Unix and Linux file permissions control who can read, write, and execute files. Permissions are set using the chmod command with either numeric (octal) notation like 755 or symbolic notation like rwxr-xr-x. This interactive calculator lets you toggle individual permission bits and see the corresponding numeric code, symbolic notation, and chmod command.
rw-r--r--
chmod 644
read, write
read
read
Toggle individual permission checkboxes for Owner, Group, and Others - each has Read (r), Write (w), and Execute (x) options. The numeric (octal) value and symbolic notation update in real time. Alternatively, enter a numeric code (e.g., 755) and see the corresponding permission checkboxes and symbolic representation. The generated chmod command is displayed, ready to copy and paste into your terminal.
Chmod calculation is essential for Linux/Unix system administrators setting file and directory permissions, web developers configuring server file permissions for security, DevOps engineers writing deployment scripts with correct permission settings, Docker container configuration, SSH key file permissions (600 for private keys), and ensuring web server files have appropriate access levels to prevent unauthorized access while maintaining functionality.
Unix permissions use a 9-bit model: 3 bits each for owner, group, and others. Each set contains read (4), write (2), and execute (1) bits. The octal representation converts each 3-bit group to a single digit (0-7). Common values: 755 (rwxr-xr-x) for executables and directories, 644 (rw-r--r--) for regular files, 600 (rw-------) for private files, 777 (rwxrwxrwx) for full access (security risk). The setuid (4000), setgid (2000), and sticky (1000) bits can add a fourth octal digit.
chmod 755 sets permissions to rwxr-xr-x: the owner can read, write, and execute (7 = 4+2+1); the group can read and execute (5 = 4+0+1); others can read and execute (5 = 4+0+1). This is the standard permission for directories and executable files.
Each file has three permission sets: owner, group, and others. Each set has three bits: read (r=4), write (w=2), and execute (x=1). The numeric value is the sum: rwx = 4+2+1 = 7, r-x = 4+0+1 = 5, r-- = 4+0+0 = 4.
644 (rw-r--r--) is for regular files: owner can read and write, everyone else can only read. 755 (rwxr-xr-x) is for directories and executables: owner has full access, everyone else can read and execute but not modify.
chmod 777 gives full read, write, and execute permissions to everyone. This is a security risk and should almost never be used in production. Use the minimum permissions necessary - typically 644 for files and 755 for directories.
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