Chmod Calc

Calculate chmod permissions — convert between numeric (octal) and symbolic notation, and generate the chmod command. Use when the user asks what chmod 755 me...

chmod Calculator

Convert file permission specifications between numeric (octal) and symbolic notation, and produce the corresponding chmod command.

Input

• Either a numeric permission string (e.g. 755, 644, 4755)
• Or a description of permissions per entity (owner/group/others with read/write/execute)
• Or a symbolic string (e.g. rwxr-xr-x)

Output

• Numeric notation (3 or 4 digits)
• Symbolic notation (9 characters, e.g. rwxr-xr-x)
• Numeric chmod command: chmod 755 filename
• Symbolic chmod command: chmod u+rwx,g+rx,o+rx filename
• Plain-English description of what each entity can do

Instructions

Permission bit values

Each entity (owner/group/others) is a sum of:

• Read (r) = 4
• Write (w) = 2
• Execute (x) = 1

Special permission bits (4th leading digit)

• Setuid = 4 (runs as owner)
• Setgid = 2 (runs as group)
• Sticky = 1 (only owner can delete in directory)

Numeric → Symbolic conversion

Given a 3-digit octet OGO:

• For each digit, decompose: r if bit 2 set, w if bit 1 set, x if bit 0 set, else -.
• With leading special digit S: setuid sets s/S in owner execute position; setgid sets s/S in group execute position; sticky sets t/T in others execute position.

Symbolic → Numeric conversion

Map each 3-char group to a digit using the bit values above.

Symbolic chmod command format

Build clauses for each entity with permissions set:

• u = owner, g = group, o = others
• Emit {entity}+{perms} for each entity that has any permissions
• Special bits: u+s (setuid), g+s (setgid), +t (sticky)

Common permission reference

Numeric	Symbolic	Description
644	rw-r--r--	Standard file (owner rw, rest r)
755	rwxr-xr-x	Standard directory/executable
600	rw-------	Private file (owner only)
700	rwx------	Private directory/executable
777	rwxrwxrwx	Full access (avoid in production)
4755	rwsr-xr-x	Setuid executable
2755	rwxr-sr-x	Setgid executable
1755	rwxr-xr-t	Sticky directory

Options

• Input can be numeric, symbolic, or a plain-English description of desired permissions.

Examples

Input: 755 Output:

Numeric:          755
Symbolic:         rwxr-xr-x
Command (numeric):   chmod 755 filename
Command (symbolic):  chmod u+rwx,g+rx,o+rx filename

Owner: read, write, execute
Group: read, execute
Others: read, execute

Input: 644 Output:

Numeric:          644
Symbolic:         rw-r--r--
Command (numeric):   chmod 644 filename
Command (symbolic):  chmod u+rw,g+r,o+r filename

Owner: read, write
Group: read
Others: read

Input: rwxr-xr-x Output:

Numeric:          755
Symbolic:         rwxr-xr-x
Command (numeric):   chmod 755 filename
Command (symbolic):  chmod u+rwx,g+rx,o+rx filename

Input: "owner can read and write, group can read, others nothing" Output:

Numeric:          640
Symbolic:         rw-r-----
Command (numeric):   chmod 640 filename
Command (symbolic):  chmod u+rw,g+r filename

Error Handling

• If numeric input has more than 4 digits or contains non-octal characters, say so.
• If a digit in the octet exceeds 7, explain that each permission digit must be 0–7.
• If symbolic input is not exactly 9 or 10 characters in valid format, ask for clarification.
• Warn the user if they request 777 that this allows full access to everyone and is a security risk.