A new process group is created every time we execute a command or a pipeline of commands in a shell. Inside a shell, a process group is usually called a job. In its turn, each process group belongs to a session. As such, a process group is a set of processes, and a session is a set of related process groups. This information about a process can be gathered from /proc/$PID/stat.

Process groups

A process group has its process group identificator PGID and a leader who created this group. The PID of the group leader is equal to the corresponding PGID. A process group lives as long as it has at least one member. All members of a process group can be killed or waited for using its PGID.

Sessions

For its part, a session is a collection of process groups. All members of a session identify themselves by the identical SID, which, as in a process group, also inherited from the session leader. All processes in the session share a single controlling terminal.

Shell job controol

Shells start processes in a single session, every pipeline gets a new job (process group). A session has shared controlling terminal, which is relied upon for listening for terminal exits and some signals (SIGINT, SIGSTP and SIGHUP). A session also has a single foreground job. Every other job is either stopped (after C-z, i.e. SIGSTP) or runs in the background (bg %1). When exiting, a shell will send SIGHUP to all jobs it has started and keeps track of. To make a job survive shell exit (when working with remote shells for example) nohup or disown can be used. nohup redirects standard fd's to files and sets SIG_INT flag, that ignores SIGHUP, before running execve(). disown makes the shell forget about a job, thus preventing it from sending SIGHUP.

Daemons

Daemons are long running processes running in the background without a controlling terminal, thus never receiving terminal-related signals from the kernel, like SIGINT, SIGSTP and SIGHUP. Daemons can be started manually using the double-fork technique, implemented in gnu's libc as daemon(), or using systemd, which handless all redirection and other nice features.