.d/99-ptmx.rules
sudo udevadm control --reload-rules

After implementing the changes, verify tmux works:

tmux new-session -d -s test_session
tmux ls

If permission changes aren't possible, consider:

• Using screen instead of tmux
• Running tmux in a container with proper permissions
• Configuring sudo rules for specific tmux commands

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When attempting to run tmux as a non-root user on your VPS, you're encountering the frustrating error:

create session failed: : No such file or directory

Through strace diagnostics (strace -f -e trace=file tmux), we can see the underlying issue is permission denial on pseudo-terminal devices:

32000 open("/dev/ptmx", O_RDWR)         = -1 EACCES (Permission denied)
32000 open("/dev/ptyp0", O_RDWR)        = -1 EACCES (Permission denied)

Modern Linux systems handle pseudo-terminals through two mechanisms:

1. The legacy BSD-style /dev/pty* devices
2. The newer Unix98 /dev/ptmx device

Your system appears to have restrictive permissions on these critical devices. Let's verify the current permissions:

ls -l /dev/ptmx
ls -l /dev/pts/

First, check and potentially fix the device group ownership:

sudo chmod 666 /dev/ptmx
sudo chown root:tty /dev/ptmx

For modern systems using devpts, ensure proper mount options in /etc/fstab:

devpts  /dev/pts  devpts  gid=5,mode=620  0  0

After making changes, verify with:

tmux new -s test_session
echo $?

Successful execution should return exit code 0. If problems persist, check kernel messages:

dmesg | grep -i pty

If you can't modify system permissions, consider these workarounds:

# Run tmux through sudo with proper permissions
sudo -u your_user tmux

# Or create an alias in ~/.bashrc
alias tmux='sudo -u $(whoami) tmux'

On modern systems using SystemD, you might need to check:

systemctl status systemd-udevd
journalctl -u systemd-udevd

Look for any errors related to device creation or permissions.