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Chmod Calculator

Convert Unix file permissions between checkboxes, octal like 755 and symbolic notation like rwxr-xr-x, all kept in sync as you edit any of them. Covers setuid, setgid and the sticky bit, and shows the matching chmod command ready to copy.

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Permissions

Owneru
Groupg
Otherso
Special

Octal and symbolic

Octal takes 3 digits, or 4 with the special bits in front (4 setuid, 2 setgid, 1 sticky). Symbolic accepts s, S, t and T in the execute slots, and a pasted ls -l string like -rwxr-xr-x works too.

Special bitsOwnerGroupOthers

Presets

In plain words

The owner can read, write and run it. The group and everyone else can read and run it.

Command

Numeric form
chmod 755 file
Symbolic form
chmod u=rwx,go=rx file

Umask

What permissions new files and directories get by default under a given umask.

New files
644 · rw-r--r--
New directories
755 · rwxr-xr-x

A umask removes permissions rather than granting them. Programs ask for 666 when creating files and 777 for directories, and each umask digit takes its bits away from the matching digit.

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About the Chmod Calculator Tool

This tool converts Unix file permissions between the three ways they are written: a checkbox grid, octal numbers like 755 and symbolic strings like rwxr-xr-x. Edit any of the three and the other two follow along live, including the setuid, setgid and sticky bits.

Matching colors tie the notations together. The special bits, owner, group and others each keep one color across the checkboxes, the octal digits, the symbolic characters and the plain-English summary, so you can see exactly which digit drives which letters.

What you can do

  • Convert octal permissions like 755 to rwxr-xr-x and back.
  • Build a permission set by ticking read, write and execute boxes per class.
  • Copy the finished command in both forms, chmod 755 file and chmod u=rwx,go=rx file.
  • Set the setuid, setgid and sticky bits with 4-digit octal like 4755.
  • Decode a permission string pasted from ls -l output.
  • Read a plain-English summary of who can do what, including what the special bits mean.
  • Apply common presets like 644, 755, 600 and 700 with one click.
  • Check which default permissions a umask like 022 gives new files and directories.

How to use the Chmod Calculator

  1. 1Tick the boxes, pick a preset, or type an octal or symbolic value. All three stay in sync, and invalid input gets an explanation instead of being ignored.
  2. 2Read the plain-words summary to check the permissions do what you expect.
  3. 3Set the file name and copy the chmod command in the form you prefer.

Octal and symbolic notation

Each class of users gets one octal digit, adding up read (4), write (2) and execute (1). So 755 means 7 for the owner (read, write and execute) and 5 for the group and everyone else (read and execute). The symbolic form spells the same nine bits out as three rwx triplets, which is what ls -l prints.

A fourth leading digit carries the special bits: 4 for setuid, 2 for setgid and 1 for the sticky bit. In symbolic notation they appear as s or t in the execute slots, capitalized when the matching execute bit is off. 4755 is the same permission set as rwsr-xr-x.

Setuid, setgid and the sticky bit

Setuid makes a program run with its owner's permissions instead of the caller's, which is how passwd can edit a root-owned password file. Setgid does the same with the group, and on a directory it makes new files inherit the directory's group. The sticky bit on a shared directory like /tmp stops users from deleting or renaming each other's files.

Careful with 777

chmod 777 gives every user on the system full control, and it is almost never the right fix for a permission error. On anything shared or reachable from the internet it lets any process rewrite your files. Most files want 644, executables and directories want 755, and private keys want 600.

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