Current Permission
555
u=rx,g=rx,o=rx
Presets
Permissions

user

group

others

Special Bits
Direct Editing
Options

Introduction:

Unix and POSIX-like operating systems gate file access through a three-tier permission model that expresses read, write, and execute rights for the file owner, members of the owner’s group, and all other users. Each right is stored as a single binary flag and surfaced in octal, symbolic, or long-listing formats.

This tool lets you toggle those flags directly, convert between octal and symbolic notations, add special setuid, setgid, or sticky bits, and instantly preview a ready-to-run chmod command alongside the matching ls -l line. All calculations run locally, enabling rapid experimentation with zero risk to production systems.

Use it before deploying shell scripts, container images, or configuration-management playbooks to verify that a binary remains runnable yet immutable for non-admins; incorrect flags can silently break application startup or expose sensitive data.

Technical Details:

Unix permissions encode three independent bit-blocks—owner, group, others—where each block holds a read (4), write (2), and execute (1) flag. Adding the flag values yields one octal digit per block; prefixing an optional special-bit digit extends the scheme to four places (0-7xxx).

Core Process:

  1. Sum the active flags for each entity: P=4×r+2×w+x.
  2. Concatenate the three digits to form an octal string.
  3. Prepend a fourth digit when setuid, setgid, or sticky is set (values 4, 2, 1 respectively).
  4. Translate each bit back to rwx or - for long listings.
OctalBinarySymbolic
0000---
1001--x
2010-w-
3011-wx
4100r--
5101r-x
6110rw-
7111rwx
  • read – allows viewing file contents or directory listings.
  • write – permits modifying contents or creating entries.
  • execute – enables running binaries or traversing directories.
  • setuid/setgid – elevates executed file to owner/group ID.
  • sticky – restricts deletion within world-writable directories.

Worked example (0755):

74+2+1=rwx
54+1=r-x
5r-x

Final symbolic: -rwxr-xr-x

Assumptions & Limitations:

  • Numeric form assumes octal; decimal input yields incorrect flags.
  • Special bits affect only executable files or directory semantics.
  • Network file systems may override local mode changes.
  • Access Control Lists extend permissions beyond the classic model.

Edge Cases & Error Sources:

  • Leading zeros dropped by some shells.
  • Applying execute without read on scripts can still fail under certain interpreters.
  • Sticky bit on non-directories is ignored by most kernels.
  • Setuid is ignored on files mounted with nosuid.

Scientific validity & references: Defined in the Single UNIX Specification v4, POSIX 1003.1-2017, and historically analysed in Bach’s “The Design of the Unix Operating System”.

The concept processes no personal data and falls outside GDPR scope.

Step-by-Step Guide:

Follow these steps to generate a permission string and command.

  1. Select the read, write, and execute rights for each entity.
  2. (Optional) Toggle setuid, setgid, or sticky.
  3. Observe the live octal, symbolic, and long-listing previews.
  4. Enable recursive or verbose modifiers if applying to directory trees.
  5. Copy the generated chmod command and run it on your target system.

FAQ:

What are special bits?

They alter default execution or deletion behaviour: setuid and setgid run the file with elevated IDs, while sticky protects directory entries.

Why use octal instead of symbolic?

Octal notation is shorter, script-friendly, and unambiguous, especially when setting all three entities in one operation.

Is my data stored?

No. All calculations occur within your browser session; nothing leaves your machine.

Can I remove execute alone?

Yes. Clear the execute box for the desired entity or subtract x from the symbolic string, then copy the updated command.

Does it handle ACLs?

No. The classic permission model is displayed; advanced ACL entries require dedicated system utilities.

Glossary:

Octal Digit
Base-8 value of a permission block.
Symbolic Mode
Text form like u=rw,g=r.
Setuid
Execute under file owner’s user ID.
Setgid
Execute under file owner’s group ID.
Sticky Bit
Restricts deletions in shared directories.

No data is transmitted or stored server-side.