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Unix File Permissions (chmod)

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Round-trips between the three forms unix uses for file permissions: the bit checkboxes, the octal (sometimes four digits when setuid/setgid/sticky are set), and the ls-style symbolic string. The bottom section emits ready-to-copy chmod and find -perm commands plus a human-readable summary, so you can see at a glance what 4755 actually grants. Common modes are one click away via the preset row.

—
Presets
Permission bits
Owner
Group
Other
Special bits
  • chmodchmod 755 <file>
  • find -permfind . -type f -perm 755
  • Meaning
    • owner: read + write + execute
    • group: read + execute
    • other: read + execute

Symbolic uses s/S for setuid/setgid and t/T for sticky — capital means the special bit is set but the matching x is not.

How to use

  1. Pick a preset, or paste an octal mode you copied off a tutorial, or type the symbolic form.
  2. Tick the special-bit boxes (setuid, setgid, sticky) if you need them.
  3. Copy the chmod command into your shell.

Frequently asked questions

Why is 755 "correct" for directories?
Directories need the execute bit to be traversable — without x on a directory you can list its name but not cd into it. 755 gives the owner full control and lets everyone else traverse + read.
What does the s in rws actually do?
On an executable file, setuid (the s bit in the user-execute slot) makes the file run with the owner's effective UID instead of the caller's. On a directory, setgid makes new files inherit the directory's group. Sticky on a directory (like /tmp) means only the file's owner can delete it.

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