chmod sets permission bits on files and directories: read (r), write (w), execute (x) for user, group, and others. You can express changes symbolically (u+x, go-w) or with octal (for example 644 or 2755 when including setgid). Directories need execute bit to traverse; files need it to run as programs. Recursive -R is powerful—mistakes propagate tree-wide.
| Mode | Meaning | Typical use |
|---|---|---|
| 644 | rw-r--r-- | Private data files, world-readable |
| 755 | rwxr-xr-x | Programs and traversable dirs |
| 1777 | sticky /tmp style | Shared writable dirs with deletion rules |
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