Learn Linux

Permissions are shown in three groups of three for owner, group, and others. Each group is read (r=4), write (w=2), execute (x=1). So 'rwxr-xr--' means the owner can read/write/execute (7), the group can read/execute (5), and others can only read (4) — i.e. mode 754. Directories need execute permission to be entered.

A hard link is another directory entry pointing to the same inode (the same data on disk); the file's data survives as long as any hard link exists, and hard links can't span filesystems or link directories. A symbolic (soft) link is a small file that stores a path to another file; it can cross filesystems and link directories but breaks if the target is moved or deleted.

Use 'ss -ltnp' (or the older 'netstat -ltnp') to list listening TCP sockets with the owning process, or 'lsof -i :8080' to see what's bound to a specific port. You typically need sudo to see process names for ports owned by other users.

A process is any running instance of a program. A daemon is a background process that runs detached from a terminal, usually started at boot and managed by the init system (systemd), providing a service — like sshd or nginx. Daemon names conventionally end in 'd'.

Start with 'df -h' to see which filesystem is full, then 'du -sh *' (or 'du -h --max-depth=1') from the top of that filesystem to drill into the largest directories. Common culprits are runaway logs (check /var/log), old package caches, and orphaned files still held open by a process (visible via 'lsof' even after deletion). Clean up or rotate logs and remove unneeded files.

It recursively searches the current directory tree for lines containing the text 'error' and prints the matching file and line. You can add -i for case-insensitive matching, -n for line numbers, -l to show only filenames, and -v to invert the match (show lines that do NOT contain the pattern). Pipe to another command for further filtering.