Master Linux Boot Autostart, Aliases, and Links with systemctl, alias, and ln

Boot Autostart: systemd and rc.local

Linux starts services automatically through a built‑in mechanism. The modern approach is systemd , used by CentOS 7+, Ubuntu 16.04+, and Debian 8+. Enabling a service is a single command: systemctl enable nginx This creates a symbolic link in systemd’s configuration directory, marking the service to be started on the next boot. To disable: systemctl disable nginx Check status with: systemctl is-enabled nginx and view runtime state with: systemctl status nginx Note that enable does not start the service immediately; use systemctl start nginx or combine both with systemctl enable --now nginx.

The legacy method rc.local runs a script at boot. Create or edit /etc/rc.local, ensure it is executable ( chmod +x /etc/rc.local) and ends with exit 0. Modern distributions treat rc.local as compatibility mode, so systemd is preferred.

Custom systemd Service

To manage a custom script with systemd, create a unit file such as /etc/systemd/system/myservice.service:

[Unit]
Description=My Custom Service
After=network.target

[Service]
ExecStart=/usr/local/bin/myscript.sh
Restart=always

[Install]
WantedBy=multi-user.target

Key fields:

After=network.target – start after the network is ready.

Restart=always – automatically restart on failure.

WantedBy=multi-user.target – equivalent to the traditional multi‑user run level.

Reload the daemon and enable the service:

systemctl daemon-reload
systemctl enable --now myservice

Alias: Shortcuts for Long Commands

The alias builtin lets you assign a short name to a longer command. Basic syntax:

alias ll='ls -alh --color=auto'
alias grep='grep --color=auto'
alias vi='vim'
alias cls='clear'

List current aliases with alias. To make an alias persistent, add it to ~/.bashrc and reload the file with source ~/.bashrc. Removing an alias uses unalias (e.g., unalias ll or unalias -a to clear all). Prefix a command with a backslash (e.g., \rm -rf /tmp/*) to bypass an alias.

ln: Soft Links vs Hard Links

Understanding ln requires the concept of an inode , the filesystem’s unique identifier for a file. The filename is merely a label pointing to an inode.

Soft (symbolic) links are created with -s: ln -s /var/log/nginx/access.log ./access A soft link is an independent file containing the target path. It can cross filesystem boundaries and link to directories, but becomes a “dead link” if the target is removed.

Hard links are created without -s: ln /etc/nginx/nginx.conf ./nginx.bak Hard links add another name to the same inode; they cannot span filesystems or point to directories. Deleting the original name does not remove the data as long as at least one hard link remains.

Practical Scenarios

Redirect a log directory to a larger disk with a soft link.

Switch Python versions by linking /usr/bin/python3 to the desired binary.

Backup a configuration file using a hard link before editing.

Summary

Systemd is the mainstream method for boot autostart; systemctl enable sets the flag, --now starts immediately. rc.local remains for legacy understanding. Aliases shorten repetitive commands; make them permanent via ~/.bashrc. Soft links ( ln -s) act like shortcuts with separate inodes, while hard links share the same inode and cannot cross filesystems. These three topics are frequent interview questions and daily operations tasks.