Some Linux binaries end with a "d", for example sshd, httpd, ppd, etc.
Why is this so?
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d at the end of some process means daemon.
Deamon means processes which works in background and services works in background.
Background here means that you don't have direct access to it and they aren't waiting for you! If you set that a service comes up after system booting it will run automatically.
A bit more technically:
Daemons are usually instantiated as processes. A process is an executing (i.e., running) instance of a program. Processes are managed by the kernel (i.e., the core of the operating system), which assigns each a unique process identification number (PID).
There are three basic types of processes in Linux: interactive, batch and daemon. Interactive processes are run interactively by a user at the command line (i.e., all-text mode). Batch processes are submitted from a queue of processes and are not associated with the command line; they are well suited for performing recurring tasks when system usage is otherwise low.
Daemons are recognized by the system as any processes whose parent process has a PID of one, which always represents the process init. init is always the first process that is started when a Linux computer is booted up (i.e., started), and it remains on the system until the computer is turned off. init adopts any process whose parent process dies (i.e., terminates) without waiting for the child process's status. Thus, the common method for launching a daemon involves forking (i.e., dividing) once or twice, and making the parent (and grandparent) processes die while the child (or grandchild) process begins performing its normal function.
Two good references:
Just to expand on https://unix.stackexchange.com/a/207724/72402:
A binary can be named whatever the author wants to call it but it is common practice to name a binary with a d at the end to indicate that it is a daemon process/invoker.