devpts

devpts is a Linux pseudo-filesystem that provides dynamic pseudo-terminal (PTY) devices used by terminal emulators, shells, and remote sessions. It is typically mounted at /dev/pts and works in conjunction with the master device /dev/ptmx. When a program opens /dev/ptmx, the kernel allocates a new PTY pair and exposes the corresponding slave device under /dev/pts, usually as /dev/pts/N. The slave acts as the controlling terminal for the process or session, enabling standard terminal behavior such as line discipline, job control, and signal delivery.

The devpts filesystem is part of the Linux TTY subsystem. The slave devices created under /dev/pts function like traditional terminals, providing the necessary interfaces for input and output that terminal-based programs rely on. This dynamic allocation replaces the older approach of static, purely device-file-based PTYs.

Namespaces and isolation: modern Linux supports separate devpts instances per user namespace or container, achievable with the newinstance mount option. This allows different namespaces to have independent sets of PTYs, improving isolation in containers and other virtualization scenarios.

Mount options and ownership: devpts accepts options to control permissions and ownership of the created PTY nodes. Common options include gid to set the group ownership, mode to define permissions for the slave devices, and ptmxmode to control the permissions of /dev/ptmx. The exact options can vary by kernel version and distribution.

In practice, most systems mount devpts automatically during boot or via /etc/fstab, enabling standard terminal functionality for local and remote logins, as well as for tools such as SSH, screen, and tmux.