What does chmod 644 mean?

Chmod 644 means the owner can read and write the file, while the group and others can only read it. No one can execute it. This is the most common setting for regular files like HTML pages or config files.

What does chmod 755 mean?

Chmod 755 gives the owner full access (read, write, and execute). The group and others can read and execute but cannot write. This is the standard setting for scripts, programs, and directories.

What is the difference between octal and symbolic notation?

Octal notation uses numbers like 755 . Each digit represents a user class (owner, group, others), and the digit is the sum of read (4), write (2), and execute (1). Symbolic notation uses letters like rwxr-xr-x . Both describe the same permissions in different formats. This calculator converts between them instantly.

Is chmod 777 safe to use?

No. Chmod 777 gives every user on the system full read, write, and execute access. This is a serious security risk . Anyone could change or delete the file. Only use 777 for temporary testing, and switch to a safer value like 755 or 644 as soon as possible.

What does the -R flag do in chmod?

The -R flag stands for recursive . It applies the permission change to the target folder and every file and subfolder inside it . Be careful with this flag, especially when combined with broad permissions, because it changes everything at once.

What is the sticky bit?

The sticky bit (octal 1000 ) is a special permission for shared folders. When it is set, only the owner of a file can delete or rename that file, even if others have write access to the folder. The /tmp directory on Linux uses the sticky bit.

What is the difference between setuid and setgid?

Setuid (octal 4000 ) makes a file run with the permissions of the file's owner , not the user who runs it. Setgid (octal 2000 ) makes a file run with the permissions of the file's group . On directories, setgid causes new files to inherit the directory's group.

How do I read an ls -l output?

The ls -l command shows a string like -rwxr-xr-x . The first character is the file type ( - for file, d for directory). The next nine characters are permissions in three groups of three: owner , group , and others . Each group shows read (r), write (w), and execute (x), or a dash if that permission is off.

What is the difference between 3-digit and 4-digit chmod values?

A 3-digit value like 755 sets the basic read, write, and execute permissions. A 4-digit value like 4755 adds a leading digit for special bits (setuid, setgid, sticky bit). If no special bits are needed, the leading digit is 0, so 0755 and 755 are the same.

What chmod should I use for a website folder?

Most website folders should use 755 . This lets the owner make changes while allowing the web server to read and access the folder. For files inside, 644 is standard. Avoid 777 on any web-facing file or folder because it opens your site to attacks.

What does the --preserve-root flag do?

The --preserve-root flag stops chmod from running recursively on the root directory ( / ). This is a safety feature. Changing permissions on the entire root filesystem can break your system. Use this flag whenever you run chmod -R to add a layer of protection.

Can I copy permissions from another file?

Yes. Use the --reference=RFILE option. This tells chmod to copy the permissions from the reference file instead of using a number or symbolic value. In this calculator, check the Reference File box under Command Modifiers to add this flag to your command.

What is the difference between verbose and changes mode?

Verbose ( -v ) prints a message for every file that chmod processes, whether the permissions changed or not. Changes ( -c ) only prints a message when a file's permissions are actually different from what they were before. Use -c if you only want to see what was updated.

What does s and S mean in symbolic notation?

A lowercase s in the owner's execute position means setuid is on and execute is on . An uppercase S means setuid is on but execute is off . The same pattern applies to the group's execute position for setgid . For others, t and T work the same way for the sticky bit .

Why does my script say permission denied?

This usually means the file does not have execute permission for your user class. Run ls -l to check the current permissions. If you are the owner, you need at least chmod 700 or chmod u+x to run the script. Use this calculator to find the right value.

How is the octal digit calculated for each user class?

Each permission has a weight: read = 4 , write = 2 , execute = 1 . Add up the weights of the permissions that are turned on. For example, read + execute = 4 + 1 = 5 . The calculator's step-by-step section shows this math for every digit.

Infinity Calculator

Chmod Calculator

Updated Jun 30, 2026 · By Jehan Wadia

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