initd

init.d is a directory commonly found on Unix-like systems that houses a collection of initialization scripts used by the legacy System V init (SysVinit) startup process. Located typically at /etc/init.d, the scripts are executable and implement actions such as start, stop, restart, and status to control services during boot, shutdown, and normal operation.

How it works: In SysVinit systems, the system boots into a specific runlevel and init uses runlevel directories (for example, /etc/rc2.d, /etc/rc3.d) containing symbolic links to the scripts in /etc/init.d. Links prefixed with S indicate a start action and a numeric order, while links prefixed with K indicate a kill/stop action. The order of these links determines the sequence in which services are started or stopped.

Script structure and dependencies: Init scripts in /etc/init.d are typically shell scripts. Many include a Linux Standard Base (LSB) header that describes dependencies, start/stop runlevels, and what the script provides. This metadata helps tools determine the correct startup order and which services must be available first.

Context in modern systems: While many distributions have migrated to systemd or other modern init systems, the init.d directory remains for compatibility. Systemd, for example, can execute SysVinit scripts via a compatibility layer and can generate corresponding unit files from them. Management utilities in SysVinit-based systems include update-rc.d (Debian-based) or chkconfig (Red Hat-based) to install or remove the appropriate links in rc?.d directories.

Usage: Administrators place a script in /etc/init.d, make it executable, and use tools or manual symlinks to enable or disable it at specific runlevels (for example, by configuring defaults or using update-rc.d). Typical scripts support at least start and stop, with restart and status commonly available.