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#

NumericSymbolicDescription644rw-r--r--Standard file (owner rw, rest r)755rwxr-xr-xStandard directory/executable600rw-------Private file (owner only)700rwx------Private directory/executable777rwxrwxrwxFull access (avoid in production)4755rwsr-xr-xSetuid executable2755rwxr-sr-xSetgid executable1755rwxr-xr-tSticky directory

Options#

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

Examples#

Input: 755 Output:

text

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:

text

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:

text

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:

text

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.