Linux Permissions

Three triples, three bits each

Permissions cover owner, group and others, each with read (4), write (2) and execute (1). Summing gives the octal digit: 7 is all three, 6 is read and write, 5 is read and execute. So 755 means the owner can do everything while group and others can read and execute, and 644 gives the owner read and write with everyone else read-only.

Directories reinterpret every bit

On a directory, read lists names, write creates and deletes entries, and execute permits entering it and accessing anything inside. A directory with read but no execute lets you see filenames and open nothing. This is why 644 on a directory is broken while 755 is normal, and why removing execute from a parent directory blocks access to files whose own permissions are unchanged.

Delete depends on the directory, not the file

Removing a file requires write on its containing directory. Someone with no write permission on a read-only file can still delete it if they can write to the folder — the reason the sticky bit exists on shared directories such as /tmp, where it restricts deletion to owners.

777 is almost never the answer

It grants every user on the system write access, which is a genuine vulnerability on any shared or internet-facing host. It appears in troubleshooting advice because it makes permission errors disappear, and it does so by removing the protection rather than fixing the cause, which is usually ownership.

SetUID, SetGID and the sticky bit

A fourth leading digit sets these: 4 for SetUID, which runs a binary as its owner, 2 for SetGID, which on a directory makes new files inherit the group, and 1 for the sticky bit. SetUID on a writable binary is a privilege escalation waiting to happen.