File access methods and protection

Access methods describe how records of a file are read. Sequential access processes records in order; direct (relative) access jumps to any record number, so record n of size r starts at byte n x r (block = n x r / block size). Indexed access uses an index to locate records.

Protection controls who may do what. UNIX gives each file three permission triples for the owner, group and others, each with read (4), write (2) and execute (1). The string rwxr-xr-- is octal 754: owner 7, group 5, others 4. The first matching class decides: if the user is the owner, owner bits apply; otherwise group; otherwise others. On a directory, read lets you list names, write lets you create or delete entries, and execute (search) lets you pass through the directory to reach entries by name. Opening /a/b/c.txt needs search permission on every directory in the path.

The umask removes bits from the default mode of new files: 666 with umask 022 gives 644.

Worked example: record 37 of 128 bytes with 512-byte blocks is at byte 4736, so block 9, offset 128.

Exam trap: execute on a directory does not mean running it; it means searching it.