Systems CourseC/C++ · Linux · Memory
LinuxLesson 29 of 441 min read6 questions

Permissions (rwx)

Read the 10-char mode; r=4 w=2 x=1.

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Concept core idea Watch out easy to get wrong

Concept

Key concepts

  1. ls -l line: 10 chars = 1 file-type (- file, d dir, l symlink) + three rwx triplets for owner / group / others.
  2. r=4, w=2, x=1; add per triplet → numeric modes like 755 (rwxr-xr-x), 644 (rw-r--r--).
  3. A binary needs the x bit to run with ./; a source .cpp is usually 644 (no x), a compiled binary 755.
Watch out

Easily confused

owner vs group vs others
Three separate rwx triplets: the file's owner, its group, everyone else. The kernel picks which triplet applies from your UID/GID.
r/w/x on a file vs a directory
On a directory, x = can enter/traverse, r = can list names, w = can add/remove entries — different from files.

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6 questions on this lesson. Write your answers from memory, then compare them with the model answers side by side.

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