Difference(s) between a background process and a daemon in linux

Question

Background processes don't belong to a user and a terminal, nor do daemon processes. What is the main difference between the two? If I were to write a server program, should I run it as a background process or a daemon?

Answer 1

When one says 'background process', it's usually in the context of a shell (like bash), which implements job control.

When a process (or a process group) is put into the background, it's still part of the session created by the shell and will still have an association with the shell's controlling terminal. The standard input/output of a background process will still be linked to the terminal (unless explicitly changed). Also, depending on how the shell exits, it may send a SIGHUP signal to all the background processes (See this answer to know exactly when). Until the shell terminates, it remains the parent of the background process.

A daemon on the other hand does not have a controlling terminal and is usually explicitly made to be a child of the init process. The standard input/output of a dare usually redirected to /dev/null

Answer 2

A Background process usually refers to a process which:

Another process is its parent; eg, a shell;
It has standard streams (input, output, error) connected to that parent

The most common type is when you run a shell program with a trailing &. It generally shares the shell’s output streams, but will get a signal and stop if it tries to read from its input stream.

More significantly (usually), a background process like this is still parented, so signals to that process group will continue to it. If the parent process terminates, the children will receive signals that will most likely terminate them, as well. (This is probably the biggest difference between the two for most users.)

A Daemon process is one that:

Has no parent, ie, its parent process is the system (or container) initial thread, commonly systemd (Linux), init (other Unix), or launchd? (MacOS);
Typically has its output disconnected, or connected to a log file;
Typically has its input disconnected.

Daemons are usually also written to accept the “user hung up” signal (SIGHUP), which would terminate a program if not handled, as a special instruction to re-read their configuration files and continue working.

Most often, these are processes created by some system-level facility that continue to operate completely independently of user activity (logins, logouts, and the like). Things that, themselves, handle logins (getty or gdm and the like), as well as other network-facing services (web servers, mail servers, etc) may be daemons, as well as self-monitoring services like cron, or smartd.