What are file permissions in Linux? Explain `chmod`, `chown`, special bits (SUID, SGID, sticky), and the meaning of 755 / 644 / 600.

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Linux file permissions control who can read (r) , write (w) , and execute (x) a file, for three categories: owner (u) , group (g) , others (o) . chmod — change permissions. Numeric (octal) or symbolic. chown — change owner / group: chown user:group file . Octal cheat sheet: r=4, w=2, x=1. 755 = rwxr-xr-x (executables, directories). 644 = rw-r--r-- (config, source files). 600 = rw------- (SSH keys, secrets). 700 = rwx------ (private executable / SSH dir). Special bits: SUID (4xxx) — file runs with the owner's privileges. Why passwd can edit /etc/shadow as a regular user. SGID (2xxx) — new files inside a directory inherit the directory's group. Sticky bit (1xxx) — only the file's owner (or root) can delete it. Why /tmp is safe to share. Most candidates can read 755/644 but stumble on special bits. SUID is why /usr/bin/passwd works without sudo. Sticky bit on /tmp prevents users from deleting each other's files. Naming these correctly signals real Linux depth.