chmod 755 means: the owner can read, write, and execute the file. Everyone else can read and execute it, but not modify it.

chmod 755 script.sh

This is the standard permission for executable scripts and programs.

Breaking Down 755

Each digit represents permissions for a different group:

Digit Who Permissions
7 Owner (you) Read + Write + Execute
5 Group Read + Execute
5 Others Read + Execute

So 755 means:

  • You can do anything with the file
  • Everyone else can run it and read it, but not change it

How the Numbers Work

Permissions are calculated by adding:

Number Permission
4 Read (r)
2 Write (w)
1 Execute (x)

So:

  • 7 = 4 + 2 + 1 = Read + Write + Execute
  • 6 = 4 + 2 = Read + Write
  • 5 = 4 + 1 = Read + Execute
  • 4 = Read only
  • 0 = No permissions

Common Permission Sets

Number Meaning When to use
755 rwxr-xr-x Executable scripts, programs
644 rw-r--r-- Regular files (documents, configs)
700 rwx------ Private executables
600 rw------- Private files (SSH keys, secrets)
777 rwxrwxrwx Everyone can do everything (rarely needed, often dangerous)

Reading Permissions

To see a file's current permissions:

ls -l script.sh

Output:

-rwxr-xr-x  1 you  staff  1234 Jan 15 10:00 script.sh

The -rwxr-xr-x breaks down as:

Characters Meaning
- It's a file (d = directory)
rwx Owner: read, write, execute
r-x Group: read, execute (no write)
r-x Others: read, execute (no write)

When to Use chmod 755

Making a script executable:

chmod 755 myscript.sh
./myscript.sh

Fixing "permission denied" on a script you wrote:

chmod 755 backup.sh

After downloading an executable:

chmod 755 downloaded-tool

When to Use chmod 644

For regular files that shouldn't be executed:

chmod 644 document.txt
chmod 644 config.json

This is the default for most files.

Private Files: 600 and 700

For files only you should access:

chmod 600 ~/.ssh/id_rsa        # SSH private key
chmod 700 ~/.ssh               # SSH directory

SSH actually requires these permissions - it refuses to work if your key is readable by others.

The Dangerous 777

chmod 777 file.sh    # Don't do this unless you know why

This lets anyone read, write, and execute. Rarely correct. If something tells you to use 777, there's usually a better solution.

Using Letters Instead of Numbers

You can also use symbolic notation:

chmod u+x script.sh      # Add execute for user (owner)
chmod go-w file.txt      # Remove write for group and others
chmod a+r document.pdf   # Add read for all
Symbol Meaning
u User (owner)
g Group
o Others
a All (everyone)
+ Add permission
- Remove permission
= Set exactly

Changing Folder Permissions

Folders work the same way, but execute means "can enter the directory":

chmod 755 myfolder/

For a folder and everything inside it:

chmod -R 755 myfolder/

The -R means recursive.

Why Permissions Matter

  • Security: Prevents unauthorized access to sensitive files
  • Functionality: Scripts need execute permission to run
  • Multi-user systems: Controls what others can do with shared files

On your personal Mac, you're mostly dealing with the "owner" permissions. The others matter more on shared or server systems.

Keep Learning

Understanding permissions is part of understanding how Unix (which macOS is based on) works. The free course covers more fundamentals.

Check it out at Mac Terminal for Humans.